[tip: sched/core] rseq: Provide static branch for time slice extensions

["tip-bot2 for Thomas Gleixner" <tip-bot2@linutronix.de>]

The following commit has been merged into the sched/core branch of tip:

Commit-ID:     f8380f976804533df4c6c3d3a0b2cd03c2d262bc
Gitweb:        https://git.kernel.org/tip/f8380f976804533df4c6c3d3a0b2cd03c2d262bc
Author:        Thomas Gleixner <tglx@linutronix.de>
AuthorDate:    Mon, 15 Dec 2025 17:52:06 +01:00
Committer:     Peter Zijlstra <peterz@infradead.org>
CommitterDate: Thu, 22 Jan 2026 11:11:16 +01:00

rseq: Provide static branch for time slice extensions

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>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20251215155708.733429292@linutronix.de
---
 Documentation/admin-guide/kernel-parameters.txt |  5 +++++-
 include/linux/rseq_entry.h                      | 11 ++++++++++-
 kernel/rseq.c                                   | 17 ++++++++++++++++-
 3 files changed, 33 insertions(+)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index a8d0afd..f2348bc 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -6600,6 +6600,11 @@ Kernel parameters
 
 	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.
 
diff --git a/include/linux/rseq_entry.h b/include/linux/rseq_entry.h
index a36b472..d0ec471 100644
--- a/include/linux/rseq_entry.h
+++ b/include/linux/rseq_entry.h
@@ -75,6 +75,17 @@ DECLARE_STATIC_KEY_MAYBE(CONFIG_RSEQ_DEBUG_DEFAULT_ENABLE, rseq_debug_enabled);
 #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);
 
diff --git a/kernel/rseq.c b/kernel/rseq.c
index 07c324d..bf75268 100644
--- a/kernel/rseq.c
+++ b/kernel/rseq.c
@@ -483,3 +483,20 @@ SYSCALL_DEFINE4(rseq, struct rseq __user *, rseq, u32, rseq_len, int, flags, u32
 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 */