linux-kernel - Re: [PATCH v6 2/2] tick/sched: Ensure quiet_vmstat() is called when the idle tick was stopped too

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Date:   Wed, 17 Aug 2022 17:48:44 +0100
From:   Aaron Tomlin <atomlin@...hat.com>
To:     Marcelo Tosatti <mtosatti@...hat.com>
Cc:     frederic@...nel.org, cl@...ux.com, tglx@...utronix.de,
        mingo@...nel.org, peterz@...radead.org, pauld@...hat.com,
        neelx@...hat.com, oleksandr@...alenko.name, atomlin@...mlin.com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v6 2/2] tick/sched: Ensure quiet_vmstat() is called when
 the idle tick was stopped too

On Tue 2022-08-16 15:11 -0300, Marcelo Tosatti wrote:
> Aaron,
> 
> It is also necessary to remove the delayed_work_pending check from
> quiet_vmstat:
> 
> @@ -1987,10 +1970,7 @@ void quiet_vmstat(void)
>         if (system_state != SYSTEM_RUNNING)
>                 return;
>  
> -       if (!delayed_work_pending(this_cpu_ptr(&vmstat_work)))
> -               return;
> -
> -       if (!need_update(smp_processor_id()))
> +       if (!__this_cpu_read(vmstat_dirty))
>                 return;
>  
> 
> Because, consider the following situation:
> 
> 
> vmstat_shepherd CPU				nohz_full CPU
> 
> 						
> 						vmstat_dirty = false
> 						enter nohz_full
> 						return to userspace
> check vmstat_dirty, unset,
> delayed work cancelled.
> 						dirties per-CPU vmstats
> 						vmstat_dirty = true
> 						return to userspace
> 						(quiet_vmstat will skip
> 						 clearing stats because
> 						 delayed work was not queued).

Hi Marcelo,

Yes, I agree - the above scenario can occur.


Kind regards,

-- 
Aaron Tomlin