linux-kernel - Re: [RFC PATCH v1] blk-mq: isolate CPUs from hctx

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Message-ID: <a840176f-2a46-4a2f-b48f-9eab40e542f9@redhat.com>
Date: Fri, 15 Nov 2024 15:16:33 -0500
From: Waiman Long <llong@...hat.com>
To: Michal Koutný <mkoutny@...e.com>,
 Costa Shulyupin <costa.shul@...hat.com>
Cc: ming.lei@...hat.com, Jens Axboe <axboe@...nel.dk>,
 Tejun Heo <tj@...nel.org>, Johannes Weiner <hannes@...xchg.org>,
 linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
 cgroups@...r.kernel.org, Daniel Wagner <dwagner@...e.de>
Subject: Re: [RFC PATCH v1] blk-mq: isolate CPUs from hctx


On 11/15/24 10:45 AM, Michal Koutný wrote:
> Hello.
>
> On Fri, Nov 08, 2024 at 07:48:30AM GMT, Costa Shulyupin <costa.shul@...hat.com> wrote:
>> Cgroups allow configuring isolated_cpus at runtime.
>> However, blk-mq may still use managed interrupts on the
>> newly isolated CPUs.
>>
>> Rebuild hctx->cpumask considering isolated CPUs to avoid
>> managed interrupts on those CPUs and reclaim non-isolated ones.
>>
>> The patch is based on
>> isolation: Exclude dynamically isolated CPUs from housekeeping masks:
>> https://lore.kernel.org/lkml/20240821142312.236970-1-longman@redhat.com/
> Even based on that this seems incomplete to me the CPUs that are part of
> isolcpus mask on boot time won't be excluded from this?
> IOW, isolating CPUs from blk_mq_hw_ctx would only be possible via cpuset
> but not "statically" throught the cmdline option, or would it?

The cpuset had already been changed to take note of the statically 
isolated CPUs and included them in its consideration. They are recorded 
in the boot_hk_cpus mask. It relies on the fact that most users will set 
both nohz_full and isolcpus boot parameters. If only nohz_full is set 
without isolcpus, it will not be recorded.

Cheers,
Longman