linux-kernel - Re: [patch 2/4] task isolation: sync vmstats on return to userspace

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Message-ID: <62a66a5744b28dfea6ff2aec4e02ca0978914819.camel@redhat.com>
Date:   Tue, 03 Aug 2021 17:13:03 +0200
From:   nsaenzju@...hat.com
To:     Marcelo Tosatti <mtosatti@...hat.com>, linux-kernel@...r.kernel.org
Cc:     Nitesh Lal <nilal@...hat.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Christoph Lameter <cl@...ux.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Alex Belits <abelits@...its.com>, Peter Xu <peterx@...hat.com>
Subject: Re: [patch 2/4] task isolation: sync vmstats on return to userspace

On Fri, 2021-07-30 at 17:18 -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, use the task isolation prctl interface to quiesce 
> deferred actions when returning to userspace.

Even though this is mostly targeted at nohz_full users, I believe I haven't
seen anything in this series that forces the feature to be run on nohz_full
CPUs (this is a good thing IMO). So, I'd suggest to reword the patch
description so it doesn't imply nohz_full is necessary to use this.

-- 
Nicolás Sáenz