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

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Message-ID: <xhsmhldi7nnuk.mognet@vschneid-thinkpadt14sgen2i.remote.csb>
Date: Fri, 09 Jan 2026 10:21:07 +0100
From: Valentin Schneider <vschneid@...hat.com>
To: Aaron Tomlin <atomlin@...mlin.com>, Daniel Vacek <neelx@...e.com>
Cc: 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 07/01/26 15:25, Aaron Tomlin wrote:
> On Tue, Jan 06, 2026 at 05:38:17PM +0100, Daniel Vacek wrote:
>> On Tue, 6 Jan 2026 at 16:38, Valentin Schneider <vschneid@...hat.com> wrote:
>> >
>> > Right, that's the part I don't get; why not use CPU isolation / cpusets to
>> > isolate the CPUs running those NOHZ_FULL applications? Regardless of the
>> > deadline server, if CFS tasks get scheduled on the same CPU as your
>> > latency-sensitive tasks then something's not right.
>>
>> Some kernel workers and threaded interrupt handlers can be local/pinned, right?
>>
>> For example this is usually (was often?) visible with DPDK
>> applications like FlexRAN/OpenRAN, etc.
>> And Aaron has mentioned high speed trading before.
>
> Hi Valentin, Daniel,
>
> I must offer my apologies for the confusion; I neglected to mention in the
> cover letter that isolcpus=domain is indeed deployed in this environment.
>
> Consequently, standard load-balancing is effectively disabled. You are
> quite right that standard CFS tasks should not appear on these cores; any
> SCHED_NORMAL entities that do appear are not the result of leakage or
> misconfiguration, but are rather unavoidable CPU-specific kthreads or
> explicit migrations initiated by user-space.
>

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.

>
> Kind regards,
> --
> Aaron Tomlin