linux-kernel - Re: [PATCH] timers: Exclude isolated cpus from timer migation

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Message-ID: <Z_fcA2uxfp52eez-@localhost.localdomain>
Date: Thu, 10 Apr 2025 16:56:03 +0200
From: Frederic Weisbecker <frederic@...nel.org>
To: Waiman Long <llong@...hat.com>
Cc: Gabriele Monaco <gmonaco@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] timers: Exclude isolated cpus from timer migation

Le Thu, Apr 10, 2025 at 10:50:39AM -0400, Waiman Long a écrit :
> 
> On 4/10/25 10:43 AM, Frederic Weisbecker wrote:
> > Le Thu, Apr 10, 2025 at 10:35:55AM -0400, Waiman Long a écrit :
> > > On 4/10/25 9:03 AM, Frederic Weisbecker wrote:
> > > > Le Thu, Apr 10, 2025 at 12:38:25PM +0200, Gabriele Monaco a écrit :
> > > > > On Thu, 2025-04-10 at 10:26 +0200, Thomas Gleixner wrote:
> > > > > > How can that happen? There is always at least _ONE_ housekeeping,
> > > > > > non-isolated, CPU online, no?
> > > > > > 
> > > > > In my understanding it shouldn't, but I'm not sure there's anything
> > > > > preventing the user from isolating everything via cpuset.
> > > > > Anyway that's something no one in their mind should do, so I guess I'd
> > > > > just opt for the cpumask_first (or actually cpumask_any, like before
> > > > > the change).
> > > > With "nohz_full=..." or "isolcpus=nohz,..." there is always at least one
> > > > housekeeping CPU. But with isolcpus=[domain] or cpusets equivalents
> > > > (v1 cpuset.sched_load_balance, v2 isolated partion) there is nothing that
> > > > prevents all CPUs from being isolated.
> > > Actually v2 won't allow users to isolate all the CPUs. Users can probably do
> > > that with v1's cpuset.sched_load_balance.
> > Perhaps, and I think isolcpus= can too.
> 
> No, I don't think so. The following code is in kernel/sched/isolation.c:
> 
> first_cpu = cpumask_first_and(cpu_present_mask, housekeeping_staging); if
> (first_cpu >= nr_cpu_ids || first_cpu >= setup_max_cpus) {
> __cpumask_set_cpu(smp_processor_id(), housekeeping_staging);
> __cpumask_clear_cpu(smp_processor_id(), non_housekeeping_mask); if
> (!housekeeping.flags) { pr_warn("Housekeeping: must include one present CPU,
> " "using boot CPU:%d\n", smp_processor_id()); } }

Ok, good then!

-- 
Frederic Weisbecker
SUSE Labs