linux-kernel - Re: [RFC PATCH 0/1] sched/fair: Feature to suppress Fair Server for NOHZ

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Message-ID: <yhy5mbricszwesctl26qcc3bqg2onxeusyxau7mzhrxyz22voi@2i4eadieg4ti>
Date: Tue, 13 Jan 2026 14:15:37 -0500
From: Aaron Tomlin <atomlin@...mlin.com>
To: Valentin Schneider <vschneid@...hat.com>
Cc: Daniel Vacek <neelx@...e.com>, Shrikanth Hegde <sshegde@...ux.ibm.com>, 
	sean@...e.io, mproche@...il.com, linux-kernel@...r.kernel.org, 
	mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com, 
	vincent.guittot@...aro.org, dietmar.eggemann@....com, rostedt@...dmis.org, 
	bsegall@...gle.com, mgorman@...e.de
Subject: Re: [RFC PATCH 0/1] sched/fair: Feature to suppress Fair Server for
 NOHZ_FULL isolation

On Fri, Jan 09, 2026 at 10:21:07AM +0100, Valentin Schneider wrote:
> Gotcha. Do you have any specific examples for these per-CPU kthreads? We
> should have features to prevent most of these (e.g. workqueue cpumasks),
> and if not then that's something we could look into.
> 
> As for userspace messing things up... Well, not much we can do here, other
> than preventing that via e.g. cpusets so only your latency-sensitive tasks
> are allowed to be migrated on the isolated CPUs.

Hi Valentin,

To your point regarding specific examples of these per-CPU kthreads, I do
not have any illustrative cases to hand at the moment. However, I shall
attempt to reproduce the scenario to identify which specific threads are
eluding our current isolation boundaries.

I certainly concur with your final observation regarding userspace
interference; there is, indeed, little to be done beyond enforcing strict
partitioning via cpusets to ensure only latency-sensitive tasks are
permitted to migrate to isolated CPUs.

That being said, the suggestion made by Peter, namely, to prevent the
enqueue on the isolated CPU - is a particularly compelling one.

Please see [1].

[1]: https://lore.kernel.org/lkml/zmjr43kk2m52huk2vvetvwefil7waletzuijiu5y34v3n4slgi@3wdtd3xckx7m/

Kind regards,
-- 
Aaron Tomlin

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