linux-kernel - Re: [PATCH v13 3/6] mm/vmstat: manage per-CPU stats from CPU context when NOHZ full

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Date:   Tue, 10 Jan 2023 08:50:28 -0300
From:   Marcelo Tosatti <mtosatti@...hat.com>
To:     Hillf Danton <hdanton@...a.com>
Cc:     atomlin@...mlin.com, frederic@...nel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v13 3/6] mm/vmstat: manage per-CPU stats from CPU context
 when NOHZ full

On Tue, Jan 10, 2023 at 10:43:56AM +0800, Hillf Danton wrote:
> On 9 Jan 2023 11:12:49 -0300 Marcelo Tosatti <mtosatti@...hat.com>
> > 
> > Yes, but if you do not return to userspace, then the per-CPU vm
> > statistics can be dirty indefinitely.
> 
> Could you specify the reasons for failing to return to userspace,
> given it is undesired intereference for the shepherd to queue work
> on the isolated CPUs.

Any system call that takes longer than the threshold to sync vmstats.

Or a long running kernel thread, for example:

https://stackoverflow.com/questions/65111483/long-running-kthread-and-synchronize-net