[PATCH] sched: Add preempt checks in preempt_schedule() code

[Steven Rostedt]

While testing the tracer preemptoff, I hit this strange trace:

#  cmd     pid   ||||| time  |   caller      
#     \   /      |||||  \    |   /         
   <...>-259     0...1    0us : schedule <-worker_thread
   <...>-259     0d..1    0us : rcu_note_context_switch <-__schedule
   <...>-259     0d..1    0us : rcu_sched_qs <-rcu_note_context_switch
   <...>-259     0d..1    0us : rcu_preempt_qs <-rcu_note_context_switch
   <...>-259     0d..1    0us : _raw_spin_lock <-__schedule
   <...>-259     0d..1    0us : preempt_count_add <-_raw_spin_lock
   <...>-259     0d..2    0us : do_raw_spin_lock <-_raw_spin_lock
   <...>-259     0d..2    1us : deactivate_task <-__schedule
   <...>-259     0d..2    1us : update_rq_clock.part.84 <-deactivate_task
   <...>-259     0d..2    1us : dequeue_task_fair <-deactivate_task
   <...>-259     0d..2    1us : dequeue_entity <-dequeue_task_fair
   <...>-259     0d..2    1us : update_curr <-dequeue_entity
   <...>-259     0d..2    1us : update_min_vruntime <-update_curr
   <...>-259     0d..2    1us : cpuacct_charge <-update_curr
   <...>-259     0d..2    1us : __rcu_read_lock <-cpuacct_charge
   <...>-259     0d..2    1us : __rcu_read_unlock <-cpuacct_charge
   <...>-259     0d..2    1us : clear_buddies <-dequeue_entity
   <...>-259     0d..2    1us : account_entity_dequeue <-dequeue_entity
   <...>-259     0d..2    2us : update_min_vruntime <-dequeue_entity
   <...>-259     0d..2    2us : update_cfs_shares <-dequeue_entity
   <...>-259     0d..2    2us : hrtick_update <-dequeue_task_fair
   <...>-259     0d..2    2us : wq_worker_sleeping <-__schedule
   <...>-259     0d..2    2us : kthread_data <-wq_worker_sleeping
   <...>-259     0d..2    2us : pick_next_task_fair <-__schedule
   <...>-259     0d..2    2us : check_cfs_rq_runtime <-pick_next_task_fair
   <...>-259     0d..2    2us : pick_next_entity <-pick_next_task_fair
   <...>-259     0d..2    2us : clear_buddies <-pick_next_entity
   <...>-259     0d..2    2us : pick_next_entity <-pick_next_task_fair
   <...>-259     0d..2    2us : clear_buddies <-pick_next_entity
   <...>-259     0d..2    2us : set_next_entity <-pick_next_task_fair
   <...>-259     0d..2    3us : put_prev_entity <-pick_next_task_fair
   <...>-259     0d..2    3us : check_cfs_rq_runtime <-put_prev_entity
   <...>-259     0d..2    3us : set_next_entity <-pick_next_task_fair
gnome-sh-1031    0d..2    3us : finish_task_switch <-__schedule
gnome-sh-1031    0d..2    3us : _raw_spin_unlock_irq <-finish_task_switch
gnome-sh-1031    0d..2    3us : do_raw_spin_unlock <-_raw_spin_unlock_irq
gnome-sh-1031    0...2    3us!: preempt_count_sub <-_raw_spin_unlock_irq
gnome-sh-1031    0...1  582us : do_raw_spin_lock <-_raw_spin_lock
gnome-sh-1031    0...1  583us : _raw_spin_unlock <-drm_gem_object_lookup
gnome-sh-1031    0...1  583us : do_raw_spin_unlock <-_raw_spin_unlock
gnome-sh-1031    0...1  583us : preempt_count_sub <-_raw_spin_unlock
gnome-sh-1031    0...1  584us : _raw_spin_unlock <-drm_gem_object_lookup
gnome-sh-1031    0...1  584us+: trace_preempt_on <-drm_gem_object_lookup
gnome-sh-1031    0...1  603us : <stack trace>
 => preempt_count_sub
 => _raw_spin_unlock
 => drm_gem_object_lookup
 => i915_gem_madvise_ioctl
 => drm_ioctl
 => do_vfs_ioctl
 => SyS_ioctl
 => entry_SYSCALL_64_fastpath

As I'm tracing preemption disabled, it seemed incorrect that the trace
would go across a schedule and report not being in the scheduler.
Looking into this I discovered the problem.

schedule() calls preempt_disable() but the preempt_schedule() calls
preempt_enable_notrace(). What happened above was that the gnome-shell
task was preempted on another CPU, migrated over to the idle cpu. The
tracer stared with idle calling schedule(), which called
preempt_disable(), but then gnome-shell finished, and it enabled
preemption with preempt_enable_notrace() that does stop the trace, even
though preemption was enabled.

