linux-kernel - Re: [PATCH 1/1] io_uring/sqpoll: inherit cpumask of creating process

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Message-ID: <8633f306-f5e0-42f8-a4c6-f6f34b85844d@kernel.dk>
Date: Fri, 6 Sep 2024 07:47:24 -0600
From: Jens Axboe <axboe@...nel.dk>
To: Felix Moessbauer <felix.moessbauer@...mens.com>, asml.silence@...il.com
Cc: linux-kernel@...r.kernel.org, io-uring@...r.kernel.org,
 cgroups@...r.kernel.org, dqminh@...udflare.com, longman@...hat.com,
 adriaan.schmidt@...mens.com, florian.bezdeka@...mens.com,
 stable@...r.kernel.org
Subject: Re: [PATCH 1/1] io_uring/sqpoll: inherit cpumask of creating process

On 9/6/24 7:44 AM, Felix Moessbauer wrote:
> The submit queue polling threads are "kernel" threads that are started

It's not a kernel thread, it's a normal userland thread that just never
exits to userspace.

> from the userland. In case the userland task is part of a cgroup with
> the cpuset controller enabled, the poller should also stay within that
> cpuset. This also holds, as the poller belongs to the same cgroup as
> the task that started it.
> 
> With the current implementation, a process can "break out" of the
> defined cpuset by creating sq pollers consuming CPU time on other CPUs,
> which is especially problematic for realtime applications.
> 
> Part of this problem was fixed in a5fc1441 by dropping the
> PF_NO_SETAFFINITY flag, but this only becomes effective after the first
> modification of the cpuset (i.e. the pollers cpuset is correct after the
> first update of the enclosing cgroups cpuset).
> 
> By inheriting the cpuset of the creating tasks, we ensure that the
> poller is created with a cpumask that is a subset of the cgroups mask.
> Inheriting the creators cpumask is reasonable, as other userland tasks
> also inherit the mask.

Looks fine to me.

-- 
Jens Axboe