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

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Message-ID: <f5241a1d-9753-4c95-b633-067ecda50c10@redhat.com>
Date: Fri, 15 Nov 2024 14:32:32 -0500
From: Waiman Long <llong@...hat.com>
To: Michal Koutný <mkoutny@...e.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

On 11/15/24 10:45 AM, Michal Koutný wrote:
> 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.)

Thanks for taking a look at this.

Yes, I am moving in this direction too. Boot time statically isolated 
CPUs have better isolation than is currently possible if we do it 
dynamically at run time, though we are trying to close the gap.

>
>
> 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.

There are currently 9 different hk_type's defined in 
include/linux/sched/isolation.h. We are now trying to reduce their 
number as some of them cannot be set independently. See [1]. I am 
thinking about doing dynamism in the best effort basis. Of course, we 
could expose some information about what aspect of dynamic isolation can 
be enabled at the moment, if necessary.

Cheers,
Longman