The purpose of the preempt_disable_notrace() in the preempt_schedule()
is to prevent function tracing from going into an infinite loop.
Because function tracing can trace the preempt_enable/disable() calls
that are traced. The problem with function tracing is:

  NEED_RESCHED set
  preempt_schedule()
    preempt_disable()
      preempt_count_inc()
        function trace (before incrementing preempt count)
          preempt_disable_notrace()
          preempt_enable_notrace()
            sees NEED_RESCHED set
               preempt_schedule() (repeat)

Now by breaking out the preempt off/on tracing into their own code:
preempt_disable_check() and preempt_enable_check(), we can add these to
the preempt_schedule() code. As preemption would then be disabled, even
if they were to be traced by the function tracer, the disabled
preemption would prevent the recursion.

Signed-off-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
---
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 9503d590e5ef..a925e1d2c4cd 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3023,6 +3023,17 @@ notrace unsigned long get_parent_ip(unsigned long addr)
 #if defined(CONFIG_PREEMPT) && (defined(CONFIG_DEBUG_PREEMPT) || \
 				defined(CONFIG_PREEMPT_TRACER))
 
+static inline void preempt_disable_check(int val)
+{
+	if (preempt_count() == val) {
+		unsigned long ip = get_parent_ip(CALLER_ADDR1);
+#ifdef CONFIG_DEBUG_PREEMPT
+		current->preempt_disable_ip = ip;
+#endif
+		trace_preempt_off(CALLER_ADDR0, ip);
+	}
+}
+
 void preempt_count_add(int val)
 {
 #ifdef CONFIG_DEBUG_PREEMPT
@@ -3040,17 +3051,17 @@ void preempt_count_add(int val)
 	DEBUG_LOCKS_WARN_ON((preempt_count() & PREEMPT_MASK) >=
 				PREEMPT_MASK - 10);
 #endif
-	if (preempt_count() == val) {
-		unsigned long ip = get_parent_ip(CALLER_ADDR1);
-#ifdef CONFIG_DEBUG_PREEMPT
-		current->preempt_disable_ip = ip;
-#endif
-		trace_preempt_off(CALLER_ADDR0, ip);
-	}
+	preempt_disable_check(val);
 }
 EXPORT_SYMBOL(preempt_count_add);
 NOKPROBE_SYMBOL(preempt_count_add);
 
+static inline void preempt_enable_check(int val)
+{
+	if (preempt_count() == val)
+		trace_preempt_on(CALLER_ADDR0, get_parent_ip(CALLER_ADDR1));
+}
+
 void preempt_count_sub(int val)
 {
 #ifdef CONFIG_DEBUG_PREEMPT
@@ -3067,13 +3078,15 @@ void preempt_count_sub(int val)
 		return;
 #endif
 
-	if (preempt_count() == val)
-		trace_preempt_on(CALLER_ADDR0, get_parent_ip(CALLER_ADDR1));
+	preempt_enable_check(val);
 	__preempt_count_sub(val);
 }
 EXPORT_SYMBOL(preempt_count_sub);
 NOKPROBE_SYMBOL(preempt_count_sub);
 
+#else
+static inline void preempt_disable_check(int val) { }
+static inline void preempt_enable_check(int val) { }
 #endif
 
 /*
@@ -3349,7 +3362,14 @@ static void __sched notrace preempt_schedule_common(void)
 {
 	do {
 		preempt_disable_notrace();
+		/*
+		 * Function tracer requires disabling preemption before
+		 * tracing functions. But we still want to trace
+		 * preemption off locations.
+		 */
+		preempt_disable_check(1);
 		__schedule(true);
+		preempt_enable_check(1);
 		preempt_enable_no_resched_notrace();
 
 		/*
@@ -3403,6 +3423,12 @@ asmlinkage __visible void __sched notrace preempt_schedule_notrace(void)
 	do {
 		preempt_disable_notrace();
 		/*
+		 * Function tracer requires disabling preemption before
+		 * tracing functions. But we still want to trace
+		 * preemption off locations.
+		 */
+		preempt_disable_check(1);
+		/*
 		 * Needs preempt disabled in case user_exit() is traced
 		 * and the tracer calls preempt_enable_notrace() causing
 		 * an infinite recursion.
@@ -3411,6 +3437,7 @@ asmlinkage __visible void __sched notrace preempt_schedule_notrace(void)
 		__schedule(true);
 		exception_exit(prev_ctx);
 
+		preempt_enable_check(1);
 		preempt_enable_no_resched_notrace();
 	} while (need_resched());
 }