linux-kernel - Re: [patch V3 8/8] mm: vmstat_refresh: avoid queueing work item if cpu stats are clean

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Date:   Wed, 1 Sep 2021 09:05:55 -0400
From:   Nitesh Lal <nilal@...hat.com>
To:     Marcelo Tosatti <mtosatti@...hat.com>
Cc:     linux-kernel@...r.kernel.org,
        Nicolas Saenz Julienne <nsaenzju@...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 V3 8/8] mm: vmstat_refresh: avoid queueing work item if
 cpu stats are clean

Hi Marcelo,

On Tue, Aug 24, 2021 at 11:42 AM Marcelo Tosatti <mtosatti@...hat.com> wrote:
>
> It is not necessary to queue work item to run refresh_vm_stats
> on a remote CPU if that CPU has no dirty stats and no per-CPU
> allocations for remote nodes.
>
> This fixes sosreport hang (which uses vmstat_refresh) with
> spinning SCHED_FIFO process.
>

I was still able to reproduce the sosreport hang with this patchset and I
am wondering if that is because right now we do vmstat_sync and then cancel
any pending jobs on a CPU in the context of one task.

However, while this task is running another process can come in and can
dirty the stats resulting in vmstat job getting placed on CPUs running
SCHED_FIFO tasks.
Am I missing something?

What we can probably do is to communicate that a CPU is running on task
isolation mode to any other process that is trying to run and schedule
jobs there.

-- 
Thanks
Nitesh