linux-kernel - Re: [PATCH] cgroup/rstat: avoid disabling irqs for O(num

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Message-ID: <454qatlzbtbfsh3vub7qrnropuyux4lxsokxt72fbiy2fpy2pu@dmfi22u5d64k>
Date: Tue, 1 Apr 2025 18:59:31 +0200
From: Michal Koutný <mkoutny@...e.com>
To: Mateusz Guzik <mjguzik@...il.com>
Cc: Yosry Ahmed <yosry.ahmed@...ux.dev>, Greg Thelen <gthelen@...gle.com>, 
	Tejun Heo <tj@...nel.org>, Johannes Weiner <hannes@...xchg.org>, 
	Andrew Morton <akpm@...ux-foundation.org>, Eric Dumazet <edumzaet@...gle.com>, cgroups@...r.kernel.org, 
	linux-mm@...ck.org, linux-kernel@...r.kernel.org, Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH] cgroup/rstat: avoid disabling irqs for O(num_cpu)

On Tue, Apr 01, 2025 at 05:46:41PM +0200, Mateusz Guzik <mjguzik@...il.com> wrote:
> Is this really going to suffer for toggling every 8 CPUs? that's a 50x
> factor reduction

As per the original patch, there's ~10x saving in max holding irqs-off,
naïevely thinking aggregating it flushing by 8 CPUs could reduce it to
(10/8) ~1.25x saving only.
(OTOH, it's not 400x effect, so it's not explained completely, not all
CPUs are same.) I can imagine the balanced value with this information
would be around 20 CPUs (sqrt(400)).
But the issue is it could as well be 4 or 32 or 8. Starting with 1 is
the simplest approach without introducing magic constants or heuristics.

> the temp changes like the to stay for a long time.

That'd mean that no one notices the performance impact there :-)
It can be easily changed later too.

> that said, there is bigger fish to fry elsewhere and I have no stake
> in this code, so I'm not going to mail any further about this.

Thank you for spending your effort on this, it's useful reference for
the future!

Michal

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