linux-kernel - Re: [PATCH 08/17] sched/core: Implement CPU soft offline/online

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Message-ID: <aTMrNXJmkzdW3CB5@linux.ibm.com>
Date: Sat, 6 Dec 2025 00:27:57 +0530
From: Srikar Dronamraju <srikar@...ux.ibm.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        Ben Segall <bsegall@...gle.com>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Ingo Molnar <mingo@...nel.org>, Juri Lelli <juri.lelli@...hat.com>,
        K Prateek Nayak <kprateek.nayak@....com>,
        Madhavan Srinivasan <maddy@...ux.ibm.com>,
        Mel Gorman <mgorman@...e.de>, Michael Ellerman <mpe@...erman.id.au>,
        Nicholas Piggin <npiggin@...il.com>,
        Shrikanth Hegde <sshegde@...ux.ibm.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Swapnil Sapkal <swapnil.sapkal@....com>,
        Thomas Huth <thuth@...hat.com>,
        Valentin Schneider <vschneid@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        virtualization@...ts.linux.dev, Yicong Yang <yangyicong@...ilicon.com>,
        Ilya Leoshkevich <iii@...ux.ibm.com>
Subject: Re: [PATCH 08/17] sched/core: Implement CPU soft offline/online

* Peter Zijlstra <peterz@...radead.org> [2025-12-05 17:07:23]:

> On Thu, Dec 04, 2025 at 11:23:56PM +0530, Srikar Dronamraju wrote:
> > Scheduler already supports CPU online/offline. However for cases where
> > scheduler has to offline a CPU temporarily, the online/offline cost is
> > too high. Hence here is an attempt to come-up with soft-offline that
> > almost looks similar to offline without actually having to do the
> > full-offline. Since CPUs are not to be used temporarily for a short
> > duration, they will continue to be part of the CPU topology.
> > 
> > In the soft-offline, CPU will be marked as inactive, i.e removed from
> > the cpu_active_mask, CPUs capacity would be reduced and non-pinned tasks
> > would be migrated out of the CPU's runqueue.
> > 
> > Similarly when onlined, CPU will be remarked as active, i.e. added to
> > cpu_active_mask, CPUs capacity would be restored.
> > 
> > Soft-offline is almost similar as 1st step of offline except rebuilding
> > the sched-domains. Since the other steps are not done including
> > rebuilding the sched-domain, the overhead of soft-offline would be less
> > compared to regular offline. A new cpumask is used to indicate
> > soft-offline is in progress and hence skips rebuilding the
> > sched-domains.
> 
> Note that your thing still very much includes the synchronize_rcu() that
> a lot of the previous 'hotplug is too slow' crowd have complained about.
> 
> So I'm taking it that your steal time thing really isn't that 'fast'.

Yes, it does have synchronize_rcu()
> 
> It might be good to mention the frequency at which you expect cores to
> come and go with your setup.

We are expecting the cores to keep changing at a 1 second to 2second
frequency.

-- 
Thanks and Regards
Srikar Dronamraju