linux-kernel - Re: [PATCH v4 2/5] timers: Add the available mask in timer migration

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Message-ID: <aBtt3C4dFIxNgO0q@localhost.localdomain>
Date: Wed, 7 May 2025 16:27:40 +0200
From: Frederic Weisbecker <frederic@...nel.org>
To: Gabriele Monaco <gmonaco@...hat.com>
Cc: linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Waiman Long <longman@...hat.com>
Subject: Re: [PATCH v4 2/5] timers: Add the available mask in timer migration

Le Wed, May 07, 2025 at 03:54:32PM +0200, Gabriele Monaco a écrit :
> 
> 
> On Wed, 2025-05-07 at 15:40 +0200, Frederic Weisbecker wrote:
> > Le Wed, May 07, 2025 at 02:46:39PM +0200, Gabriele Monaco a écrit :
> > > 
> > > I'm not so sure about this one though.
> > > As far as I understand [1], is preventing the user from setting
> > > different CPUs while doing isolcpus=nohz, and nohz_full= (which is
> > > now
> > > equivalent). But I seem to be able to do isolcpus=0-3 and
> > > nohz_full=4-7
> > > without any problem and I believe I'd hit the issue you're
> > > mentioning.
> > 
> > Duh!
> > 
> > > (The same would work if I swap the masks as 0 cannot be nohz_full).
> > 
> > Unfortunately 0 can be nohz_full...
> 
> Well, I haven't found what enforces it, but I wasn't able to set 0 as
> nohz_full, no matter what I tried on x86 and arm64. Not sure if 0 was
> just by chance in this case (I'm guessing it has something to do with
> tick_do_timer_cpu, I'm not quite familiar with this code).

Ah looks like you need CONFIG_PM_SLEEP_SMP_NONZERO_CPU. IIUC it's only
powerpc.

You can try to deactivate CONFIG_SUSPEND and CONFIG_HIBERNATE_CALLBACKS
otherwise.

Thanks.

-- 
Frederic Weisbecker
SUSE Labs