linux-kernel - Re: [RFC PATCH v3] tick/sched: Ensure quiet_vmstat() is called when the idle tick was stopped too

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Message-ID: <alpine.DEB.2.22.394.2204251603570.25814@gentwo.de>
Date:   Mon, 25 Apr 2022 16:06:04 +0200 (CEST)
From:   Christoph Lameter <cl@...two.de>
To:     Peter Zijlstra <peterz@...radead.org>
cc:     Aaron Tomlin <atomlin@...hat.com>, frederic@...nel.org,
        mtosatti@...hat.com, tglx@...utronix.de, mingo@...nel.org,
        pauld@...hat.com, neelx@...hat.com, oleksandr@...alenko.name,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [RFC PATCH v3] tick/sched: Ensure quiet_vmstat() is called when
 the idle tick was stopped too

On Mon, 25 Apr 2022, Peter Zijlstra wrote:

> > Folding the vmstat diffs *always* when entering idle prevents unnecessary
> > wakeups and processing in the future and also provides more accurate
> > counters for the VM allowing better decision to be made on reclaim.
>
> I'm thinking you're going to find a ton of regressions if you try it
> though; some workloads go idle *very* shortly, doing all this accounting
> is going to be counter-productive.

Well there is usually not much to do in terms of accounting. If there are
a lot of updates then it is worthwhile because if the numbers are off too
much then the VM has trouble assessing its own situation.

It may depend though on how long the idle periods are. Do we have
statistics on the duration? Always folding the vmstat deltas may also
increase the length of the idle periods.