linux-kernel - Re: [PATCH 1/2] sched/isolation: Exclude dynamically isolated CPUs from housekeeping masks

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Date: Thu, 28 Mar 2024 20:56:15 -0400
From: Waiman Long <longman@...hat.com>
To: Frederic Weisbecker <frederic@...nel.org>
Cc: Zefan Li <lizefan.x@...edance.com>, Tejun Heo <tj@...nel.org>,
 Johannes Weiner <hannes@...xchg.org>, Ingo Molnar <mingo@...hat.com>,
 Peter Zijlstra <peterz@...radead.org>, Juri Lelli <juri.lelli@...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>,
 linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
 Cestmir Kalina <ckalina@...hat.com>, Costa Shulyupin <cshulyup@...hat.com>
Subject: Re: [PATCH 1/2] sched/isolation: Exclude dynamically isolated CPUs
 from housekeeping masks


On 3/25/24 19:21, Frederic Weisbecker wrote:
> Le Wed, Feb 28, 2024 at 09:14:13PM -0500, Waiman Long a écrit :
>> The housekeeping CPU masks, set up by the "isolcpus" and "nohz_full"
>> boot command line options, are used at boot time to exclude selected CPUs
>> from running some kernel background processes to minimize disturbance
>> to latency sensitive userspace applications. Some of housekeeping CPU
>> masks are also checked at run time to avoid using those isolated CPUs.
>>
>> The cpuset subsystem is now able to dynamically create a set of isolated
>> CPUs to be used in isolated cpuset partitions. The long term goal is
>> to make the degree of isolation as close as possible to what can be
>> done statically using those boot command line options.
>>
>> This patch is a step in that direction by making the housekeeping CPU
>> mask APIs exclude the dynamically isolated CPUs when they are called
>> at run time. The housekeeping CPU masks will fall back to the bootup
>> default when all the dynamically isolated CPUs are released.
>>
>> A new housekeeping_exlude_isolcpus() function is added which is to be
>> called by the cpuset subsystem to provide a list of isolated CPUs to
>> be excluded.
> Can we instead overwrite housekeeping_boot.cpumasks from cpusets entirely and
> forget about the original boot value forever?

That is an interesting question.

I think we will still need to keep the boot values somewhere, at least 
until we can dynamically isolate CPUs as good as what can be done at 
boot time. At this point, what can be done dynamically is still limited. 
So we want to keep the statically isolated CPUs around without breaking 
its isolation. Of course, we can do all these within cpuset with minimal 
change in kernel/sched/isolation.c.

Cheers,
Longman