linux-kernel - Re: [PATCH v8 5/5] tick/sched: Ensure quiet_vmstat() is called when the idle tick was stopped too

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Message-ID: <20221012124101.zod6m2brlzpgx6nb@ava.usersys.com>
Date:   Wed, 12 Oct 2022 13:41:01 +0100
From:   Aaron Tomlin <atomlin@...hat.com>
To:     Hillf Danton <hdanton@...a.com>
Cc:     frederic@...nel.org, mtosatti@...hat.com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        atomlin@...mlin.com
Subject: Re: [PATCH v8 5/5] tick/sched: Ensure quiet_vmstat() is called when
 the idle tick was stopped too

> Given work canceled because per-CPU pages can be freed remotely from
> housekeeping CPUs (see patch 3/5), what is added here is not needed.

Hi Hillf,

Firstly, apologies for the delay!

The concern is to ensure CPU-specific vmstat counters are reset and folded
into NUMA node and zone specific and global counters too before entering
idle. It is necessary to invoke quiet_vmstat() on return to idle even if
the scheduling-clock tick has been previously stopped.
Please refer to the complete scenario I described again.

If I understand correctly, indeed the remote drain/or free of zone
CPU-specific pages can be initiated by a "housekeeping" CPU i.e.
refresh_cpu_vm_stats(true) via a worker thread/or kworker, yet the actual
free will only occur when the nohz_full CPU exits idle code and calls
schedule_idle().

Kind regards,

-- 
Aaron Tomlin