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

[Boqun Feng]

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@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());
>  }

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