* [patch V5 01/11] rseq: Add fields and constants for time slice extension
2025-12-01 7:05 [patch V5 00/11] rseq: Implement time slice extension mechanism Thomas Gleixner
@ 2025-12-01 7:05 ` Thomas Gleixner
2025-12-04 16:35 ` [PATCH] rseq: Correct typo in the documentation Sebastian Andrzej Siewior
2025-12-01 7:05 ` [patch V5 02/11] rseq: Provide static branch for time slice extensions Thomas Gleixner
` (9 subsequent siblings)
10 siblings, 1 reply; 13+ messages in thread
From: Thomas Gleixner @ 2025-12-01 7:05 UTC (permalink / raw)
To: LKML
Cc: Mathieu Desnoyers, Paul E. McKenney, Boqun Feng, Jonathan Corbet,
Prakash Sangappa, Madadi Vineeth Reddy, K Prateek Nayak,
Steven Rostedt, Sebastian Andrzej Siewior, Arnd Bergmann,
linux-arch, Randy Dunlap, Peter Zijlstra, Ron Geva, Waiman Long
Aside of a Kconfig knob add the following items:
- Two flag bits for the rseq user space ABI, which allow user space to
query the availability and enablement without a syscall.
- A new member to the user space ABI struct rseq, which is going to be
used to communicate request and grant between kernel and user space.
- A rseq state struct to hold the kernel state of this
- Documentation of the new mechanism
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Prakash Sangappa <prakash.sangappa@oracle.com>
Cc: Madadi Vineeth Reddy <vineethr@linux.ibm.com>
Cc: K Prateek Nayak <kprateek.nayak@amd.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
V5: Document behaviour of arbitrary syscalls
V4: Make the example correct - Prakash
V3: Fix more typos and expressions - Randy
V2: Fix Kconfig indentation, fix typos and expressions - Randy
Make the control fields a struct and remove the atomicity requirement - Mathieu
---
Documentation/userspace-api/index.rst | 1
Documentation/userspace-api/rseq.rst | 135 ++++++++++++++++++++++++++++++++++
include/linux/rseq_types.h | 28 ++++++-
include/uapi/linux/rseq.h | 38 +++++++++
init/Kconfig | 12 +++
kernel/rseq.c | 7 +
6 files changed, 220 insertions(+), 1 deletion(-)
--- a/Documentation/userspace-api/index.rst
+++ b/Documentation/userspace-api/index.rst
@@ -21,6 +21,7 @@ System calls
ebpf/index
ioctl/index
mseal
+ rseq
Security-related interfaces
===========================
--- /dev/null
+++ b/Documentation/userspace-api/rseq.rst
@@ -0,0 +1,135 @@
+=====================
+Restartable Sequences
+=====================
+
+Restartable Sequences allow to register a per thread userspace memory area
+to be used as an ABI between kernel and userspace for three purposes:
+
+ * userspace restartable sequences
+
+ * quick access to read the current CPU number, node ID from userspace
+
+ * scheduler time slice extensions
+
+Restartable sequences (per-cpu atomics)
+---------------------------------------
+
+Restartable sequences allow userspace to perform update operations on
+per-cpu data without requiring heavyweight atomic operations. The actual
+ABI is unfortunately only available in the code and selftests.
+
+Quick access to CPU number, node ID
+-----------------------------------
+
+Allows to implement per CPU data efficiently. Documentation is in code and
+selftests. :(
+
+Scheduler time slice extensions
+-------------------------------
+
+This allows a thread to request a time slice extension when it enters a
+critical section to avoid contention on a resource when the thread is
+scheduled out inside of the critical section.
+
+The prerequisites for this functionality are:
+
+ * Enabled in Kconfig
+
+ * Enabled at boot time (default is enabled)
+
+ * A rseq userspace pointer has been registered for the thread
+
+The thread has to enable the functionality via prctl(2)::
+
+ prctl(PR_RSEQ_SLICE_EXTENSION, PR_RSEQ_SLICE_EXTENSION_SET,
+ PR_RSEQ_SLICE_EXT_ENABLE, 0, 0);
+
+prctl() returns 0 on success or otherwise with the following error codes:
+
+========= ==============================================================
+Errorcode Meaning
+========= ==============================================================
+EINVAL Functionality not available or invalid function arguments.
+ Note: arg4 and arg5 must be zero
+ENOTSUPP Functionality was disabled on the kernel command line
+ENXIO Available, but no rseq user struct registered
+========= ==============================================================
+
+The state can be also queried via prctl(2)::
+
+ prctl(PR_RSEQ_SLICE_EXTENSION, PR_RSEQ_SLICE_EXTENSION_GET, 0, 0, 0);
+
+prctl() returns ``PR_RSEQ_SLICE_EXT_ENABLE`` when it is enabled or 0 if
+disabled. Otherwise it returns with the following error codes:
+
+========= ==============================================================
+Errorcode Meaning
+========= ==============================================================
+EINVAL Functionality not available or invalid function arguments.
+ Note: arg3 and arg4 and arg5 must be zero
+========= ==============================================================
+
+The availability and status is also exposed via the rseq ABI struct flags
+field via the ``RSEQ_CS_FLAG_SLICE_EXT_AVAILABLE_BIT`` and the
+``RSEQ_CS_FLAG_SLICE_EXT_ENABLED_BIT``. These bits are read-only for user
+space and only for informational purposes.
+
+If the mechanism was enabled via prctl(), the thread can request a time
+slice extension by setting rseq::slice_ctrl.request to 1. If the thread is
+interrupted and the interrupt results in a reschedule request in the
+kernel, then the kernel can grant a time slice extension and return to
+userspace instead of scheduling out. The length of the extension is
+determined by the ``rseq_slice_extension_nsec`` sysctl.
+
+The kernel indicates the grant by clearing rseq::slice_ctrl::reqeust and
+setting rseq::slice_ctrl::granted to 1. If there is a reschedule of the
+thread after granting the extension, the kernel clears the granted bit to
+indicate that to userspace.
+
+If the request bit is still set when the leaving the critical section,
+userspace can clear it and continue.
+
+If the granted bit is set, then userspace invokes rseq_slice_yield(2) when
+leaving the critical section to relinquish the CPU. The kernel enforces
+this by arming a timer to prevent misbehaving userspace from abusing this
+mechanism.
+
+If both the request bit and the granted bit are false when leaving the
+critical section, then this indicates that a grant was revoked and no
+further action is required by userspace.
+
+The required code flow is as follows::
+
+ rseq->slice_ctrl.request = 1;
+ barrier(); // Prevent compiler reordering
+ critical_section();
+ barrier(); // Prevent compiler reordering
+ rseq->slice_ctrl.request = 0;
+ if (rseq->slice_ctrl.granted)
+ rseq_slice_yield();
+
+As all of this is strictly CPU local, there are no atomicity requirements.
+Checking the granted state is racy, but that cannot be avoided at all::
+
+ if (rseq->slice_ctrl.granted)
+ -> Interrupt results in schedule and grant revocation
+ rseq_slice_yield();
+
+So there is no point in pretending that this might be solved by an atomic
+operation.
+
+If the thread issues a syscall other than rseq_slice_yield(2) within the
+granted timeslice extension, the grant is also revoked and the CPU is
+relinquished immediately when entering the kernel. This is required as
+syscalls might consume arbitrary CPU time until they reach a scheduling
+point when the preemption model is either NONE or VOLUNTARY and therefore
+might exceed the grant by far.
+
+The preferred solution for user space is to use rseq_slice_yield(2) which
+is side effect free. The support for arbitrary syscalls is required to
+support onion layer architectured applications, where the code handling the
+critical section and requesting the time slice extension has no control
+over the code within the critical section.
+
+The kernel enforces flag consistency and terminates the thread with SIGSEGV
+if it detects a violation.
--- a/include/linux/rseq_types.h
+++ b/include/linux/rseq_types.h
@@ -73,12 +73,35 @@ struct rseq_ids {
};
/**
+ * union rseq_slice_state - Status information for rseq time slice extension
+ * @state: Compound to access the overall state
+ * @enabled: Time slice extension is enabled for the task
+ * @granted: Time slice extension was granted to the task
+ */
+union rseq_slice_state {
+ u16 state;
+ struct {
+ u8 enabled;
+ u8 granted;
+ };
+};
+
+/**
+ * struct rseq_slice - Status information for rseq time slice extension
+ * @state: Time slice extension state
+ */
+struct rseq_slice {
+ union rseq_slice_state state;
+};
+
+/**
* struct rseq_data - Storage for all rseq related data
* @usrptr: Pointer to the registered user space RSEQ memory
* @len: Length of the RSEQ region
- * @sig: Signature of critial section abort IPs
+ * @sig: Signature of critical section abort IPs
* @event: Storage for event management
* @ids: Storage for cached CPU ID and MM CID
+ * @slice: Storage for time slice extension data
*/
struct rseq_data {
struct rseq __user *usrptr;
@@ -86,6 +109,9 @@ struct rseq_data {
u32 sig;
struct rseq_event event;
struct rseq_ids ids;
+#ifdef CONFIG_RSEQ_SLICE_EXTENSION
+ struct rseq_slice slice;
+#endif
};
#else /* CONFIG_RSEQ */
--- a/include/uapi/linux/rseq.h
+++ b/include/uapi/linux/rseq.h
@@ -23,9 +23,15 @@ enum rseq_flags {
};
enum rseq_cs_flags_bit {
+ /* Historical and unsupported bits */
RSEQ_CS_FLAG_NO_RESTART_ON_PREEMPT_BIT = 0,
RSEQ_CS_FLAG_NO_RESTART_ON_SIGNAL_BIT = 1,
RSEQ_CS_FLAG_NO_RESTART_ON_MIGRATE_BIT = 2,
+ /* (3) Intentional gap to put new bits into a separate byte */
+
+ /* User read only feature flags */
+ RSEQ_CS_FLAG_SLICE_EXT_AVAILABLE_BIT = 4,
+ RSEQ_CS_FLAG_SLICE_EXT_ENABLED_BIT = 5,
};
enum rseq_cs_flags {
@@ -35,6 +41,11 @@ enum rseq_cs_flags {
(1U << RSEQ_CS_FLAG_NO_RESTART_ON_SIGNAL_BIT),
RSEQ_CS_FLAG_NO_RESTART_ON_MIGRATE =
(1U << RSEQ_CS_FLAG_NO_RESTART_ON_MIGRATE_BIT),
+
+ RSEQ_CS_FLAG_SLICE_EXT_AVAILABLE =
+ (1U << RSEQ_CS_FLAG_SLICE_EXT_AVAILABLE_BIT),
+ RSEQ_CS_FLAG_SLICE_EXT_ENABLED =
+ (1U << RSEQ_CS_FLAG_SLICE_EXT_ENABLED_BIT),
};
/*
@@ -53,6 +64,27 @@ struct rseq_cs {
__u64 abort_ip;
} __attribute__((aligned(4 * sizeof(__u64))));
+/**
+ * rseq_slice_ctrl - Time slice extension control structure
+ * @all: Compound value
+ * @request: Request for a time slice extension
+ * @granted: Granted time slice extension
+ *
+ * @request is set by user space and can be cleared by user space or kernel
+ * space. @granted is set and cleared by the kernel and must only be read
+ * by user space.
+ */
+struct rseq_slice_ctrl {
+ union {
+ __u32 all;
+ struct {
+ __u8 request;
+ __u8 granted;
+ __u16 __reserved;
+ };
+ };
+};
+
/*
* struct rseq is aligned on 4 * 8 bytes to ensure it is always
* contained within a single cache-line.
@@ -142,6 +174,12 @@ struct rseq {
__u32 mm_cid;
/*
+ * Time slice extension control structure. CPU local updates from
+ * kernel and user space.
+ */
+ struct rseq_slice_ctrl slice_ctrl;
+
+ /*
* Flexible array member at end of structure, after last feature field.
