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Subject: Re: [PATCH] lib/cpumask: Boot option to disable tasks distribution
 within cpumask
From: Ankit Jain <ankit-aj.jain@broadcom.com>
In-Reply-To: <F4B1A5C9-FD5E-42EF-9DA7-4EB394D10408@broadcom.com>
Date: Thu, 2 May 2024 15:09:34 +0530
Cc: linux@rasmusvillemoes.dk,
 akpm@linux-foundation.org,
 LKML <linux-kernel@vger.kernel.org>,
 Juri Lelli <juri.lelli@redhat.com>,
 ajay.kaher@broadcom.com,
 alexey.makhalov@broadcom.com,
 vasavi.sirnapalli@broadcom.com,
 Paul Turner <pjt@google.com>,
 Ingo Molnar <mingo@redhat.com>,
 Vincent Guittot <vincent.guittot@linaro.org>,
 Dietmar Eggemann <dietmar.eggemann@arm.com>,
 Steven Rostedt <rostedt@goodmis.org>,
 Ben Segall <bsegall@google.com>,
 Mel Gorman <mgorman@suse.de>,
 Daniel Bristot de Oliveira <bristot@redhat.com>,
 Valentin Schneider <vschneid@redhat.com>,
 Phil Auld <pauld@redhat.com>,
 yury.norov@gmail.com
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References: <20240430090431.1619622-1-ankit-aj.jain@broadcom.com>
 <ZjE3C9UgeZR02Jyy@yury-ThinkPad>
 <20240501133608.GB39737@lorien.usersys.redhat.com>
 <ZjJfftWMbjT9r8iT@yury-ThinkPad>
 <20240501153911.GD39737@lorien.usersys.redhat.com>
 <F4B1A5C9-FD5E-42EF-9DA7-4EB394D10408@broadcom.com>
To: Peter Zijlstra <peterz@infradead.org>


> On 02-May-2024, at 1:39 PM, Ankit Jain <ankit-aj.jain@broadcom.com> =
wrote:
>=20
> 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.
>>=20
>> So let's add the authors in CC list?
>>=20
>>> 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).
>>=20
>> OK... It looks like adding scheduler maintainers is also a necessity =
to
>> cater here...
>=20
> So 14e292f8d453 is very specifically only using sched_domain_span(), =
and
> if you're using partitioned CPUs they should not show up there.
>=20
> 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.
>=20
> 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.
>=20
> I even explained what the problem was and where to look for solutions.
>=20
> =
https://www.google.com/url?q=3Dhttps://lkml.kernel.org/r/20231011135238.GG=
6337@noisy.programming.kicks-ass.net&source=3Dgmail-imap&ust=3D17152442350=
00000&usg=3DAOvVaw03JVPjd7QleOP3JrOjP8pO


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=