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Date: Sat, 19 Mar 2016 12:10:30 +0800
From: Boqun Feng <boqun.feng@...il.com>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: Peter Zijlstra <peterz@...radead.org>,
LKML <linux-kernel@...r.kernel.org>,
Ingo Molnar <mingo@...nel.org>,
Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] sched: Add preempt checks in preempt_schedule() code
Hi Steven,
On Fri, Mar 18, 2016 at 01:06:02PM -0400, Steven Rostedt wrote:
> 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()
Just out of curiosity, could this be solved by adding a barrier() here?
> function trace (before incrementing preempt count)
> preempt_disable_notrace()
> preempt_enable_notrace()
> sees NEED_RESCHED set
> preempt_schedule() (repeat)
>
I'm asking this because it seems to me if PREEMPT_COUNT=y and
PREEMPT_TRACER=y, then
preempt_disable_notrace();
preempt_disable_check(1);
is actually
__preempt_count_inc();
barrier();
preempt_disable_check(1);
whereas
preempt_disable()
is actually
__preempt_count_inc();
preempt_disable_check(1);
barrier();
so I think adding barrier() (or even moving the barrier() up) in
preempt_disable() could solve the same problem.
If so, seems we don't need to introduce preempt_disable_check() and
preempt_enable_check(), even better, we can use preempt_disable() and
preempt_enable_no_resched() in preempt_schedule().
Or Am I missing something subtle here?
Regards,
Boqun
> 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@...dmis.org>
> ---
> 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());
> }
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