*/
char end[];
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -1913,6 +1913,18 @@ config RSEQ
If unsure, say Y.
+config RSEQ_SLICE_EXTENSION
+ bool "Enable rseq-based time slice extension mechanism"
+ depends on RSEQ && HIGH_RES_TIMERS && GENERIC_ENTRY && HAVE_GENERIC_TIF_BITS
+ help
+ Allows userspace to request a limited time slice extension when
+ returning from an interrupt to user space via the RSEQ shared
+ data ABI. If granted, that allows to complete a critical section,
+ so that other threads are not stuck on a conflicted resource,
+ while the task is scheduled out.
+
+ If unsure, say N.
+
config RSEQ_STATS
default n
bool "Enable lightweight statistics of restartable sequences" if EXPERT
--- a/kernel/rseq.c
+++ b/kernel/rseq.c
@@ -389,6 +389,8 @@ static bool rseq_reset_ids(void)
*/
SYSCALL_DEFINE4(rseq, struct rseq __user *, rseq, u32, rseq_len, int, flags, u32, sig)
{
+ u32 rseqfl = 0;
+
if (flags & RSEQ_FLAG_UNREGISTER) {
if (flags & ~RSEQ_FLAG_UNREGISTER)
return -EINVAL;
@@ -440,6 +442,9 @@ SYSCALL_DEFINE4(rseq, struct rseq __user
if (!access_ok(rseq, rseq_len))
return -EFAULT;
+ if (IS_ENABLED(CONFIG_RSEQ_SLICE_EXTENSION))
+ rseqfl |= RSEQ_CS_FLAG_SLICE_EXT_AVAILABLE;
+
scoped_user_write_access(rseq, efault) {
/*
* If the rseq_cs pointer is non-NULL on registration, clear it to
@@ -449,11 +454,13 @@ SYSCALL_DEFINE4(rseq, struct rseq __user
* clearing the fields. Don't bother reading it, just reset it.
*/
unsafe_put_user(0UL, &rseq->rseq_cs, efault);
+ unsafe_put_user(rseqfl, &rseq->flags, efault);
/* Initialize IDs in user space */
unsafe_put_user(RSEQ_CPU_ID_UNINITIALIZED, &rseq->cpu_id_start, efault);
unsafe_put_user(RSEQ_CPU_ID_UNINITIALIZED, &rseq->cpu_id, efault);
unsafe_put_user(0U, &rseq->node_id, efault);
unsafe_put_user(0U, &rseq->mm_cid, efault);
+ unsafe_put_user(0U, &rseq->slice_ctrl.all, efault);
}
/*
^ permalink raw reply [flat|nested] 13+ messages in thread* [PATCH] rseq: Correct typo in the documentation.
2025-12-01 7:05 ` [patch V5 01/11] rseq: Add fields and constants for time slice extension Thomas Gleixner
@ 2025-12-04 16:35 ` Sebastian Andrzej Siewior
0 siblings, 0 replies; 13+ messages in thread
From: Sebastian Andrzej Siewior @ 2025-12-04 16:35 UTC (permalink / raw)
To: Thomas Gleixner
Cc: LKML, Mathieu Desnoyers, Paul E. McKenney, Boqun Feng,
Jonathan Corbet, Prakash Sangappa, Madadi Vineeth Reddy,
K Prateek Nayak, Steven Rostedt, Arnd Bergmann, linux-arch,
Randy Dunlap, Peter Zijlstra, Ron Geva, Waiman Long
for consistency:
s/slice_ctrl.request/slice_ctrl::request/
s/reqeust/request
Please fold.
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
Documentation/userspace-api/rseq.rst | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/Documentation/userspace-api/rseq.rst b/Documentation/userspace-api/rseq.rst
index c7d1c651514a5..e1fdb0d5ce695 100644
--- a/Documentation/userspace-api/rseq.rst
+++ b/Documentation/userspace-api/rseq.rst
@@ -75,13 +75,13 @@ field via the ``RSEQ_CS_FLAG_SLICE_EXT_AVAILABLE_BIT`` and the
space and only for informational purposes.
If the mechanism was enabled via prctl(), the thread can request a time
-slice extension by setting rseq::slice_ctrl.request to 1. If the thread is
+slice extension by setting rseq::slice_ctrl::request to 1. If the thread is
interrupted and the interrupt results in a reschedule request in the
kernel, then the kernel can grant a time slice extension and return to
userspace instead of scheduling out. The length of the extension is
determined by the ``rseq_slice_extension_nsec`` sysctl.
-The kernel indicates the grant by clearing rseq::slice_ctrl::reqeust and
+The kernel indicates the grant by clearing rseq::slice_ctrl::request and
setting rseq::slice_ctrl::granted to 1. If there is a reschedule of the
thread after granting the extension, the kernel clears the granted bit to
indicate that to userspace.
--
2.51.0
^ permalink raw reply [flat|nested] 13+ messages in thread
* [patch V5 02/11] rseq: Provide static branch for time slice extensions
2025-12-01 7:05 [patch V5 00/11] rseq: Implement time slice extension mechanism Thomas Gleixner
2025-12-01 7:05 ` [patch V5 01/11] rseq: Add fields and constants for time slice extension Thomas Gleixner
@ 2025-12-01 7:05 ` Thomas Gleixner
2025-12-01 7:05 ` [patch V5 03/11] rseq: Add statistics " Thomas Gleixner
` (8 subsequent siblings)
10 siblings, 0 replies; 13+ messages in thread
From: Thomas Gleixner @ 2025-12-01 7:05 UTC (permalink / raw)
To: LKML
Cc: Mathieu Desnoyers, Paul E. McKenney, Boqun Feng, Jonathan Corbet,
Prakash Sangappa, Madadi Vineeth Reddy, K Prateek Nayak,
Steven Rostedt, Sebastian Andrzej Siewior, Arnd Bergmann,
linux-arch, Randy Dunlap, Peter Zijlstra, Ron Geva, Waiman Long
Guard the time slice extension functionality with a static key, which can
be disabled on the kernel command line.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
---
V4: Return 0 on error in __setup() - Randy
V3: Document command line parameter - Sebastian
V2: Return 1 from __setup() - Prateek
---
Documentation/admin-guide/kernel-parameters.txt | 5 +++++
include/linux/rseq_entry.h | 11 +++++++++++
kernel/rseq.c | 17 +++++++++++++++++
3 files changed, 33 insertions(+)
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -6482,6 +6482,11 @@
rootflags= [KNL] Set root filesystem mount option string
+ rseq_slice_ext= [KNL] RSEQ based time slice extension
+ Format: boolean
+ Control enablement of RSEQ based time slice extension.
+ Default is 'on'.
+
initramfs_options= [KNL]
Specify mount options for for the initramfs mount.
--- a/include/linux/rseq_entry.h
+++ b/include/linux/rseq_entry.h
@@ -75,6 +75,17 @@ DECLARE_STATIC_KEY_MAYBE(CONFIG_RSEQ_DEB
#define rseq_inline __always_inline
#endif
+#ifdef CONFIG_RSEQ_SLICE_EXTENSION
+DECLARE_STATIC_KEY_TRUE(rseq_slice_extension_key);
+
+static __always_inline bool rseq_slice_extension_enabled(void)
+{
+ return static_branch_likely(&rseq_slice_extension_key);
+}
+#else /* CONFIG_RSEQ_SLICE_EXTENSION */
+static inline bool rseq_slice_extension_enabled(void) { return false; }
+#endif /* !CONFIG_RSEQ_SLICE_EXTENSION */
+
bool rseq_debug_update_user_cs(struct task_struct *t, struct pt_regs *regs, unsigned long csaddr);
bool rseq_debug_validate_ids(struct task_struct *t);
--- a/kernel/rseq.c
+++ b/kernel/rseq.c
@@ -483,3 +483,20 @@ SYSCALL_DEFINE4(rseq, struct rseq __user
efault:
return -EFAULT;
}
+
+#ifdef CONFIG_RSEQ_SLICE_EXTENSION
+DEFINE_STATIC_KEY_TRUE(rseq_slice_extension_key);
+
+static int __init rseq_slice_cmdline(char *str)
+{
+ bool on;
+
+ if (kstrtobool(str, &on))
+ return 0;
+
+ if (!on)
+ static_branch_disable(&rseq_slice_extension_key);
+ return 1;
+}
+__setup("rseq_slice_ext=", rseq_slice_cmdline);
+#endif /* CONFIG_RSEQ_SLICE_EXTENSION */
^ permalink raw reply [flat|nested] 13+ messages in thread* [patch V5 03/11] rseq: Add statistics for time slice extensions
2025-12-01 7:05 [patch V5 00/11] rseq: Implement time slice extension mechanism Thomas Gleixner
2025-12-01 7:05 ` [patch V5 01/11] rseq: Add fields and constants for time slice extension Thomas Gleixner
2025-12-01 7:05 ` [patch V5 02/11] rseq: Provide static branch for time slice extensions Thomas Gleixner
@ 2025-12-01 7:05 ` Thomas Gleixner
2025-12-01 7:06 ` [patch V5 04/11] rseq: Add prctl() to enable " Thomas Gleixner
` (7 subsequent siblings)
10 siblings, 0 replies; 13+ messages in thread
From: Thomas Gleixner @ 2025-12-01 7:05 UTC (permalink / raw)
To: LKML
Cc: Mathieu Desnoyers, Paul E. McKenney, Boqun Feng, Jonathan Corbet,
Prakash Sangappa, Madadi Vineeth Reddy, K Prateek Nayak,
Steven Rostedt, Sebastian Andrzej Siewior, Arnd Bergmann,
linux-arch, Randy Dunlap, Peter Zijlstra, Ron Geva, Waiman Long
Extend the quick statistics with time slice specific fields.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
---
V5: Add s_aborted to account for arbitrary syscalls
---
include/linux/rseq_entry.h | 5 +++++
kernel/rseq.c | 14 ++++++++++++++
2 files changed, 19 insertions(+)
--- a/include/linux/rseq_entry.h
+++ b/include/linux/rseq_entry.h
@@ -15,6 +15,11 @@ struct rseq_stats {
unsigned long cs;
unsigned long clear;
unsigned long fixup;
+ unsigned long s_granted;
+ unsigned long s_expired;
+ unsigned long s_revoked;
+ unsigned long s_yielded;
+ unsigned long s_aborted;
};
DECLARE_PER_CPU(struct rseq_stats, rseq_stats);
--- a/kernel/rseq.c
+++ b/kernel/rseq.c
@@ -138,6 +138,13 @@ static int rseq_stats_show(struct seq_fi
stats.cs += data_race(per_cpu(rseq_stats.cs, cpu));
stats.clear += data_race(per_cpu(rseq_stats.clear, cpu));
stats.fixup += data_race(per_cpu(rseq_stats.fixup, cpu));
+ if (IS_ENABLED(CONFIG_RSEQ_SLICE_EXTENSION)) {
+ stats.s_granted += data_race(per_cpu(rseq_stats.s_granted, cpu));
+ stats.s_expired += data_race(per_cpu(rseq_stats.s_expired, cpu));
+ stats.s_revoked += data_race(per_cpu(rseq_stats.s_revoked, cpu));
+ stats.s_yielded += data_race(per_cpu(rseq_stats.s_yielded, cpu));
+ stats.s_aborted += data_race(per_cpu(rseq_stats.s_aborted, cpu));
+ }
}
seq_printf(m, "exit: %16lu\n", stats.exit);
@@ -148,6 +155,13 @@ static int rseq_stats_show(struct seq_fi
seq_printf(m, "cs: %16lu\n", stats.cs);
seq_printf(m, "clear: %16lu\n", stats.clear);
seq_printf(m, "fixup: %16lu\n", stats.fixup);
+ if (IS_ENABLED(CONFIG_RSEQ_SLICE_EXTENSION)) {
+ seq_printf(m, "sgrant: %16lu\n", stats.s_granted);
+ seq_printf(m, "sexpir: %16lu\n", stats.s_expired);
+ seq_printf(m, "srevok: %16lu\n", stats.s_revoked);
+ seq_printf(m, "syield: %16lu\n", stats.s_yielded);
+ seq_printf(m, "sabort: %16lu\n", stats.s_aborted);
+ }
return 0;
}
^ permalink raw reply [flat|nested] 13+ messages in thread* [patch V5 04/11] rseq: Add prctl() to enable time slice extensions
2025-12-01 7:05 [patch V5 00/11] rseq: Implement time slice extension mechanism Thomas Gleixner
` (2 preceding siblings ...)
