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

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Message-ID: <aBtix0VHFCRI_Y-c@localhost.localdomain>
Date: Wed, 7 May 2025 15:40:23 +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 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...

> 
>   # vng -a isolcpus=0-7 -a nohz_full=8-15 head
> /sys/devices/system/cpu/{isolated,nohz_full}
> 
>   ==> /sys/devices/system/cpu/isolated <==
>   0-7
> 
>   ==> /sys/devices/system/cpu/nohz_full <==
>   8-15
> 
> (where probably some CPUs are set up to do housekeeping stuff anyway,
> but if we just look at the masks, we won't notice)
> 
> Then I assume this should not be allowed either, should it?
> Or am I missing something here?

Exactly then. housekeeping_setup() already handles cases when
there is no housekeeping left. I guess that section could be
made aware of nohz_full + isolcpus not leaving any housekeeping left.

> 
> > 
> > But if nohz_full= is passed on boot and cpusets later create an
> > isolated
> > partition which spans the housekeeping set, then the isolated
> > partition must
> > be rejected.
> 
> Mmh, that would make things easier actually.
> I assume there's no real use case for that kind of hybrid setup with
> half CPUs nohz_full and half domain isolated..

I guess we can accept nohz_full + isolated partition as long as a housekeeping
CPU remains.

Thanks.

-- 
Frederic Weisbecker
SUSE Labs