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

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Message-ID: <20210901173204.GA48995@fuller.cnet>
Date:   Wed, 1 Sep 2021 14:32:04 -0300
From:   Marcelo Tosatti <mtosatti@...hat.com>
To:     Nitesh Lal <nilal@...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

On Wed, Sep 01, 2021 at 09:05:55AM -0400, Nitesh Lal wrote:
> 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.

Hi Nitesh,

Did you use chisol (with proper flags) and the modified oslat?

Tested with "echo 1 > /proc/sys/vmstat_refresh" and it was successful
(no hangs).

> 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.

No, that can happen. Can use sched notifiers to handle this problem.
Good point.