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

[Boqun Feng]

On Sat, Mar 19, 2016 at 12:10:30PM +0800, Boqun Feng wrote:
> 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?
> 

Oops, seems I have the question because I really don't understand how
function tracing works. I now understand the problem here, Please ignore
this, sorry for the noise.

Regards,
Boqun

> 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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