2025-12-01 7:05 ` [patch V5 03/11] rseq: Add statistics " Thomas Gleixner
@ 2025-12-01 7:06 ` Thomas Gleixner
2025-12-01 7:06 ` [patch V5 05/11] rseq: Implement sys_rseq_slice_yield() Thomas Gleixner
` (6 subsequent siblings)
10 siblings, 0 replies; 13+ messages in thread
From: Thomas Gleixner @ 2025-12-01 7:06 UTC (permalink / raw)
To: LKML
Cc: Mathieu Desnoyers, Paul E. McKenney, Boqun Feng, Jonathan Corbet,
Prakash Sangappa, Madadi Vineeth Reddy, K Prateek Nayak,
Steven Rostedt, Sebastian Andrzej Siewior, Arnd Bergmann,
linux-arch, Randy Dunlap, Peter Zijlstra, Ron Geva, Waiman Long
Implement a prctl() so that tasks can enable the time slice extension
mechanism. This fails, when time slice extensions are disabled at compile
time or on the kernel command line and when no rseq pointer is registered
in the kernel.
That allows to implement a single trivial check in the exit to user mode
hotpath, to decide whether the whole mechanism needs to be invoked.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
---
V3: Use -ENOTSUPP for the stub inline - Sebastian
---
include/linux/rseq.h | 9 +++++++
include/uapi/linux/prctl.h | 10 ++++++++
kernel/rseq.c | 52 +++++++++++++++++++++++++++++++++++++++++++++
kernel/sys.c | 6 +++++
4 files changed, 77 insertions(+)
--- a/include/linux/rseq.h
+++ b/include/linux/rseq.h
@@ -163,4 +163,13 @@ void rseq_syscall(struct pt_regs *regs);
static inline void rseq_syscall(struct pt_regs *regs) { }
#endif /* !CONFIG_DEBUG_RSEQ */
+#ifdef CONFIG_RSEQ_SLICE_EXTENSION
+int rseq_slice_extension_prctl(unsigned long arg2, unsigned long arg3);
+#else /* CONFIG_RSEQ_SLICE_EXTENSION */
+static inline int rseq_slice_extension_prctl(unsigned long arg2, unsigned long arg3)
+{
+ return -ENOTSUPP;
+}
+#endif /* !CONFIG_RSEQ_SLICE_EXTENSION */
+
#endif /* _LINUX_RSEQ_H */
--- a/include/uapi/linux/prctl.h
+++ b/include/uapi/linux/prctl.h
@@ -386,4 +386,14 @@ struct prctl_mm_map {
# define PR_FUTEX_HASH_SET_SLOTS 1
# define PR_FUTEX_HASH_GET_SLOTS 2
+/* RSEQ time slice extensions */
+#define PR_RSEQ_SLICE_EXTENSION 79
+# define PR_RSEQ_SLICE_EXTENSION_GET 1
+# define PR_RSEQ_SLICE_EXTENSION_SET 2
+/*
+ * Bits for RSEQ_SLICE_EXTENSION_GET/SET
+ * PR_RSEQ_SLICE_EXT_ENABLE: Enable
+ */
+# define PR_RSEQ_SLICE_EXT_ENABLE 0x01
+
#endif /* _LINUX_PRCTL_H */
--- a/kernel/rseq.c
+++ b/kernel/rseq.c
@@ -71,6 +71,7 @@
#define RSEQ_BUILD_SLOW_PATH
#include <linux/debugfs.h>
+#include <linux/prctl.h>
#include <linux/ratelimit.h>
#include <linux/rseq_entry.h>
#include <linux/sched.h>
@@ -501,6 +502,57 @@ SYSCALL_DEFINE4(rseq, struct rseq __user
#ifdef CONFIG_RSEQ_SLICE_EXTENSION
DEFINE_STATIC_KEY_TRUE(rseq_slice_extension_key);
+int rseq_slice_extension_prctl(unsigned long arg2, unsigned long arg3)
+{
+ switch (arg2) {
+ case PR_RSEQ_SLICE_EXTENSION_GET:
+ if (arg3)
+ return -EINVAL;
+ return current->rseq.slice.state.enabled ? PR_RSEQ_SLICE_EXT_ENABLE : 0;
+
+ case PR_RSEQ_SLICE_EXTENSION_SET: {
+ u32 rflags, valid = RSEQ_CS_FLAG_SLICE_EXT_AVAILABLE;
+ bool enable = !!(arg3 & PR_RSEQ_SLICE_EXT_ENABLE);
+
+ if (arg3 & ~PR_RSEQ_SLICE_EXT_ENABLE)
+ return -EINVAL;
+ if (!rseq_slice_extension_enabled())
+ return -ENOTSUPP;
+ if (!current->rseq.usrptr)
+ return -ENXIO;
+
+ /* No change? */
+ if (enable == !!current->rseq.slice.state.enabled)
+ return 0;
+
+ if (get_user(rflags, ¤t->rseq.usrptr->flags))
+ goto die;
+
+ if (current->rseq.slice.state.enabled)
+ valid |= RSEQ_CS_FLAG_SLICE_EXT_ENABLED;
+
+ if ((rflags & valid) != valid)
+ goto die;
+
+ rflags &= ~RSEQ_CS_FLAG_SLICE_EXT_ENABLED;
+ rflags |= RSEQ_CS_FLAG_SLICE_EXT_AVAILABLE;
+ if (enable)
+ rflags |= RSEQ_CS_FLAG_SLICE_EXT_ENABLED;
+
+ if (put_user(rflags, ¤t->rseq.usrptr->flags))
+ goto die;
+
+ current->rseq.slice.state.enabled = enable;
+ return 0;
+ }
+ default:
+ return -EINVAL;
+ }
+die:
+ force_sig(SIGSEGV);
+ return -EFAULT;
+}
+
static int __init rseq_slice_cmdline(char *str)
{
bool on;
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -53,6 +53,7 @@
#include <linux/time_namespace.h>
#include <linux/binfmts.h>
#include <linux/futex.h>
+#include <linux/rseq.h>
#include <linux/sched.h>
#include <linux/sched/autogroup.h>
@@ -2868,6 +2869,11 @@ SYSCALL_DEFINE5(prctl, int, option, unsi
case PR_FUTEX_HASH:
error = futex_hash_prctl(arg2, arg3, arg4);
break;
+ case PR_RSEQ_SLICE_EXTENSION:
+ if (arg4 || arg5)
+ return -EINVAL;
+ error = rseq_slice_extension_prctl(arg2, arg3);
+ break;
default:
trace_task_prctl_unknown(option, arg2, arg3, arg4, arg5);
error = -EINVAL;
^ permalink raw reply [flat|nested] 13+ messages in thread* [patch V5 05/11] rseq: Implement sys_rseq_slice_yield()
2025-12-01 7:05 [patch V5 00/11] rseq: Implement time slice extension mechanism Thomas Gleixner
` (3 preceding siblings ...)
2025-12-01 7:06 ` [patch V5 04/11] rseq: Add prctl() to enable " Thomas Gleixner
@ 2025-12-01 7:06 ` Thomas Gleixner
2025-12-01 7:06 ` [patch V5 06/11] rseq: Implement syscall entry work for time slice extensions Thomas Gleixner
` (5 subsequent siblings)
10 siblings, 0 replies; 13+ messages in thread
From: Thomas Gleixner @ 2025-12-01 7:06 UTC (permalink / raw)
To: LKML
Cc: Mathieu Desnoyers, Paul E. McKenney, Boqun Feng, Jonathan Corbet,
Prakash Sangappa, Madadi Vineeth Reddy, K Prateek Nayak,
Steven Rostedt, Sebastian Andrzej Siewior, Arnd Bergmann,
linux-arch, Randy Dunlap, Peter Zijlstra, Ron Geva, Waiman Long
Provide a new syscall which has the only purpose to yield the CPU after the
kernel granted a time slice extension.
sched_yield() is not suitable for that because it unconditionally
schedules, but the end of the time slice extension is not required to
schedule when the task was already preempted. This also allows to have a
strict check for termination to catch user space invoking random syscalls
including sched_yield() from a time slice extension region.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Cc: linux-arch@vger.kernel.org
---
Note: This still uses 470 which conflicts with -next, but this is scheduled
for post -rc1 and basing it on -next makes it more complicated for
now. Will be changed in the final submission.
