linux-kernel - Re: [patch 0/5] optionally sync per-CPU vmstats counter on return to userspace

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Message-ID: <20210702123032.GA72061@lothringen>
Date:   Fri, 2 Jul 2021 14:30:32 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Marcelo Tosatti <mtosatti@...hat.com>
Cc:     linux-kernel@...r.kernel.org, Christoph Lameter <cl@...ux.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Juri Lelli <juri.lelli@...hat.com>,
        Nitesh Lal <nilal@...hat.com>
Subject: Re: [patch 0/5] optionally sync per-CPU vmstats counter on return to
 userspace

On Thu, Jul 01, 2021 at 06:03:36PM -0300, Marcelo Tosatti wrote:
> The logic to disable vmstat worker thread, when entering
> nohz full, does not cover all scenarios. For example, it is possible
> for the following to happen:
> 
> 1) enter nohz_full, which calls refresh_cpu_vm_stats, syncing the stats.
> 2) app runs mlock, which increases counters for mlock'ed pages.
> 3) start -RT loop
> 
> Since refresh_cpu_vm_stats from nohz_full logic can happen _before_
> the mlock, vmstat shepherd can restart vmstat worker thread on
> the CPU in question.
>  
> To fix this, optionally sync the vmstat counters when returning
> from userspace, controllable by a new "vmstat_sync" isolcpus
> flags (default off).

Wasn't the plan for such finegrained isolation features to do it at
the per task level using prctl()?

Thanks.

> 
> See individual patches for details.
> 
>