linux-kernel - Re: [PATCH v2 1/2] sched/isolation: Exclude dynamically isolated CPUs from housekeeping masks

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Message-ID: <qicmttz6sqccty6jha7s22wi6bc2agps44qrqwhm4hhorcluyp@nl734io7qnl5>
Date: Fri, 15 Nov 2024 16:45:14 +0100
From: Michal Koutný <mkoutny@...e.com>
To: Waiman Long <longman@...hat.com>
Cc: 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>, 
	Valentin Schneider <vschneid@...hat.com>, Frederic Weisbecker <frederic@...nel.org>, 
	linux-kernel@...r.kernel.org, cgroups@...r.kernel.org, Costa Shulyupin <cshulyup@...hat.com>, 
	Daniel Wagner <dwagner@...e.de>
Subject: Re: [PATCH v2 1/2] sched/isolation: Exclude dynamically isolated
 CPUs from housekeeping masks

Hello.

I recently liked the idea of considering isolated CPUs a static (boot
time) resource and only use cpusets to place (or remove) sensitive
workload from those selected CPUs depending on current needs. (Yes, this
may not efficiently utilize the isolated CPUs when reserve them based on
maximum needs of a node.)

On Wed, Aug 21, 2024 at 10:23:11AM GMT, Waiman Long <longman@...hat.com> wrote:
> 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.

But when I look at it with the dynamism in mind, I would expect that
some API like housekeeping_setup_type(), i.e. modify the set of isolated
CPUs are requested and leave it up to the isolation implementation to
propagate any changes to respective subsystems. And return an error of
type contains a flag for which dynamism isn't implemented yet or not
possible.

The boot time value would only be the initial default, but the value
would be mutated by this API. There's IMO no need to revert to that.

(Also someone mentioned that this could share lots of code with CPU
offlining/onlining.)

> A new housekeeping_exlude_isolcpus() function is added which is to be
                     exclude

> called by the cpuset subsystem to provide a list of isolated CPUs to
> be excluded.

HTH,
Michal

(-Cc: lizefan.x@...edance.com)

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