linux-kernel - Re: [PATCH 02/27] sched/isolation: Introduce housekeeping per-cpu rwsem

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Message-ID: <878ql9o06j.ffs@tglx>
Date: Mon, 30 Jun 2025 14:59:00 +0200
From: Thomas Gleixner <tglx@...utronix.de>
To: Phil Auld <pauld@...hat.com>, Waiman Long <llong@...hat.com>
Cc: Frederic Weisbecker <frederic@...nel.org>, LKML
 <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...hat.com>, Marco
 Crivellari <marco.crivellari@...e.com>, Michal Hocko <mhocko@...e.com>,
 Peter Zijlstra <peterz@...radead.org>, Tejun Heo <tj@...nel.org>,
 Vlastimil Babka <vbabka@...e.cz>
Subject: Re: [PATCH 02/27] sched/isolation: Introduce housekeeping per-cpu
 rwsem

On Thu, Jun 26 2025 at 20:48, Phil Auld wrote:
> On Thu, Jun 26, 2025 at 08:11:54PM -0400 Waiman Long wrote:
>> > My understanding is that it's the stop machine issue. If you have a way
>> > around that then great!
>> 
>> My current thinking is to just run a selected set of CPUHP teardown and
>> startup methods relevant to housekeeping cpumasks usage without calling the
>> full set from CPUHP_ONLINE to CPUHP_OFFLINE. I don't know if it is possible
>> or not or how much additional changes will be needed to make that possible.
>> That will skip the CPUHP_TEARDOWN_CPU teardown method that is likely the
>> cause of most the latency spike experienced by other CPUs.
>>
>
> Yes, CPUHP_TEARDOWN_CPU is the source of the stop_machine I believe.

Correct.

> It'll be interesting to see if you can safely use the cpuhp machinery
> selectively like that :)

It is supposed to work that way and you can exercise it from userspace
via sysfs already. If it fails, then there are bugs in hotplug callbacks
or ordering or ..., which need to be fixed anyway :)

Thanks,

        tglx