---
V5: Rework to adjust to support for arbitrary syscall changes
Use n32/n64/o32 for MIPS - Arnd
V2: Use the proper name in sys_ni.c and add comment - Prateek
---
arch/alpha/kernel/syscalls/syscall.tbl | 1 +
arch/arm/tools/syscall.tbl | 1 +
arch/arm64/tools/syscall_32.tbl | 1 +
arch/m68k/kernel/syscalls/syscall.tbl | 1 +
arch/microblaze/kernel/syscalls/syscall.tbl | 1 +
arch/mips/kernel/syscalls/syscall_n32.tbl | 1 +
arch/mips/kernel/syscalls/syscall_n64.tbl | 1 +
arch/mips/kernel/syscalls/syscall_o32.tbl | 1 +
arch/parisc/kernel/syscalls/syscall.tbl | 1 +
arch/powerpc/kernel/syscalls/syscall.tbl | 1 +
arch/s390/kernel/syscalls/syscall.tbl | 1 +
arch/sh/kernel/syscalls/syscall.tbl | 1 +
arch/sparc/kernel/syscalls/syscall.tbl | 1 +
arch/x86/entry/syscalls/syscall_32.tbl | 1 +
arch/x86/entry/syscalls/syscall_64.tbl | 1 +
arch/xtensa/kernel/syscalls/syscall.tbl | 1 +
include/linux/rseq_types.h | 2 ++
include/linux/syscalls.h | 1 +
include/uapi/asm-generic/unistd.h | 5 ++++-
kernel/rseq.c | 21 +++++++++++++++++++++
kernel/sys_ni.c | 1 +
scripts/syscall.tbl | 1 +
22 files changed, 46 insertions(+), 1 deletion(-)
--- a/arch/alpha/kernel/syscalls/syscall.tbl
+++ b/arch/alpha/kernel/syscalls/syscall.tbl
@@ -509,3 +509,4 @@
577 common open_tree_attr sys_open_tree_attr
578 common file_getattr sys_file_getattr
579 common file_setattr sys_file_setattr
+580 common rseq_slice_yield sys_rseq_slice_yield
--- a/arch/arm/tools/syscall.tbl
+++ b/arch/arm/tools/syscall.tbl
@@ -484,3 +484,4 @@
467 common open_tree_attr sys_open_tree_attr
468 common file_getattr sys_file_getattr
469 common file_setattr sys_file_setattr
+470 common rseq_slice_yield sys_rseq_slice_yield
--- a/arch/arm64/tools/syscall_32.tbl
+++ b/arch/arm64/tools/syscall_32.tbl
@@ -481,3 +481,4 @@
467 common open_tree_attr sys_open_tree_attr
468 common file_getattr sys_file_getattr
469 common file_setattr sys_file_setattr
+470 common rseq_slice_yield sys_rseq_slice_yield
--- a/arch/m68k/kernel/syscalls/syscall.tbl
+++ b/arch/m68k/kernel/syscalls/syscall.tbl
@@ -469,3 +469,4 @@
467 common open_tree_attr sys_open_tree_attr
468 common file_getattr sys_file_getattr
469 common file_setattr sys_file_setattr
+470 common rseq_slice_yield sys_rseq_slice_yield
--- a/arch/microblaze/kernel/syscalls/syscall.tbl
+++ b/arch/microblaze/kernel/syscalls/syscall.tbl
@@ -475,3 +475,4 @@
467 common open_tree_attr sys_open_tree_attr
468 common file_getattr sys_file_getattr
469 common file_setattr sys_file_setattr
+470 common rseq_slice_yield sys_rseq_slice_yield
--- a/arch/mips/kernel/syscalls/syscall_n32.tbl
+++ b/arch/mips/kernel/syscalls/syscall_n32.tbl
@@ -408,3 +408,4 @@
467 n32 open_tree_attr sys_open_tree_attr
468 n32 file_getattr sys_file_getattr
469 n32 file_setattr sys_file_setattr
+470 n32 rseq_slice_yield sys_rseq_slice_yield
--- a/arch/mips/kernel/syscalls/syscall_n64.tbl
+++ b/arch/mips/kernel/syscalls/syscall_n64.tbl
@@ -384,3 +384,4 @@
467 n64 open_tree_attr sys_open_tree_attr
468 n64 file_getattr sys_file_getattr
469 n64 file_setattr sys_file_setattr
+470 n64 rseq_slice_yield sys_rseq_slice_yield
--- a/arch/mips/kernel/syscalls/syscall_o32.tbl
+++ b/arch/mips/kernel/syscalls/syscall_o32.tbl
@@ -457,3 +457,4 @@
467 o32 open_tree_attr sys_open_tree_attr
468 o32 file_getattr sys_file_getattr
469 o32 file_setattr sys_file_setattr
+470 o32 rseq_slice_yield sys_rseq_slice_yield
--- a/arch/parisc/kernel/syscalls/syscall.tbl
+++ b/arch/parisc/kernel/syscalls/syscall.tbl
@@ -468,3 +468,4 @@
467 common open_tree_attr sys_open_tree_attr
468 common file_getattr sys_file_getattr
469 common file_setattr sys_file_setattr
+470 common rseq_slice_yield sys_rseq_slice_yield
--- a/arch/powerpc/kernel/syscalls/syscall.tbl
+++ b/arch/powerpc/kernel/syscalls/syscall.tbl
@@ -560,3 +560,4 @@
467 common open_tree_attr sys_open_tree_attr
468 common file_getattr sys_file_getattr
469 common file_setattr sys_file_setattr
+470 nospu rseq_slice_yield sys_rseq_slice_yield
--- a/arch/s390/kernel/syscalls/syscall.tbl
+++ b/arch/s390/kernel/syscalls/syscall.tbl
@@ -472,3 +472,4 @@
467 common open_tree_attr sys_open_tree_attr sys_open_tree_attr
468 common file_getattr sys_file_getattr sys_file_getattr
469 common file_setattr sys_file_setattr sys_file_setattr
+470 common rseq_slice_yield sys_rseq_slice_yield sys_rseq_slice_yield
--- a/arch/sh/kernel/syscalls/syscall.tbl
+++ b/arch/sh/kernel/syscalls/syscall.tbl
@@ -473,3 +473,4 @@
467 common open_tree_attr sys_open_tree_attr
468 common file_getattr sys_file_getattr
469 common file_setattr sys_file_setattr
+470 common rseq_slice_yield sys_rseq_slice_yield
--- a/arch/sparc/kernel/syscalls/syscall.tbl
+++ b/arch/sparc/kernel/syscalls/syscall.tbl
@@ -515,3 +515,4 @@
467 common open_tree_attr sys_open_tree_attr
468 common file_getattr sys_file_getattr
469 common file_setattr sys_file_setattr
+470 common rseq_slice_yield sys_rseq_slice_yield
--- a/arch/x86/entry/syscalls/syscall_32.tbl
+++ b/arch/x86/entry/syscalls/syscall_32.tbl
@@ -475,3 +475,4 @@
467 i386 open_tree_attr sys_open_tree_attr
468 i386 file_getattr sys_file_getattr
469 i386 file_setattr sys_file_setattr
+470 i386 rseq_slice_yield sys_rseq_slice_yield
--- a/arch/x86/entry/syscalls/syscall_64.tbl
+++ b/arch/x86/entry/syscalls/syscall_64.tbl
@@ -394,6 +394,7 @@
467 common open_tree_attr sys_open_tree_attr
468 common file_getattr sys_file_getattr
469 common file_setattr sys_file_setattr
+470 common rseq_slice_yield sys_rseq_slice_yield
#
# Due to a historical design error, certain syscalls are numbered differently
--- a/arch/xtensa/kernel/syscalls/syscall.tbl
+++ b/arch/xtensa/kernel/syscalls/syscall.tbl
@@ -440,3 +440,4 @@
467 common open_tree_attr sys_open_tree_attr
468 common file_getattr sys_file_getattr
469 common file_setattr sys_file_setattr
+470 common rseq_slice_yield sys_rseq_slice_yield
--- a/include/linux/rseq_types.h
+++ b/include/linux/rseq_types.h
@@ -89,9 +89,11 @@ union rseq_slice_state {
/**
* struct rseq_slice - Status information for rseq time slice extension
* @state: Time slice extension state
+ * @yielded: Indicator for rseq_slice_yield()
*/
struct rseq_slice {
union rseq_slice_state state;
+ u8 yielded;
};
/**
--- a/include/linux/syscalls.h
+++ b/include/linux/syscalls.h
@@ -957,6 +957,7 @@ asmlinkage long sys_statx(int dfd, const
unsigned mask, struct statx __user *buffer);
asmlinkage long sys_rseq(struct rseq __user *rseq, uint32_t rseq_len,
int flags, uint32_t sig);
+asmlinkage long sys_rseq_slice_yield(void);
asmlinkage long sys_open_tree(int dfd, const char __user *path, unsigned flags);
asmlinkage long sys_open_tree_attr(int dfd, const char __user *path,
unsigned flags,
--- a/include/uapi/asm-generic/unistd.h
+++ b/include/uapi/asm-generic/unistd.h
@@ -858,8 +858,11 @@
#define __NR_file_setattr 469
__SYSCALL(__NR_file_setattr, sys_file_setattr)
+#define __NR_rseq_slice_yield 470
+__SYSCALL(__NR_rseq_slice_yield, sys_rseq_slice_yield)
+
#undef __NR_syscalls
-#define __NR_syscalls 470
+#define __NR_syscalls 471
/*
* 32 bit systems traditionally used different
--- a/kernel/rseq.c
+++ b/kernel/rseq.c
@@ -553,6 +553,27 @@ int rseq_slice_extension_prctl(unsigned
return -EFAULT;
}
+/**
+ * sys_rseq_slice_yield - yield the current processor side effect free if a
+ * task granted with a time slice extension is done with
+ * the critical work before being forced out.
+ *
+ * Return: 1 if the task successfully yielded the CPU within the granted slice.
+ * 0 if the slice extension was either never granted or was revoked by
+ * going over the granted extension, using a syscall other than this one
+ * or being scheduled out earlier due to a subsequent interrupt.
+ *
+ * The syscall does not schedule because the syscall entry work immediately
+ * relinquishes the CPU and schedules if required.
+ */
+SYSCALL_DEFINE0(rseq_slice_yield)
+{
+ int yielded = !!current->rseq.slice.yielded;
+
+ current->rseq.slice.yielded = 0;
+ return yielded;
+}
+
static int __init rseq_slice_cmdline(char *str)
{
bool on;
--- a/kernel/sys_ni.c
+++ b/kernel/sys_ni.c
@@ -390,6 +390,7 @@ COND_SYSCALL(setuid16);
/* restartable sequence */
COND_SYSCALL(rseq);
+COND_SYSCALL(rseq_slice_yield);
COND_SYSCALL(uretprobe);
COND_SYSCALL(uprobe);
--- a/scripts/syscall.tbl
+++ b/scripts/syscall.tbl
@@ -410,3 +410,4 @@
467 common open_tree_attr sys_open_tree_attr
468 common file_getattr sys_file_getattr
469 common file_setattr sys_file_setattr
+470 common rseq_slice_yield sys_rseq_slice_yield
^ permalink raw reply [flat|nested] 13+ messages in thread* [patch V5 06/11] rseq: Implement syscall entry work for time slice extensions
2025-12-01 7:05 [patch V5 00/11] rseq: Implement time slice extension mechanism Thomas Gleixner
` (4 preceding siblings ...)
2025-12-01 7:06 ` [patch V5 05/11] rseq: Implement sys_rseq_slice_yield() Thomas Gleixner
@ 2025-12-01 7:06 ` Thomas Gleixner
2025-12-01 7:06 ` [patch V5 07/11] rseq: Implement time slice extension enforcement timer Thomas Gleixner
` (4 subsequent siblings)
10 siblings, 0 replies; 13+ messages in thread
From: Thomas Gleixner @ 2025-12-01 7:06 UTC (permalink / raw)
To: LKML
Cc: Mathieu Desnoyers, Paul E. McKenney, Boqun Feng, Jonathan Corbet,
Prakash Sangappa, Madadi Vineeth Reddy, K Prateek Nayak,
Steven Rostedt, Sebastian Andrzej Siewior, Arnd Bergmann,
linux-arch, Randy Dunlap, Peter Zijlstra, Ron Geva, Waiman Long
The kernel sets SYSCALL_WORK_RSEQ_SLICE when it grants a time slice
extension. This allows to handle the rseq_slice_yield() syscall, which is
used by user space to relinquish the CPU after finishing the critical
section for which it requested an extension.
In case the kernel state is still GRANTED, the kernel resets both kernel
and user space state with a set of sanity checks. If the kernel state is
already cleared, then this raced against the timer or some other interrupt
and just clears the work bit.
Doing it in syscall entry work allows to catch misbehaving user space,
which issues an arbitrary syscall, i.e. not rseq_slice_yield(), from the
critical section. Contrary to the initial strict requirement to use
rseq_slice_yield() arbitrary syscalls are not considered a violation of the
ABI contract anymore to allow onion architecture applications, which cannot
control the code inside a critical section, to utilize this as well.
If the code detects inconsistent user space that result in a SIGSEGV for
the application.
