[PATCH v10 07/12] task_isolation: add debug boot flag

[Chris Metcalf <cmetcalf@ezchip.com>]

The new "task_isolation_debug" flag simplifies debugging
of TASK_ISOLATION kernels when processes are running in
PR_TASK_ISOLATION_ENABLE mode.  Such processes should get no
interrupts from the kernel, and if they do, when this boot flag is
specified a kernel stack dump on the console is generated.

It's possible to use ftrace to simply detect whether a task_isolation
core has unexpectedly entered the kernel.  But what this boot flag
does is allow the kernel to provide better diagnostics, e.g. by
reporting in the IPI-generating code what remote core and context
is preparing to deliver an interrupt to a task_isolation core.

It may be worth considering other ways to generate useful debugging
output rather than console spew, but for now that is simple and direct.

Signed-off-by: Chris Metcalf <cmetcalf@ezchip.com>
---
 Documentation/kernel-parameters.txt    |  8 ++++
 include/linux/context_tracking_state.h |  6 +++
 include/linux/isolation.h              |  5 +++
 kernel/irq_work.c                      |  5 ++-
 kernel/isolation.c                     | 77 ++++++++++++++++++++++++++++++++++
 kernel/sched/core.c                    | 18 ++++++++
 kernel/signal.c                        |  5 +++
 kernel/smp.c                           |  6 ++-
 kernel/softirq.c                       | 33 +++++++++++++++
 9 files changed, 161 insertions(+), 2 deletions(-)

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index c8d0b42d984a..ea0434fa906e 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -3755,6 +3755,14 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 			also sets up nohz_full and isolcpus mode for the
 			listed set of cpus.
 
+	task_isolation_debug	[KNL]
+			In kernels built with CONFIG_TASK_ISOLATION
+			and booted in task_isolation= mode, this
+			setting will generate console backtraces when
+			the kernel is about to interrupt a task that
+			has requested PR_TASK_ISOLATION_ENABLE and is
+			running on a task_isolation core.
+
 	tcpmhash_entries= [KNL,NET]
 			Set the number of tcp_metrics_hash slots.
 			Default value is 8192 or 16384 depending on total
diff --git a/include/linux/context_tracking_state.h b/include/linux/context_tracking_state.h
index 1d34fe68f48a..4e2c4b900b82 100644
--- a/include/linux/context_tracking_state.h
+++ b/include/linux/context_tracking_state.h
@@ -39,8 +39,14 @@ static inline bool context_tracking_in_user(void)
 {
 	return __this_cpu_read(context_tracking.state) == CONTEXT_USER;
 }
+
+static inline bool context_tracking_cpu_in_user(int cpu)
+{
+	return per_cpu(context_tracking.state, cpu) == CONTEXT_USER;
+}
 #else
 static inline bool context_tracking_in_user(void) { return false; }
+static inline bool context_tracking_cpu_in_user(int cpu) { return false; }
 static inline bool context_tracking_active(void) { return false; }
 static inline bool context_tracking_is_enabled(void) { return false; }
 static inline bool context_tracking_cpu_is_enabled(void) { return false; }
diff --git a/include/linux/isolation.h b/include/linux/isolation.h
index ba6c4d510db8..f1ae7b663746 100644
--- a/include/linux/isolation.h
+++ b/include/linux/isolation.h
@@ -45,6 +45,9 @@ static inline void task_isolation_enter(void)
 extern bool task_isolation_syscall(int nr);
 extern void task_isolation_exception(const char *fmt, ...);
 extern void task_isolation_interrupt(struct task_struct *, const char *buf);
+extern void task_isolation_debug(int cpu);
+extern void task_isolation_debug_cpumask(const struct cpumask *);
+extern void task_isolation_debug_task(int cpu, struct task_struct *p);
 
 static inline bool task_isolation_strict(void)
 {
@@ -73,6 +76,8 @@ static inline bool task_isolation_ready(void) { return true; }
 static inline void task_isolation_enter(void) { }
 static inline bool task_isolation_check_syscall(int nr) { return false; }
 static inline void task_isolation_check_exception(const char *fmt, ...) { }
+static inline void task_isolation_debug(int cpu) { }
+#define task_isolation_debug_cpumask(mask) do {} while (0)
 #endif
 
