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

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Message-ID: <2b8a231a7eea592797bbbd853d85ef443d1e58d4.camel@redhat.com>
Date: Thu, 10 Apr 2025 16:49:09 +0200
From: Gabriele Monaco <gmonaco@...hat.com>
To: Frederic Weisbecker <frederic@...nel.org>, Waiman Long <llong@...hat.com>
Cc: Thomas Gleixner <tglx@...utronix.de>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] timers: Exclude isolated cpus from timer migation



On Thu, 2025-04-10 at 16:43 +0200, 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.
> 
  # vng -a isolcpus=0-15 cat /sys/devices/system/cpu/isolated
  1-15

Seems not..