If the grant was still active and the task was not preempted yet, the work
code reschedules immediately before continuing through the syscall.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
---
V5: Allow arbitrary syscalls
V3: Use get/put_user()
---
include/linux/entry-common.h | 2
include/linux/rseq.h | 2
include/linux/thread_info.h | 16 ++++---
kernel/entry/syscall-common.c | 11 ++++-
kernel/rseq.c | 91 ++++++++++++++++++++++++++++++++++++++++++
5 files changed, 112 insertions(+), 10 deletions(-)
--- a/include/linux/entry-common.h
+++ b/include/linux/entry-common.h
@@ -36,8 +36,8 @@
SYSCALL_WORK_SYSCALL_EMU | \
SYSCALL_WORK_SYSCALL_AUDIT | \
SYSCALL_WORK_SYSCALL_USER_DISPATCH | \
+ SYSCALL_WORK_SYSCALL_RSEQ_SLICE | \
ARCH_SYSCALL_WORK_ENTER)
-
#define SYSCALL_WORK_EXIT (SYSCALL_WORK_SYSCALL_TRACEPOINT | \
SYSCALL_WORK_SYSCALL_TRACE | \
SYSCALL_WORK_SYSCALL_AUDIT | \
--- a/include/linux/rseq.h
+++ b/include/linux/rseq.h
@@ -164,8 +164,10 @@ static inline void rseq_syscall(struct p
#endif /* !CONFIG_DEBUG_RSEQ */
#ifdef CONFIG_RSEQ_SLICE_EXTENSION
+void rseq_syscall_enter_work(long syscall);
int rseq_slice_extension_prctl(unsigned long arg2, unsigned long arg3);
#else /* CONFIG_RSEQ_SLICE_EXTENSION */
+static inline void rseq_syscall_enter_work(long syscall) { }
static inline int rseq_slice_extension_prctl(unsigned long arg2, unsigned long arg3)
{
return -ENOTSUPP;
--- a/include/linux/thread_info.h
+++ b/include/linux/thread_info.h
@@ -46,15 +46,17 @@ enum syscall_work_bit {
SYSCALL_WORK_BIT_SYSCALL_AUDIT,
SYSCALL_WORK_BIT_SYSCALL_USER_DISPATCH,
SYSCALL_WORK_BIT_SYSCALL_EXIT_TRAP,
+ SYSCALL_WORK_BIT_SYSCALL_RSEQ_SLICE,
};
-#define SYSCALL_WORK_SECCOMP BIT(SYSCALL_WORK_BIT_SECCOMP)
-#define SYSCALL_WORK_SYSCALL_TRACEPOINT BIT(SYSCALL_WORK_BIT_SYSCALL_TRACEPOINT)
-#define SYSCALL_WORK_SYSCALL_TRACE BIT(SYSCALL_WORK_BIT_SYSCALL_TRACE)
-#define SYSCALL_WORK_SYSCALL_EMU BIT(SYSCALL_WORK_BIT_SYSCALL_EMU)
-#define SYSCALL_WORK_SYSCALL_AUDIT BIT(SYSCALL_WORK_BIT_SYSCALL_AUDIT)
-#define SYSCALL_WORK_SYSCALL_USER_DISPATCH BIT(SYSCALL_WORK_BIT_SYSCALL_USER_DISPATCH)
-#define SYSCALL_WORK_SYSCALL_EXIT_TRAP BIT(SYSCALL_WORK_BIT_SYSCALL_EXIT_TRAP)
+#define SYSCALL_WORK_SECCOMP BIT(SYSCALL_WORK_BIT_SECCOMP)
+#define SYSCALL_WORK_SYSCALL_TRACEPOINT BIT(SYSCALL_WORK_BIT_SYSCALL_TRACEPOINT)
+#define SYSCALL_WORK_SYSCALL_TRACE BIT(SYSCALL_WORK_BIT_SYSCALL_TRACE)
+#define SYSCALL_WORK_SYSCALL_EMU BIT(SYSCALL_WORK_BIT_SYSCALL_EMU)
+#define SYSCALL_WORK_SYSCALL_AUDIT BIT(SYSCALL_WORK_BIT_SYSCALL_AUDIT)
+#define SYSCALL_WORK_SYSCALL_USER_DISPATCH BIT(SYSCALL_WORK_BIT_SYSCALL_USER_DISPATCH)
+#define SYSCALL_WORK_SYSCALL_EXIT_TRAP BIT(SYSCALL_WORK_BIT_SYSCALL_EXIT_TRAP)
+#define SYSCALL_WORK_SYSCALL_RSEQ_SLICE BIT(SYSCALL_WORK_BIT_SYSCALL_RSEQ_SLICE)
#endif
#include <asm/thread_info.h>
--- a/kernel/entry/syscall-common.c
+++ b/kernel/entry/syscall-common.c
@@ -17,8 +17,7 @@ static inline void syscall_enter_audit(s
}
}
-long syscall_trace_enter(struct pt_regs *regs, long syscall,
- unsigned long work)
+long syscall_trace_enter(struct pt_regs *regs, long syscall, unsigned long work)
{
long ret = 0;
@@ -32,6 +31,14 @@ long syscall_trace_enter(struct pt_regs
return -1L;
}
+ /*
+ * User space got a time slice extension granted and relinquishes
+ * the CPU. The work stops the slice timer to avoid an extra round
+ * through hrtimer_interrupt().
+ */
+ if (work & SYSCALL_WORK_SYSCALL_RSEQ_SLICE)
+ rseq_syscall_enter_work(syscall);
+
/* Handle ptrace */
if (work & (SYSCALL_WORK_SYSCALL_TRACE | SYSCALL_WORK_SYSCALL_EMU)) {
ret = ptrace_report_syscall_entry(regs);
--- a/kernel/rseq.c
+++ b/kernel/rseq.c
@@ -502,6 +502,97 @@ SYSCALL_DEFINE4(rseq, struct rseq __user
#ifdef CONFIG_RSEQ_SLICE_EXTENSION
DEFINE_STATIC_KEY_TRUE(rseq_slice_extension_key);
+static inline void rseq_slice_set_need_resched(struct task_struct *curr)
+{
+ /*
+ * The interrupt guard is required to prevent inconsistent state in
+ * this case:
+ *
+ * set_tsk_need_resched()
+ * --> Interrupt
+ * wakeup()
+ * set_tsk_need_resched()
+ * set_preempt_need_resched()
+ * schedule_on_return()
+ * clear_tsk_need_resched()
+ * clear_preempt_need_resched()
+ * set_preempt_need_resched() <- Inconsistent state
+ *
+ * This is safe vs. a remote set of TIF_NEED_RESCHED because that
+ * only sets the already set bit and does not create inconsistent
+ * state.
+ */
+ scoped_guard(irq)
+ set_need_resched_current();
+}
+
+static void rseq_slice_validate_ctrl(u32 expected)
+{
+ u32 __user *sctrl = ¤t->rseq.usrptr->slice_ctrl.all;
+ u32 uval;
+
+ if (get_user(uval, sctrl) || uval != expected)
+ force_sig(SIGSEGV);
+}
+
+/*
+ * Invoked from syscall entry if a time slice extension was granted and the
+ * kernel did not clear it before user space left the critical section.
+ *
+ * While the recommended way to relinquish the CPU side effect free is
+ * rseq_slice_yield(2), any syscall within a granted slice terminates the
+ * grant and immediately reschedules if required. This supports onion layer
+ * applications, where the code requesting the grant cannot control the
+ * code within the critical section.
+ */
+void rseq_syscall_enter_work(long syscall)
+{
+ struct task_struct *curr = current;
+ struct rseq_slice_ctrl ctrl = { .granted = curr->rseq.slice.state.granted };
+
+ clear_task_syscall_work(curr, SYSCALL_RSEQ_SLICE);
+
+ if (static_branch_unlikely(&rseq_debug_enabled))
+ rseq_slice_validate_ctrl(ctrl.all);
+
+ /*
+ * The kernel might have raced, revoked the grant and updated
+ * userspace, but kept the SLICE work set.
+ */
+ if (!ctrl.granted)
+ return;
+
+ /*
+ * Required to make set_tsk_need_resched() correct on PREEMPT[RT]
+ * kernels. Leaving the scope will reschedule on preemption models
+ * FULL, LAZY and RT if necessary.
+ */
+ scoped_guard(preempt) {
+ /*
+ * Now that preemption is disabled, quickly check whether
+ * the task was already rescheduled before arriving here.
+ */
+ if (!curr->rseq.event.sched_switch) {
+ rseq_slice_set_need_resched(curr);
+
+ if (syscall == __NR_rseq_slice_yield) {
+ rseq_stat_inc(rseq_stats.s_yielded);
+ /* Update the yielded state for syscall return */
+ curr->rseq.slice.yielded = 1;
+ } else {
+ rseq_stat_inc(rseq_stats.s_aborted);
+ }
+ }
+ }
+ /* Reschedule on NONE/VOLUNTARY preemption models */
+ cond_resched();
+
+ /* Clear the grant in kernel state and user space */
+ curr->rseq.slice.state.granted = false;
+ if (put_user(0U, &curr->rseq.usrptr->slice_ctrl.all))
+ force_sig(SIGSEGV);
+}
+
int rseq_slice_extension_prctl(unsigned long arg2, unsigned long arg3)
{
switch (arg2) {
^ permalink raw reply [flat|nested] 13+ messages in thread* [patch V5 07/11] rseq: Implement time slice extension enforcement timer
2025-12-01 7:05 [patch V5 00/11] rseq: Implement time slice extension mechanism Thomas Gleixner
` (5 preceding siblings ...)
2025-12-01 7:06 ` [patch V5 06/11] rseq: Implement syscall entry work for time slice extensions Thomas Gleixner
@ 2025-12-01 7:06 ` Thomas Gleixner
2025-12-01 7:06 ` [patch V5 08/11] rseq: Reset slice extension when scheduled Thomas Gleixner
` (3 subsequent siblings)
10 siblings, 0 replies; 13+ messages in thread
From: Thomas Gleixner @ 2025-12-01 7:06 UTC (permalink / raw)
To: LKML
Cc: Mathieu Desnoyers, Paul E. McKenney, Boqun Feng, Jonathan Corbet,
Prakash Sangappa, Madadi Vineeth Reddy, K Prateek Nayak,
Steven Rostedt, Sebastian Andrzej Siewior, Arnd Bergmann,
linux-arch, Randy Dunlap, Peter Zijlstra, Ron Geva, Waiman Long
If a time slice extension is granted and the reschedule delayed, the kernel
has to ensure that user space cannot abuse the extension and exceed the
maximum granted time.
It was suggested to implement this via the existing hrtick() timer in the
scheduler, but that turned out to be problematic for several reasons:
1) It creates a dependency on CONFIG_SCHED_HRTICK, which can be disabled
independently of CONFIG_HIGHRES_TIMERS
2) HRTICK usage in the scheduler can be runtime disabled or is only used
for certain aspects of scheduling.
3) The function is calling into the scheduler code and that might have
unexpected consequences when this is invoked due to a time slice
enforcement expiry. Especially when the task managed to clear the
grant via sched_yield(0).
It would be possible to address #2 and #3 by storing state in the
scheduler, but that is extra complexity and fragility for no value.
Implement a dedicated per CPU hrtimer instead, which is solely used for the
purpose of time slice enforcement.
The timer is armed when an extension was granted right before actually
returning to user mode in rseq_exit_to_user_mode_restart().
It is disarmed, when the task relinquishes the CPU. This is expensive as
the timer is probably the first expiring timer on the CPU, which means it
has to reprogram the hardware. But that's less expensive than going through
a full hrtimer interrupt cycle for nothing.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
---
V5: Document the slice extension range - PeterZ
V4: Update comment - Steven
V3: Add sysctl documentation, simplify timer cancelation - Sebastian
---
Documentation/admin-guide/sysctl/kernel.rst | 8 +
include/linux/rseq_entry.h | 38 +++++---
include/linux/rseq_types.h | 2
kernel/rseq.c | 132 +++++++++++++++++++++++++++-
4 files changed, 167 insertions(+), 13 deletions(-)
--- a/Documentation/admin-guide/sysctl/kernel.rst
+++ b/Documentation/admin-guide/sysctl/kernel.rst
@@ -1228,6 +1228,14 @@ reboot-cmd (SPARC only)
ROM/Flash boot loader. Maybe to tell it what to do after
rebooting. ???
+rseq_slice_extension_nsec
+=========================
+
+A task can request to delay its scheduling if it is in a critical section
+via the prctl(PR_RSEQ_SLICE_EXTENSION_SET) mechanism. This sets the maximum
+allowed extension in nanoseconds before scheduling of the task is enforced.
