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

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Message-ID: <xhsmhzf6qoip7.mognet@vschneid-thinkpadt14sgen2i.remote.csb>
Date: Tue, 06 Jan 2026 16:37:56 +0100
From: Valentin Schneider <vschneid@...hat.com>
To: Shrikanth Hegde <sshegde@...ux.ibm.com>, Aaron Tomlin <atomlin@...mlin.com>
Cc: neelx@...e.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 06/01/26 14:37, Shrikanth Hegde wrote:
> On 1/6/26 9:12 AM, Aaron Tomlin wrote:
>> Hi Ingo, Peter, Juri, Vincent,
>>
>> This patch introduces a new scheduler feature, RT_SUPPRESS_FAIR_SERVER,
>> designed to ensure strict NOHZ_FULL isolation for SCHED_FIFO workloads,
>> particularly in the presence of resident CFS tasks.
>>
>> In strictly partitioned, latency-critical environments (such as High
>> Frequency Trading platforms) administrators frequently employ fully
>> adaptive-tick CPUs to execute pinned SCHED_FIFO workloads. The fundamental
>> requirement is "zero OS noise"; specifically, the scheduler clock-tick must
>> remain suppressed ("offloaded"), given that standard SCHED_FIFO semantics
>> dictate no forced preemption between tasks of identical priority.
>
> If all your SCHED_FIFO is pinned and their scheduling decisions
> are managed in userspace, using isolcpus would offer you better
> isolations compared to nohz_full.
>

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.