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

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Date:   Tue, 3 Aug 2021 13:44:02 -0300
From:   Marcelo Tosatti <mtosatti@...hat.com>
To:     nsaenzju@...hat.com
Cc:     linux-kernel@...r.kernel.org, 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 Tue, Aug 03, 2021 at 05:13:03PM +0200, nsaenzju@...hat.com wrote:
> 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). 

I don't think there is such a dependency either.

> So, I'd suggest to reword the patch
> description so it doesn't imply nohz_full is necessary to use this.

Its describing a fact from nohz_full where it can't guarantee entering
userspace with vmstat turned off (which is a reply to Christopher's
earlier comment that "this should just work with nohz_full and
logic to shutdown the vmstat delayed work timer").

Will add a comment to make it explicit that the series does not depend
on nohz_full.

Thanks.