linux-kernel - Re: [PATCH] lib/cpumask: Boot option to disable tasks distribution within cpumask

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Date: Thu, 2 May 2024 15:09:34 +0530
From: Ankit Jain <ankit-aj.jain@...adcom.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: linux@...musvillemoes.dk,
 akpm@...ux-foundation.org,
 LKML <linux-kernel@...r.kernel.org>,
 Juri Lelli <juri.lelli@...hat.com>,
 ajay.kaher@...adcom.com,
 alexey.makhalov@...adcom.com,
 vasavi.sirnapalli@...adcom.com,
 Paul Turner <pjt@...gle.com>,
 Ingo Molnar <mingo@...hat.com>,
 Vincent Guittot <vincent.guittot@...aro.org>,
 Dietmar Eggemann <dietmar.eggemann@....com>,
 Steven Rostedt <rostedt@...dmis.org>,
 Ben Segall <bsegall@...gle.com>,
 Mel Gorman <mgorman@...e.de>,
 Daniel Bristot de Oliveira <bristot@...hat.com>,
 Valentin Schneider <vschneid@...hat.com>,
 Phil Auld <pauld@...hat.com>,
 yury.norov@...il.com
Subject: Re: [PATCH] lib/cpumask: Boot option to disable tasks distribution
 within cpumask



> On 02-May-2024, at 1:39 PM, Ankit Jain <ankit-aj.jain@...adcom.com> wrote:
> 
> On Tue, Apr 30, 2024 at 11:23:07AM -0700, Yury Norov wrote:
>> On Tue, Apr 30, 2024 at 02:34:31PM +0530, Ankit Jain wrote:
>>> commit 46a87b3851f0 ("sched/core: Distribute tasks within affinity masks")
>>> and commit 14e292f8d453 ("sched,rt: Use cpumask_any*_distribute()")
>>> introduced the logic to distribute the tasks within cpumask upon initial
>>> wakeup.
>> 
>> So let's add the authors in CC list?
>> 
>>> For Telco RAN deployments, isolcpus are a necessity to cater to
>>> the requirement of low latency applications. These isolcpus are generally
>>> tickless so that high priority SCHED_FIFO tasks can execute without any
>>> OS jitter. Since load balancing is disabled on isocpus, any task
>>> which gets placed on these CPUs can not be migrated on its own.
>>> For RT applications to execute on isolcpus, a guaranteed kubernetes pod
>>> with all isolcpus becomes the requirement and these RT applications are
>>> affine to execute on a specific isolcpu within the kubernetes pod.
>>> However, there may be some non-RT tasks which could also schedule in the
>>> same kubernetes pod without being affine to any specific CPU(inherits the
>>> pod cpuset affinity).
>> 
>> OK... It looks like adding scheduler maintainers is also a necessity to
>> cater here...
> 
> So 14e292f8d453 is very specifically only using sched_domain_span(), and
> if you're using partitioned CPUs they should not show up there.
> 
> As to 46a87b3851f0, if you're explicitly creating tasks with an affinity
> masks that spans your partition then you're getting what you ask for.
> 
> In fact, I already explained this to you earlier, so why are you
> suggesting horrible hacks again? This behaviour toggle you suggest is
> absolutely unacceptable.
> 
> I even explained what the problem was and where to look for solutions.
> 
> https://www.google.com/url?q=https://lkml.kernel.org/r/20231011135238.GG6337@noisy.programming.kicks-ass.net&source=gmail-imap&ust=1715244235000000&usg=AOvVaw03JVPjd7QleOP3JrOjP8pO


Hi Peter,

Apologies for not trying out the solution you suggested earlier. I will test this
solution for our use case and share the result.

Thanks,
Ankit