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

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Date:   Fri, 2 Jul 2021 10:00:11 +0200 (CEST)
From:   Christoph Lameter <cl@...two.de>
To:     Marcelo Tosatti <mtosatti@...hat.com>
cc:     linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Frederic Weisbecker <frederic@...nel.org>,
        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, 1 Jul 2021, 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.

Can we enter nohz_full after the app runs mlock?

> To fix this, optionally sync the vmstat counters when returning
> from userspace, controllable by a new "vmstat_sync" isolcpus
> flags (default off).
>
> See individual patches for details.

Wow... This is going into some performance sensitive VM counters here and
adds code to their primitives.

Isnt there a simpler solution that does not require this amount of
changes?