Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Wed, 17 Aug 2022 17:48:44 +0100
From:   Aaron Tomlin <atomlin@redhat.com>
To:     Marcelo Tosatti <mtosatti@redhat.com>
Cc:     frederic@kernel.org, cl@linux.com, tglx@linutronix.de,
        mingo@kernel.org, peterz@infradead.org, pauld@redhat.com,
        neelx@redhat.com, oleksandr@natalenko.name, atomlin@atomlin.com,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH v6 2/2] tick/sched: Ensure quiet_vmstat() is called when
 the idle tick was stopped too
Message-ID: <20220817164844.zyypokjmzjfznuhu@ava.usersys.com>
References: <20220808194820.676246-1-atomlin@redhat.com>
 <20220808194820.676246-3-atomlin@redhat.com>
 <YvvdtwrIX+TUgn3A@fuller.cnet>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Disposition: inline
In-Reply-To: <YvvdtwrIX+TUgn3A@fuller.cnet>
Precedence: bulk

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