 #endif
diff --git a/kernel/irq_work.c b/kernel/irq_work.c
index bcf107ce0854..a9b95ce00667 100644
--- a/kernel/irq_work.c
+++ b/kernel/irq_work.c
@@ -17,6 +17,7 @@
 #include <linux/cpu.h>
 #include <linux/notifier.h>
 #include <linux/smp.h>
+#include <linux/isolation.h>
 #include <asm/processor.h>
 
 
@@ -75,8 +76,10 @@ bool irq_work_queue_on(struct irq_work *work, int cpu)
 	if (!irq_work_claim(work))
 		return false;
 
-	if (llist_add(&work->llnode, &per_cpu(raised_list, cpu)))
+	if (llist_add(&work->llnode, &per_cpu(raised_list, cpu))) {
+		task_isolation_debug(cpu);
 		arch_send_call_function_single_ipi(cpu);
+	}
 
 	return true;
 }
diff --git a/kernel/isolation.c b/kernel/isolation.c
index 5621fdf15b17..6ac35988b49a 100644
--- a/kernel/isolation.c
+++ b/kernel/isolation.c
@@ -11,6 +11,7 @@
 #include <linux/vmstat.h>
 #include <linux/isolation.h>
 #include <linux/syscalls.h>
+#include <linux/ratelimit.h>
 #include <asm/unistd.h>
 #include "time/tick-sched.h"
 
@@ -182,3 +183,79 @@ bool task_isolation_syscall(int syscall)
 	task_isolation_exception("syscall %d", syscall);
 	return true;
 }
+
+/* Enable debugging of any interrupts of task_isolation cores. */
+static int task_isolation_debug_flag;
+static int __init task_isolation_debug_func(char *str)
+{
+	task_isolation_debug_flag = true;
+	return 1;
+}
+__setup("task_isolation_debug", task_isolation_debug_func);
+
+void task_isolation_debug_task(int cpu, struct task_struct *p)
+{
+	static DEFINE_RATELIMIT_STATE(console_output, HZ, 1);
+	bool force_debug = false;
+
+	/*
+	 * Our caller made sure the task was running on a task isolation
+	 * core, but make sure the task has enabled isolation.
+	 */
+	if (!(p->task_isolation_flags & PR_TASK_ISOLATION_ENABLE))
+		return;
+
+	/*
+	 * Ensure the task is actually in userspace; if it is in kernel
+	 * mode, it is expected that it may receive interrupts, and in
+	 * any case they don't affect the isolation.  Note that there
+	 * is a race condition here as a task may have committed
+	 * to returning to user space but not yet set the context
+	 * tracking state to reflect it, and the check here is before
+	 * we trigger the interrupt, so we might fail to warn about a
+	 * legitimate interrupt.  However, the race window is narrow
+	 * and hitting it does not cause any incorrect behavior other
+	 * than failing to send the warning.
+	 */
+	if (!context_tracking_cpu_in_user(cpu))
+		return;
+
+	/*
+	 * If the task was in strict mode, deliver a signal to it.
+	 * We disable task isolation mode when we deliver a signal
+	 * so we won't end up recursing back here again.
+	 * If we are in an NMI, we don't try delivering the signal
+	 * and instead just treat it as if "debug" mode was enabled,
+	 * since that's pretty much all we can do.
+	 */
+	if (p->task_isolation_flags & PR_TASK_ISOLATION_STRICT) {
+		if (in_nmi())
+			force_debug = true;
+		else
+			task_isolation_interrupt(p, "interrupt");
+	}
+
+	/*
+	 * If (for example) the timer interrupt starts ticking
+	 * unexpectedly, we will get an unmanageable flow of output,
+	 * so limit to one backtrace per second.
+	 */
+	if (force_debug ||
+	    (task_isolation_debug_flag && __ratelimit(&console_output))) {
+		pr_err("Interrupt detected for task_isolation cpu %d, %s/%d\n",
+		       cpu, p->comm, p->pid);
+		dump_stack();
+	}
+}
+
+void task_isolation_debug_cpumask(const struct cpumask *mask)
+{
+	int cpu, thiscpu = get_cpu();
+
+	/* No need to report on this cpu since we're already in the kernel. */
+	for_each_cpu(cpu, mask)
+		if (cpu != thiscpu)
+			task_isolation_debug(cpu);
+
+	put_cpu();
+}
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 9503d590e5ef..f53e1417aa75 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -74,6 +74,7 @@
 #include <linux/binfmts.h>
 #include <linux/context_tracking.h>
 #include <linux/compiler.h>
+#include <linux/isolation.h>
 
