Re: [PATCH v3 0/2] sched/tracing: sched_switch prev_state reported as TASK_RUNNING when it's not

[Steven Rostedt <rostedt@goodmis.org>]

On Thu, 20 Jan 2022 16:25:18 +0000
Valentin Schneider <valentin.schneider@arm.com> wrote:

> Hi folks,
> 
> Problem
> =======
> 
> Abhijeet pointed out that the following sequence of trace events can be
> observed:
> 
>   cat-1676  [001] d..3 103.010411: sched_waking: comm=grep pid=1677 prio=120 target_cpu=004
>   grep-1677 [004] d..2 103.010440: sched_switch: prev_comm=grep prev_pid=1677 prev_prio=120 prev_state=R 0x0 ==> next_comm=swapper/4 next_pid=0 next_prio=120
>   <idle>-0  [004] dNh3 103.0100459: sched_wakeup: comm=grep pid=1677 prio=120 target_cpu=004
> 
> IOW, not-yet-woken task gets presented as runnable and switched out in
> favor of the idle task... Dietmar and I had a look, turns out this sequence
> can happen: 
> 
> 		      p->__state = TASK_INTERRUPTIBLE;
> 		      __schedule()
> 			deactivate_task(p);
>   ttwu()
>     READ !p->on_rq
>     p->__state=TASK_WAKING
> 			trace_sched_switch()
> 			  __trace_sched_switch_state()
> 			    task_state_index()
> 			      return 0;
> 
> TASK_WAKING isn't in TASK_REPORT, hence why task_state_index(p) returns 0.
> This can happen as of commit c6e7bd7afaeb ("sched/core: Optimize ttwu()
> spinning on p->on_cpu") which punted the TASK_WAKING write to the other
> side of
> 
>   smp_cond_load_acquire(&p->on_cpu, !VAL);
> 
> in ttwu().
> 
> Uwe reported on #linux-rt what I think is a similar issue with
> TASK_RTLOCK_WAIT on PREEMPT_RT; again that state isn't in TASK_REPORT so
> you get prev_state=0 despite the task blocking on a lock.
> 
> Both of those are very confusing for tooling or even human observers.



This all looks fine to me:

Reviewed-by: Steven Rostedt (Google) <rostedt@goodmis.org>

Peter, want to take this through your tree?

-- Steve