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 15:33:45 -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 02:32:04PM -0300, Marcelo Tosatti wrote:
> 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.

Well, actually: the goal is an isolated CPU.

So task A (latency sensitive, in task isolated mode, polling) executing on
an isolated CPU will require something like stalld to execute this other
task that dirties the vmstats.

Therefore stalld can also sched-in/sched-out vmstat_worker, with similar
impact to latency.