 #include <asm/switch_to.h>
 #include <asm/tlb.h>
@@ -746,6 +747,23 @@ bool sched_can_stop_tick(void)
 }
 #endif /* CONFIG_NO_HZ_FULL */
 
+#ifdef CONFIG_TASK_ISOLATION
+void task_isolation_debug(int cpu)
+{
+	struct task_struct *p;
+
+	if (!task_isolation_possible(cpu))
+		return;
+
+	rcu_read_lock();
+	p = cpu_curr(cpu);
+	get_task_struct(p);
+	rcu_read_unlock();
+	task_isolation_debug_task(cpu, p);
+	put_task_struct(p);
+}
+#endif
+
 void sched_avg_update(struct rq *rq)
 {
 	s64 period = sched_avg_period();
diff --git a/kernel/signal.c b/kernel/signal.c
index 0508544c8ced..b35ce19c01c4 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -638,6 +638,11 @@ int dequeue_signal(struct task_struct *tsk, sigset_t *mask, siginfo_t *info)
  */
 void signal_wake_up_state(struct task_struct *t, unsigned int state)
 {
+#ifdef CONFIG_TASK_ISOLATION
+	/* If the task is being killed, don't complain about task_isolation. */
+	if (state & TASK_WAKEKILL)
+		t->task_isolation_flags = 0;
+#endif
 	set_tsk_thread_flag(t, TIF_SIGPENDING);
 	/*
 	 * TASK_WAKEKILL also means wake it up in the stopped/traced/killable
diff --git a/kernel/smp.c b/kernel/smp.c
index d903c02223af..a61894409645 100644
--- a/kernel/smp.c
+++ b/kernel/smp.c
@@ -14,6 +14,7 @@
 #include <linux/smp.h>
 #include <linux/cpu.h>
 #include <linux/sched.h>
+#include <linux/isolation.h>
 
 #include "smpboot.h"
 
@@ -178,8 +179,10 @@ static int generic_exec_single(int cpu, struct call_single_data *csd,
 	 * locking and barrier primitives. Generic code isn't really
 	 * equipped to do the right thing...
 	 */
-	if (llist_add(&csd->llist, &per_cpu(call_single_queue, cpu)))
+	if (llist_add(&csd->llist, &per_cpu(call_single_queue, cpu))) {
+		task_isolation_debug(cpu);
 		arch_send_call_function_single_ipi(cpu);
+	}
 
 	return 0;
 }
@@ -457,6 +460,7 @@ void smp_call_function_many(const struct cpumask *mask,
 	}
 
 	/* Send a message to all CPUs in the map */
+	task_isolation_debug_cpumask(cfd->cpumask);
 	arch_send_call_function_ipi_mask(cfd->cpumask);
 
 	if (wait) {
diff --git a/kernel/softirq.c b/kernel/softirq.c
index 479e4436f787..f249b71cddf4 100644
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -26,6 +26,7 @@
 #include <linux/smpboot.h>
 #include <linux/tick.h>
 #include <linux/irq.h>
+#include <linux/isolation.h>
 
 #define CREATE_TRACE_POINTS
 #include <trace/events/irq.h>
@@ -319,6 +320,37 @@ asmlinkage __visible void do_softirq(void)
 	local_irq_restore(flags);
 }
 
+/* Determine whether this IRQ is something task isolation cares about. */
+static void task_isolation_irq(void)
+{
+#ifdef CONFIG_TASK_ISOLATION
+	struct pt_regs *regs;
+
+	if (!context_tracking_cpu_is_enabled())
+		return;
+
+	/*
+	 * We have not yet called __irq_enter() and so we haven't
+	 * adjusted the hardirq count.  This test will allow us to
+	 * avoid false positives for nested IRQs.
+	 */
+	if (in_interrupt())
+		return;
+
+	/*
+	 * If we were already in the kernel, not from an irq but from
+	 * a syscall or synchronous exception/fault, this test should
+	 * avoid a false positive as well.  Note that this requires
+	 * architecture support for calling set_irq_regs() prior to
+	 * calling irq_enter(), and if it's not done consistently, we
+	 * will not consistently avoid false positives here.
+	 */
+	regs = get_irq_regs();
+	if (regs && user_mode(regs))
+		task_isolation_debug(smp_processor_id());
+#endif
+}
+
 /*
  * Enter an interrupt context.
  */
@@ -335,6 +367,7 @@ void irq_enter(void)
 		_local_bh_enable();
 	}
 
+	task_isolation_irq();
 	__irq_enter();
 }
 
-- 
2.1.2