+Default value is 30000ns (30us). The possible range is 10000ns (10us) to
+50000ns (50us).
sched_energy_aware
==================
--- a/include/linux/rseq_entry.h
+++ b/include/linux/rseq_entry.h
@@ -87,8 +87,24 @@ static __always_inline bool rseq_slice_e
{
return static_branch_likely(&rseq_slice_extension_key);
}
+
+extern unsigned int rseq_slice_ext_nsecs;
+bool __rseq_arm_slice_extension_timer(void);
+
+static __always_inline bool rseq_arm_slice_extension_timer(void)
+{
+ if (!rseq_slice_extension_enabled())
+ return false;
+
+ if (likely(!current->rseq.slice.state.granted))
+ return false;
+
+ return __rseq_arm_slice_extension_timer();
+}
+
#else /* CONFIG_RSEQ_SLICE_EXTENSION */
static inline bool rseq_slice_extension_enabled(void) { return false; }
+static inline bool rseq_arm_slice_extension_timer(void) { return false; }
#endif /* !CONFIG_RSEQ_SLICE_EXTENSION */
bool rseq_debug_update_user_cs(struct task_struct *t, struct pt_regs *regs, unsigned long csaddr);
@@ -543,17 +559,19 @@ static __always_inline void clear_tif_rs
static __always_inline bool
rseq_exit_to_user_mode_restart(struct pt_regs *regs, unsigned long ti_work)
{
- if (likely(!test_tif_rseq(ti_work)))
- return false;
-
- if (unlikely(__rseq_exit_to_user_mode_restart(regs))) {
- current->rseq.event.slowpath = true;
- set_tsk_thread_flag(current, TIF_NOTIFY_RESUME);
- return true;
+ if (unlikely(test_tif_rseq(ti_work))) {
+ if (unlikely(__rseq_exit_to_user_mode_restart(regs))) {
+ current->rseq.event.slowpath = true;
+ set_tsk_thread_flag(current, TIF_NOTIFY_RESUME);
+ return true;
+ }
+ clear_tif_rseq();
}
-
- clear_tif_rseq();
- return false;
+ /*
+ * Arm the slice extension timer if nothing to do anymore and the
+ * task really goes out to user space.
+ */
+ return rseq_arm_slice_extension_timer();
}
#else /* CONFIG_GENERIC_ENTRY */
--- a/include/linux/rseq_types.h
+++ b/include/linux/rseq_types.h
@@ -89,10 +89,12 @@ union rseq_slice_state {
/**
* struct rseq_slice - Status information for rseq time slice extension
* @state: Time slice extension state
+ * @expires: The time when a grant expires
* @yielded: Indicator for rseq_slice_yield()
*/
struct rseq_slice {
union rseq_slice_state state;
+ u64 expires;
u8 yielded;
};
--- a/kernel/rseq.c
+++ b/kernel/rseq.c
@@ -71,6 +71,8 @@
#define RSEQ_BUILD_SLOW_PATH
#include <linux/debugfs.h>
+#include <linux/hrtimer.h>
+#include <linux/percpu.h>
#include <linux/prctl.h>
#include <linux/ratelimit.h>
#include <linux/rseq_entry.h>
@@ -500,8 +502,91 @@ SYSCALL_DEFINE4(rseq, struct rseq __user
}
#ifdef CONFIG_RSEQ_SLICE_EXTENSION
+struct slice_timer {
+ struct hrtimer timer;
+ void *cookie;
+};
+
+unsigned int rseq_slice_ext_nsecs __read_mostly = 30 * NSEC_PER_USEC;
+static DEFINE_PER_CPU(struct slice_timer, slice_timer);
DEFINE_STATIC_KEY_TRUE(rseq_slice_extension_key);
+/*
+ * When the timer expires and the task is still in user space, the return
+ * from interrupt will revoke the grant and schedule. If the task already
+ * entered the kernel via a syscall and the timer fires before the syscall
+ * work was able to cancel it, then depending on the preemption model this
+ * will either reschedule on return from interrupt or in the syscall work
+ * below.
+ */
+static enum hrtimer_restart rseq_slice_expired(struct hrtimer *tmr)
+{
+ struct slice_timer *st = container_of(tmr, struct slice_timer, timer);
+
+ /*
+ * Validate that the task which armed the timer is still on the
+ * CPU. It could have been scheduled out without canceling the
+ * timer.
+ */
+ if (st->cookie == current && current->rseq.slice.state.granted) {
+ rseq_stat_inc(rseq_stats.s_expired);
+ set_need_resched_current();
+ }
+ return HRTIMER_NORESTART;
+}
+
+bool __rseq_arm_slice_extension_timer(void)
+{
+ struct slice_timer *st = this_cpu_ptr(&slice_timer);
+ struct task_struct *curr = current;
+
+ lockdep_assert_irqs_disabled();
+
+ /*
+ * This check prevents a task, which got a time slice extension
+ * granted, from exceeding the maximum scheduling latency when the
+ * grant expired before going out to user space. Don't bother to
+ * clear the grant here, it will be cleaned up automatically before
+ * going out to user space after being scheduled back in.
+ */
+ if ((unlikely(curr->rseq.slice.expires < ktime_get_mono_fast_ns()))) {
+ set_need_resched_current();
+ return true;
+ }
+
+ /*
+ * Store the task pointer as a cookie for comparison in the timer
+ * function. This is safe as the timer is CPU local and cannot be
+ * in the expiry function at this point.
+ */
+ st->cookie = curr;
+ hrtimer_start(&st->timer, curr->rseq.slice.expires, HRTIMER_MODE_ABS_PINNED_HARD);
+ /* Arm the syscall entry work */
+ set_task_syscall_work(curr, SYSCALL_RSEQ_SLICE);
+ return false;
+}
+
+static void rseq_cancel_slice_extension_timer(void)
+{
+ struct slice_timer *st = this_cpu_ptr(&slice_timer);
+
+ /*
+ * st->cookie can be safely read as preemption is disabled and the
+ * timer is CPU local.
+ *
+ * As this is most probably the first expiring timer, the cancel is
+ * expensive as it has to reprogram the hardware, but that's less
+ * expensive than going through a full hrtimer_interrupt() cycle
+ * for nothing.
+ *
+ * hrtimer_try_to_cancel() is sufficient here as the timer is CPU
+ * local and once the hrtimer code disabled interrupts the timer
+ * callback cannot be running.
+ */
+ if (st->cookie == current)
+ hrtimer_try_to_cancel(&st->timer);
+}
+
static inline void rseq_slice_set_need_resched(struct task_struct *curr)
{
/*
@@ -563,11 +648,14 @@ void rseq_syscall_enter_work(long syscal
return;
/*
- * Required to make set_tsk_need_resched() correct on PREEMPT[RT]
- * kernels. Leaving the scope will reschedule on preemption models
- * FULL, LAZY and RT if necessary.
+ * Required to stabilize the per CPU timer pointer and to make
+ * set_tsk_need_resched() correct on PREEMPT[RT] kernels.
+ *
+ * Leaving the scope will reschedule on preemption models FULL,
+ * LAZY and RT if necessary.
*/
scoped_guard(preempt) {
+ rseq_cancel_slice_extension_timer();
/*
* Now that preemption is disabled, quickly check whether
* the task was already rescheduled before arriving here.
@@ -665,6 +753,31 @@ SYSCALL_DEFINE0(rseq_slice_yield)
return yielded;
}
+#ifdef CONFIG_SYSCTL
+static const unsigned int rseq_slice_ext_nsecs_min = 10 * NSEC_PER_USEC;
+static const unsigned int rseq_slice_ext_nsecs_max = 50 * NSEC_PER_USEC;
+
+static const struct ctl_table rseq_slice_ext_sysctl[] = {
+ {
+ .procname = "rseq_slice_extension_nsec",
+ .data = &rseq_slice_ext_nsecs,
+ .maxlen = sizeof(unsigned int),
+ .mode = 0644,
+ .proc_handler = proc_douintvec_minmax,
+ .extra1 = (unsigned int *)&rseq_slice_ext_nsecs_min,
+ .extra2 = (unsigned int *)&rseq_slice_ext_nsecs_max,
+ },
+};
+
+static void rseq_slice_sysctl_init(void)
+{
+ if (rseq_slice_extension_enabled())
+ register_sysctl_init("kernel", rseq_slice_ext_sysctl);
+}
+#else /* CONFIG_SYSCTL */
+static inline void rseq_slice_sysctl_init(void) { }
+#endif /* !CONFIG_SYSCTL */
+
static int __init rseq_slice_cmdline(char *str)
{
bool on;
@@ -677,4 +790,17 @@ static int __init rseq_slice_cmdline(cha
return 1;
}
__setup("rseq_slice_ext=", rseq_slice_cmdline);
+
+static int __init rseq_slice_init(void)
+{
+ unsigned int cpu;
+
+ for_each_possible_cpu(cpu) {
+ hrtimer_setup(per_cpu_ptr(&slice_timer.timer, cpu), rseq_slice_expired,
+ CLOCK_MONOTONIC, HRTIMER_MODE_REL_PINNED_HARD);
+ }
+ rseq_slice_sysctl_init();
+ return 0;
+}
+device_initcall(rseq_slice_init);
#endif /* CONFIG_RSEQ_SLICE_EXTENSION */
^ permalink raw reply [flat|nested] 13+ messages in thread* [patch V5 08/11] rseq: Reset slice extension when scheduled
2025-12-01 7:05 [patch V5 00/11] rseq: Implement time slice extension mechanism Thomas Gleixner
` (6 preceding siblings ...)
2025-12-01 7:06 ` [patch V5 07/11] rseq: Implement time slice extension enforcement timer Thomas Gleixner
@ 2025-12-01 7:06 ` Thomas Gleixner
2025-12-01 7:06 ` [patch V5 09/11] rseq: Implement rseq_grant_slice_extension() Thomas Gleixner
` (2 subsequent siblings)
10 siblings, 0 replies; 13+ messages in thread
From: Thomas Gleixner @ 2025-12-01 7:06 UTC (permalink / raw)
To: LKML
Cc: Mathieu Desnoyers, Paul E. McKenney, Boqun Feng, Jonathan Corbet,
Prakash Sangappa, Madadi Vineeth Reddy, K Prateek Nayak,
Steven Rostedt, Sebastian Andrzej Siewior, Arnd Bergmann,
linux-arch, Randy Dunlap, Peter Zijlstra, Ron Geva, Waiman Long
When a time slice extension was granted in the need_resched() check on exit
to user space, the task can still be scheduled out in one of the other
pending work items. When it gets scheduled back in, and need_resched() is
not set, then the stale grant would be preserved, which is just wrong.
RSEQ already keeps track of that and sets TIF_RSEQ, which invokes the
critical section and ID update mechanisms.
Utilize them and clear the user space slice control member of struct rseq
unconditionally within the existing user access sections. That's just an
unconditional store more in that path.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
---
include/linux/rseq_entry.h | 30 ++++++++++++++++++++++++++++--
1 file changed, 28 insertions(+), 2 deletions(-)
--- a/include/linux/rseq_entry.h
+++ b/include/linux/rseq_entry.h
@@ -102,9 +102,17 @@ static __always_inline bool rseq_arm_sli
return __rseq_arm_slice_extension_timer();
}
+static __always_inline void rseq_slice_clear_grant(struct task_struct *t)
+{
+ if (IS_ENABLED(CONFIG_RSEQ_STATS) && t->rseq.slice.state.granted)
+ rseq_stat_inc(rseq_stats.s_revoked);
+ t->rseq.slice.state.granted = false;
+}
+
#else /* CONFIG_RSEQ_SLICE_EXTENSION */
static inline bool rseq_slice_extension_enabled(void) { return false; }
static inline bool rseq_arm_slice_extension_timer(void) { return false; }
+static inline void rseq_slice_clear_grant(struct task_struct *t) { }
#endif /* !CONFIG_RSEQ_SLICE_EXTENSION */
bool rseq_debug_update_user_cs(struct task_struct *t, struct pt_regs *regs, unsigned long csaddr);
@@ -391,8 +399,15 @@ bool rseq_set_ids_get_csaddr(struct task
unsafe_put_user(ids->mm_cid, &rseq->mm_cid, efault);
if (csaddr)
unsafe_get_user(*csaddr, &rseq->rseq_cs, efault);
+
+ /* Open coded, so it's in the same user access region */
+ if (rseq_slice_extension_enabled()) {
+ /* Unconditionally clear it, no point in conditionals */
+ unsafe_put_user(0U, &rseq->slice_ctrl.all, efault);
+ }
}
+ rseq_slice_clear_grant(t);
/* Cache the new values */
t->rseq.ids.cpu_cid = ids->cpu_cid;
rseq_stat_inc(rseq_stats.ids);
@@ -488,8 +503,17 @@ static __always_inline bool rseq_exit_us
*/
u64 csaddr;
- if (unlikely(get_user_inline(csaddr, &rseq->rseq_cs)))
- return false;
+ scoped_user_rw_access(rseq, efault) {
+ unsafe_get_user(csaddr, &rseq->rseq_cs, efault);
+
+ /* Open coded, so it's in the same user access region */
+ if (rseq_slice_extension_enabled()) {
+ /* Unconditionally clear it, no point in conditionals */
+ unsafe_put_user(0U, &rseq->slice_ctrl.all, efault);
+ }
+ }
+
+ rseq_slice_clear_grant(t);
if (static_branch_unlikely(&rseq_debug_enabled) || unlikely(csaddr)) {
if (unlikely(!rseq_update_user_cs(t, regs, csaddr)))
@@ -505,6 +529,8 @@ static __always_inline bool rseq_exit_us
u32 node_id = cpu_to_node(ids.cpu_id);
return rseq_update_usr(t, regs, &ids, node_id);
+efault:
+ return false;
}
static __always_inline bool __rseq_exit_to_user_mode_restart(struct pt_regs *regs)
^ permalink raw reply [flat|nested] 13+ messages in thread* [patch V5 09/11] rseq: Implement rseq_grant_slice_extension()
2025-12-01 7:05 [patch V5 00/11] rseq: Implement time slice extension mechanism Thomas Gleixner
` (7 preceding siblings ...)
