From 47252cfbac03644ee4a3adfa50c77896aa94f2bb Mon Sep 17 00:00:00 2001 From: Steven Rostedt Date: Mon, 21 Mar 2016 11:23:39 -0400 Subject: [PATCH] sched/core: Add preempt checks in preempt_schedule() code While testing the tracer preemptoff, I hit this strange trace: <...>-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 : => 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 Signed-off-by: Peter Zijlstra (Intel) Cc: Linus Torvalds Cc: Mike Galbraith Cc: Peter Zijlstra Cc: Thomas Gleixner Link: http://lkml.kernel.org/r/20160321112339.6dc78ad6@gandalf.local.home Signed-off-by: Ingo Molnar --- kernel/sched/core.c | 68 +++++++++++++++++++++++++++++++++++++++------ 1 file changed, 59 insertions(+), 9 deletions(-) diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 8b489fcac37b..b5cf01d2368e 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -2958,6 +2958,20 @@ u64 scheduler_tick_max_deferment(void) #if defined(CONFIG_PREEMPT) && (defined(CONFIG_DEBUG_PREEMPT) || \ defined(CONFIG_PREEMPT_TRACER)) +/* + * If the value passed in is equal to the current preempt count + * then we just disabled preemption. Start timing the latency. + */ +static inline void preempt_latency_start(int val) +{ + if (preempt_count() == val) { + unsigned long ip = get_lock_parent_ip(); +#ifdef CONFIG_DEBUG_PREEMPT + current->preempt_disable_ip = ip; +#endif + trace_preempt_off(CALLER_ADDR0, ip); + } +} void preempt_count_add(int val) { @@ -2976,17 +2990,21 @@ 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_lock_parent_ip(); -#ifdef CONFIG_DEBUG_PREEMPT - current->preempt_disable_ip = ip; -#endif - trace_preempt_off(CALLER_ADDR0, ip); - } + preempt_latency_start(val); } EXPORT_SYMBOL(preempt_count_add); NOKPROBE_SYMBOL(preempt_count_add); +/* + * If the value passed in equals to the current preempt count + * then we just enabled preemption. Stop timing the latency. + */ +static inline void preempt_latency_stop(int val) +{ + if (preempt_count() == val) + trace_preempt_on(CALLER_ADDR0, get_lock_parent_ip()); +} + void preempt_count_sub(int val) { #ifdef CONFIG_DEBUG_PREEMPT @@ -3003,13 +3021,15 @@ void preempt_count_sub(int val) return; #endif - if (preempt_count() == val) - trace_preempt_on(CALLER_ADDR0, get_lock_parent_ip()); + preempt_latency_stop(val); __preempt_count_sub(val); } EXPORT_SYMBOL(preempt_count_sub); NOKPROBE_SYMBOL(preempt_count_sub); +#else +static inline void preempt_latency_start(int val) { } +static inline void preempt_latency_stop(int val) { } #endif /* @@ -3284,8 +3304,23 @@ void __sched schedule_preempt_disabled(void) static void __sched notrace preempt_schedule_common(void) { do { + /* + * Because the function tracer can trace preempt_count_sub() + * and it also uses preempt_enable/disable_notrace(), if + * NEED_RESCHED is set, the preempt_enable_notrace() called + * by the function tracer will call this function again and + * cause infinite recursion. + * + * Preemption must be disabled here before the function + * tracer can trace. Break up preempt_disable() into two + * calls. One to disable preemption without fear of being + * traced. The other to still record the preemption latency, + * which can also be traced by the function tracer. + */ preempt_disable_notrace(); + preempt_latency_start(1); __schedule(true); + preempt_latency_stop(1); preempt_enable_no_resched_notrace(); /* @@ -3337,7 +3372,21 @@ asmlinkage __visible void __sched notrace preempt_schedule_notrace(void) return; do { + /* + * Because the function tracer can trace preempt_count_sub() + * and it also uses preempt_enable/disable_notrace(), if + * NEED_RESCHED is set, the preempt_enable_notrace() called + * by the function tracer will call this function again and + * cause infinite recursion. + * + * Preemption must be disabled here before the function + * tracer can trace. Break up preempt_disable() into two + * calls. One to disable preemption without fear of being + * traced. The other to still record the preemption latency, + * which can also be traced by the function tracer. + */ preempt_disable_notrace(); + preempt_latency_start(1); /* * Needs preempt disabled in case user_exit() is traced * and the tracer calls preempt_enable_notrace() causing @@ -3347,6 +3396,7 @@ asmlinkage __visible void __sched notrace preempt_schedule_notrace(void) __schedule(true); exception_exit(prev_ctx); + preempt_latency_stop(1); preempt_enable_no_resched_notrace(); } while (need_resched()); }