2025-12-01 7:06 ` [patch V5 08/11] rseq: Reset slice extension when scheduled Thomas Gleixner
@ 2025-12-01 7:06 ` Thomas Gleixner
2025-12-01 7:06 ` [patch V5 10/11] entry: Hook up rseq time slice extension Thomas Gleixner
2025-12-01 7:06 ` [patch V5 11/11] selftests/rseq: Implement time slice extension test Thomas Gleixner
10 siblings, 0 replies; 13+ messages in thread
From: Thomas Gleixner @ 2025-12-01 7:06 UTC (permalink / raw)
To: LKML
Cc: Mathieu Desnoyers, Paul E. McKenney, Boqun Feng, Jonathan Corbet,
Prakash Sangappa, Madadi Vineeth Reddy, K Prateek Nayak,
Steven Rostedt, Sebastian Andrzej Siewior, Arnd Bergmann,
linux-arch, Randy Dunlap, Peter Zijlstra, Ron Geva, Waiman Long
Provide the actual decision function, which decides whether a time slice
extension is granted in the exit to user mode path when NEED_RESCHED is
evaluated.
The decision is made in two stages. First an inline quick check to avoid
going into the actual decision function. This checks whether:
#1 the functionality is enabled
#2 the exit is a return from interrupt to user mode
#3 any TIF bit, which causes extra work is set. That includes TIF_RSEQ,
which means the task was already scheduled out.
The slow path, which implements the actual user space ABI, is invoked
when:
A) #1 is true, #2 is true and #3 is false
It checks whether user space requested a slice extension by setting
the request bit in the rseq slice_ctrl field. If so, it grants the
extension and stores the slice expiry time, so that the actual exit
code can double check whether the slice is already exhausted before
going back.
B) #1 - #3 are true _and_ a slice extension was granted in a previous
loop iteration
In this case the grant is revoked.
In case that the user space access faults or invalid state is detected, the
task is terminated with SIGSEGV.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
---
V2: Provide an extra stub for the !RSEQ case - Prateek
---
include/linux/rseq_entry.h | 108 +++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 108 insertions(+)
--- a/include/linux/rseq_entry.h
+++ b/include/linux/rseq_entry.h
@@ -42,6 +42,7 @@ DECLARE_PER_CPU(struct rseq_stats, rseq_
#ifdef CONFIG_RSEQ
#include <linux/jump_label.h>
#include <linux/rseq.h>
+#include <linux/sched/signal.h>
#include <linux/uaccess.h>
#include <linux/tracepoint-defs.h>
@@ -109,10 +110,116 @@ static __always_inline void rseq_slice_c
t->rseq.slice.state.granted = false;
}
+static __always_inline bool rseq_grant_slice_extension(bool work_pending)
+{
+ struct task_struct *curr = current;
+ struct rseq_slice_ctrl usr_ctrl;
+ union rseq_slice_state state;
+ struct rseq __user *rseq;
+
+ if (!rseq_slice_extension_enabled())
+ return false;
+
+ /* If not enabled or not a return from interrupt, nothing to do. */
+ state = curr->rseq.slice.state;
+ state.enabled &= curr->rseq.event.user_irq;
+ if (likely(!state.state))
+ return false;
+
+ rseq = curr->rseq.usrptr;
+ scoped_user_rw_access(rseq, efault) {
+
+ /*
+ * Quick check conditions where a grant is not possible or
+ * needs to be revoked.
+ *
+ * 1) Any TIF bit which needs to do extra work aside of
+ * rescheduling prevents a grant.
+ *
+ * 2) A previous rescheduling request resulted in a slice
+ * extension grant.
+ */
+ if (unlikely(work_pending || state.granted)) {
+ /* Clear user control unconditionally. No point for checking */
+ unsafe_put_user(0U, &rseq->slice_ctrl.all, efault);
+ rseq_slice_clear_grant(curr);
+ return false;
+ }
+
+ unsafe_get_user(usr_ctrl.all, &rseq->slice_ctrl.all, efault);
+ if (likely(!(usr_ctrl.request)))
+ return false;
+
+ /* Grant the slice extention */
+ usr_ctrl.request = 0;
+ usr_ctrl.granted = 1;
+ unsafe_put_user(usr_ctrl.all, &rseq->slice_ctrl.all, efault);
+ }
+
+ rseq_stat_inc(rseq_stats.s_granted);
+
+ curr->rseq.slice.state.granted = true;
+ /* Store expiry time for arming the timer on the way out */
+ curr->rseq.slice.expires = data_race(rseq_slice_ext_nsecs) + ktime_get_mono_fast_ns();
+ /*
+ * This is racy against a remote CPU setting TIF_NEED_RESCHED in
+ * several ways:
+ *
+ * 1)
+ * CPU0 CPU1
+ * clear_tsk()
+ * set_tsk()
+ * clear_preempt()
+ * Raise scheduler IPI on CPU0
+ * --> IPI
+ * fold_need_resched() -> Folds correctly
+ * 2)
+ * CPU0 CPU1
+ * set_tsk()
+ * clear_tsk()
+ * clear_preempt()
+ * Raise scheduler IPI on CPU0
+ * --> IPI
+ * fold_need_resched() <- NOOP as TIF_NEED_RESCHED is false
+ *
+ * #1 is not any different from a regular remote reschedule as it
+ * sets the previously not set bit and then raises the IPI which
+ * folds it into the preempt counter
+ *
+ * #2 is obviously incorrect from a scheduler POV, but it's not
+ * differently incorrect than the code below clearing the
+ * reschedule request with the safety net of the timer.
+ *
+ * The important part is that the clearing is protected against the
+ * scheduler IPI and also against any other interrupt which might
+ * end up waking up a task and setting the bits in the middle of
+ * the operation:
+ *
+ * clear_tsk()
+ * ---> Interrupt
+ * wakeup_on_this_cpu()
+ * set_tsk()
+ * set_preempt()
+ * clear_preempt()
+ *
+ * which would be inconsistent state.
+ */
+ scoped_guard(irq) {
+ clear_tsk_need_resched(curr);
+ clear_preempt_need_resched();
+ }
+ return true;
+
+efault:
+ force_sig(SIGSEGV);
+ return false;
+}
+
#else /* CONFIG_RSEQ_SLICE_EXTENSION */
static inline bool rseq_slice_extension_enabled(void) { return false; }
static inline bool rseq_arm_slice_extension_timer(void) { return false; }
static inline void rseq_slice_clear_grant(struct task_struct *t) { }
+static inline bool rseq_grant_slice_extension(bool work_pending) { return false; }
#endif /* !CONFIG_RSEQ_SLICE_EXTENSION */
bool rseq_debug_update_user_cs(struct task_struct *t, struct pt_regs *regs, unsigned long csaddr);
@@ -671,6 +778,7 @@ static inline void rseq_syscall_exit_to_
static inline void rseq_irqentry_exit_to_user_mode(void) { }
static inline void rseq_exit_to_user_mode_legacy(void) { }
static inline void rseq_debug_syscall_return(struct pt_regs *regs) { }
+static inline bool rseq_grant_slice_extension(bool work_pending) { return false; }
#endif /* !CONFIG_RSEQ */
#endif /* _LINUX_RSEQ_ENTRY_H */
^ permalink raw reply [flat|nested] 13+ messages in thread* [patch V5 10/11] entry: Hook up rseq time slice extension
2025-12-01 7:05 [patch V5 00/11] rseq: Implement time slice extension mechanism Thomas Gleixner
` (8 preceding siblings ...)
2025-12-01 7:06 ` [patch V5 09/11] rseq: Implement rseq_grant_slice_extension() Thomas Gleixner
@ 2025-12-01 7:06 ` Thomas Gleixner
2025-12-01 7:06 ` [patch V5 11/11] selftests/rseq: Implement time slice extension test Thomas Gleixner
10 siblings, 0 replies; 13+ messages in thread
From: Thomas Gleixner @ 2025-12-01 7:06 UTC (permalink / raw)
To: LKML
Cc: Mathieu Desnoyers, Paul E. McKenney, Boqun Feng, Jonathan Corbet,
Prakash Sangappa, Madadi Vineeth Reddy, K Prateek Nayak,
Steven Rostedt, Sebastian Andrzej Siewior, Arnd Bergmann,
linux-arch, Randy Dunlap, Peter Zijlstra, Ron Geva, Waiman Long
Wire the grant decision function up in exit_to_user_mode_loop()
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
---
kernel/entry/common.c | 14 ++++++++++++--
1 file changed, 12 insertions(+), 2 deletions(-)
--- a/kernel/entry/common.c
+++ b/kernel/entry/common.c
@@ -17,6 +17,14 @@ void __weak arch_do_signal_or_restart(st
#define EXIT_TO_USER_MODE_WORK_LOOP (EXIT_TO_USER_MODE_WORK)
#endif
+/* TIF bits, which prevent a time slice extension. */
+#ifdef CONFIG_PREEMPT_RT
+# define TIF_SLICE_EXT_SCHED (_TIF_NEED_RESCHED_LAZY)
+#else
+# define TIF_SLICE_EXT_SCHED (_TIF_NEED_RESCHED | _TIF_NEED_RESCHED_LAZY)
+#endif
+#define TIF_SLICE_EXT_DENY (EXIT_TO_USER_MODE_WORK & ~TIF_SLICE_EXT_SCHED)
+
static __always_inline unsigned long __exit_to_user_mode_loop(struct pt_regs *regs,
unsigned long ti_work)
{
@@ -28,8 +36,10 @@ static __always_inline unsigned long __e
local_irq_enable_exit_to_user(ti_work);
- if (ti_work & (_TIF_NEED_RESCHED | _TIF_NEED_RESCHED_LAZY))
- schedule();
+ if (ti_work & (_TIF_NEED_RESCHED | _TIF_NEED_RESCHED_LAZY)) {
+ if (!rseq_grant_slice_extension(ti_work & TIF_SLICE_EXT_DENY))
+ schedule();
+ }
if (ti_work & _TIF_UPROBE)
uprobe_notify_resume(regs);
^ permalink raw reply [flat|nested] 13+ messages in thread* [patch V5 11/11] selftests/rseq: Implement time slice extension test
2025-12-01 7:05 [patch V5 00/11] rseq: Implement time slice extension mechanism Thomas Gleixner
` (9 preceding siblings ...)
2025-12-01 7:06 ` [patch V5 10/11] entry: Hook up rseq time slice extension Thomas Gleixner
@ 2025-12-01 7:06 ` Thomas Gleixner
10 siblings, 0 replies; 13+ messages in thread
From: Thomas Gleixner @ 2025-12-01 7:06 UTC (permalink / raw)
To: LKML
Cc: Mathieu Desnoyers, Paul E. McKenney, Boqun Feng, Jonathan Corbet,
Prakash Sangappa, Madadi Vineeth Reddy, K Prateek Nayak,
Steven Rostedt, Sebastian Andrzej Siewior, Arnd Bergmann,
linux-arch, Randy Dunlap, Peter Zijlstra, Ron Geva, Waiman Long
Provide an initial test case to evaluate the functionality. This needs to be
extended to cover the ABI violations and expose the race condition between
observing granted and arriving in rseq_slice_yield().
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
---
V5: Add a test for a random syscall
---
tools/testing/selftests/rseq/.gitignore | 1
tools/testing/selftests/rseq/Makefile | 5
tools/testing/selftests/rseq/rseq-abi.h | 27 +++
tools/testing/selftests/rseq/slice_test.c | 219 ++++++++++++++++++++++++++++++
4 files changed, 251 insertions(+), 1 deletion(-)
--- a/tools/testing/selftests/rseq/.gitignore
+++ b/tools/testing/selftests/rseq/.gitignore
@@ -10,3 +10,4 @@ param_test_mm_cid
param_test_mm_cid_benchmark
param_test_mm_cid_compare_twice
syscall_errors_test
+slice_test
--- a/tools/testing/selftests/rseq/Makefile
+++ b/tools/testing/selftests/rseq/Makefile
@@ -17,7 +17,7 @@ OVERRIDE_TARGETS = 1
TEST_GEN_PROGS = basic_test basic_percpu_ops_test basic_percpu_ops_mm_cid_test param_test \
param_test_benchmark param_test_compare_twice param_test_mm_cid \
param_test_mm_cid_benchmark param_test_mm_cid_compare_twice \
- syscall_errors_test
+ syscall_errors_test slice_test
TEST_GEN_PROGS_EXTENDED = librseq.so
@@ -59,3 +59,6 @@ include ../lib.mk
$(OUTPUT)/syscall_errors_test: syscall_errors_test.c $(TEST_GEN_PROGS_EXTENDED) \
rseq.h rseq-*.h
$(CC) $(CFLAGS) $< $(LDLIBS) -lrseq -o $@
+
+$(OUTPUT)/slice_test: slice_test.c $(TEST_GEN_PROGS_EXTENDED) rseq.h rseq-*.h
+ $(CC) $(CFLAGS) $< $(LDLIBS) -lrseq -o $@
--- a/tools/testing/selftests/rseq/rseq-abi.h
+++ b/tools/testing/selftests/rseq/rseq-abi.h
@@ -53,6 +53,27 @@ struct rseq_abi_cs {
__u64 abort_ip;
} __attribute__((aligned(4 * sizeof(__u64))));
+/**
+ * rseq_slice_ctrl - Time slice extension control structure
+ * @all: Compound value
+ * @request: Request for a time slice extension
+ * @granted: Granted time slice extension
+ *
+ * @request is set by user space and can be cleared by user space or kernel
+ * space. @granted is set and cleared by the kernel and must only be read
+ * by user space.
+ */
+struct rseq_slice_ctrl {
+ union {
+ __u32 all;
+ struct {
+ __u8 request;
+ __u8 granted;
+ __u16 __reserved;
+ };
+ };
+};
+
/*
* struct rseq_abi is aligned on 4 * 8 bytes to ensure it is always
* contained within a single cache-line.
@@ -165,6 +186,12 @@ struct rseq_abi {
__u32 mm_cid;
/*
+ * Time slice extension control structure. CPU local updates from
+ * kernel and user space.
+ */
+ struct rseq_slice_ctrl slice_ctrl;
+
+ /*
* Flexible array member at end of structure, after last feature field.
*/
char end[];
--- /dev/null
+++ b/tools/testing/selftests/rseq/slice_test.c
@@ -0,0 +1,219 @@
+// SPDX-License-Identifier: LGPL-2.1
+#define _GNU_SOURCE
+#include <assert.h>
+#include <pthread.h>
+#include <sched.h>
+#include <signal.h>
+#include <stdbool.h>
+#include <stdio.h>
+#include <string.h>
+#include <syscall.h>
+#include <unistd.h>
+
+#include <linux/prctl.h>
+#include <sys/prctl.h>
+#include <sys/time.h>
+
+#include "rseq.h"
+
+#include "../kselftest_harness.h"
+
+#ifndef __NR_rseq_slice_yield
+# define __NR_rseq_slice_yield 470
+#endif
+
+#define BITS_PER_INT 32
+#define BITS_PER_BYTE 8
+
+#ifndef PR_RSEQ_SLICE_EXTENSION
+# define PR_RSEQ_SLICE_EXTENSION 79
+# define PR_RSEQ_SLICE_EXTENSION_GET 1
+# define PR_RSEQ_SLICE_EXTENSION_SET 2
+# define PR_RSEQ_SLICE_EXT_ENABLE 0x01
+#endif
+
+#ifndef RSEQ_SLICE_EXT_REQUEST_BIT
+# define RSEQ_SLICE_EXT_REQUEST_BIT 0
+# define RSEQ_SLICE_EXT_GRANTED_BIT 1
+#endif
+
+#ifndef asm_inline
+# define asm_inline asm __inline
+#endif
+
+#define NSEC_PER_SEC 1000000000L
+#define NSEC_PER_USEC 1000L
+
+struct noise_params {
+ int64_t noise_nsecs;
+ int64_t sleep_nsecs;
+ int64_t run;
+};
+
+FIXTURE(slice_ext)
+{
+ pthread_t noise_thread;
+ struct noise_params noise_params;
+};
+
+FIXTURE_VARIANT(slice_ext)
+{
+ int64_t total_nsecs;
+ int64_t slice_nsecs;
+ int64_t noise_nsecs;
+ int64_t sleep_nsecs;
+ bool no_yield;
+};
+
+FIXTURE_VARIANT_ADD(slice_ext, n2_2_50)
+{
+ .total_nsecs = 5 * NSEC_PER_SEC,
+ .slice_nsecs = 2 * NSEC_PER_USEC,
+ .noise_nsecs = 2 * NSEC_PER_USEC,
+ .sleep_nsecs = 50 * NSEC_PER_USEC,
+};
+
+FIXTURE_VARIANT_ADD(slice_ext, n50_2_50)
+{
+ .total_nsecs = 5 * NSEC_PER_SEC,
+ .slice_nsecs = 50 * NSEC_PER_USEC,
+ .noise_nsecs = 2 * NSEC_PER_USEC,
+ .sleep_nsecs = 50 * NSEC_PER_USEC,
+};
+
+FIXTURE_VARIANT_ADD(slice_ext, n2_2_50_no_yield)
+{
+ .total_nsecs = 5 * NSEC_PER_SEC,
+ .slice_nsecs = 2 * NSEC_PER_USEC,
+ .noise_nsecs = 2 * NSEC_PER_USEC,
+ .sleep_nsecs = 50 * NSEC_PER_USEC,
+ .no_yield = true,
+};
+
+
+static inline bool elapsed(struct timespec *start, struct timespec *now,
+ int64_t span)
+{
+ int64_t delta = now->tv_sec - start->tv_sec;
+
+ delta *= NSEC_PER_SEC;
+ delta += now->tv_nsec - start->tv_nsec;
+ return delta >= span;
+}
+
+static void *noise_thread(void *arg)
+{
+ struct noise_params *p = arg;
+
+ while (RSEQ_READ_ONCE(p->run)) {
+ struct timespec ts_start, ts_now;
+
+ clock_gettime(CLOCK_MONOTONIC, &ts_start);
+ do {
+ clock_gettime(CLOCK_MONOTONIC, &ts_now);
+ } while (!elapsed(&ts_start, &ts_now, p->noise_nsecs));
+
+ ts_start.tv_sec = 0;
+ ts_start.tv_nsec = p->sleep_nsecs;
+ clock_nanosleep(CLOCK_MONOTONIC, 0, &ts_start, NULL);
+ }
+ return NULL;
+}
+
+FIXTURE_SETUP(slice_ext)
+{
+ cpu_set_t affinity;
+
+ ASSERT_EQ(sched_getaffinity(0, sizeof(affinity), &affinity), 0);
+
+ /* Pin it on a single CPU. Avoid CPU 0 */
+ for (int i = 1; i < CPU_SETSIZE; i++) {
+ if (!CPU_ISSET(i, &affinity))
+ continue;
+
+ CPU_ZERO(&affinity);
+ CPU_SET(i, &affinity);
+ ASSERT_EQ(sched_setaffinity(0, sizeof(affinity), &affinity), 0);
+ break;
+ }
+
+ ASSERT_EQ(rseq_register_current_thread(), 0);
+
+ ASSERT_EQ(prctl(PR_RSEQ_SLICE_EXTENSION, PR_RSEQ_SLICE_EXTENSION_SET,
+ PR_RSEQ_SLICE_EXT_ENABLE, 0, 0), 0);
+
+ self->noise_params.noise_nsecs = variant->noise_nsecs;
+ self->noise_params.sleep_nsecs = variant->sleep_nsecs;
+ self->noise_params.run = 1;
+
+ ASSERT_EQ(pthread_create(&self->noise_thread, NULL, noise_thread, &self->noise_params), 0);
+}
+
+FIXTURE_TEARDOWN(slice_ext)
+{
+ self->noise_params.run = 0;
+ pthread_join(self->noise_thread, NULL);
+}
+
+TEST_F(slice_ext, slice_test)
+{
+ unsigned long success = 0, yielded = 0, scheduled = 0, raced = 0;
+ unsigned long total = 0, aborted = 0;
+ struct rseq_abi *rs = rseq_get_abi();
+ struct timespec ts_start, ts_now;
+
+ ASSERT_NE(rs, NULL);
+
+ clock_gettime(CLOCK_MONOTONIC, &ts_start);
+ do {
+ struct timespec ts_cs;
+ bool req = false;
+
+ clock_gettime(CLOCK_MONOTONIC, &ts_cs);
+
+ total++;
+ RSEQ_WRITE_ONCE(rs->slice_ctrl.request, 1);
+ do {
+ clock_gettime(CLOCK_MONOTONIC, &ts_now);
+ } while (!elapsed(&ts_cs, &ts_now, variant->slice_nsecs));
+
+ /*
+ * request can be cleared unconditionally, but for making
+ * the stats work this is actually checking it first
+ */
+ if (RSEQ_READ_ONCE(rs->slice_ctrl.request)) {
+ RSEQ_WRITE_ONCE(rs->slice_ctrl.request, 0);
+ /* Race between check and clear! */
+ req = true;
+ success++;
+ }
+
+ if (RSEQ_READ_ONCE(rs->slice_ctrl.granted)) {
+ /* The above raced against a late grant */
+ if (req)
+ success--;
+ if (variant->no_yield) {
+ syscall(__NR_getpid);
+ aborted++;
+ } else {
+ yielded++;
+ if (!syscall(__NR_rseq_slice_yield))
+ raced++;
+ }
+ } else {
+ if (!req)
+ scheduled++;
+ }
+
+ clock_gettime(CLOCK_MONOTONIC, &ts_now);
+ } while (!elapsed(&ts_start, &ts_now, variant->total_nsecs));
+
+ printf("# Total %12ld\n", total);
+ printf("# Success %12ld\n", success);
+ printf("# Yielded %12ld\n", yielded);
+ printf("# Aborted %12ld\n", aborted);
+ printf("# Scheduled %12ld\n", scheduled);
+ printf("# Raced %12ld\n", raced);
+}
+
+TEST_HARNESS_MAIN
^ permalink raw reply [flat|nested] 13+ messages in thread