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

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Message-ID: <Z_fTmzdvLEmrAth6@localhost.localdomain>
Date: Thu, 10 Apr 2025 16:20:11 +0200
From: Frederic Weisbecker <frederic@...nel.org>
To: Gabriele Monaco <gmonaco@...hat.com>
Cc: Thomas Gleixner <tglx@...utronix.de>, linux-kernel@...r.kernel.org,
	Waiman Long <longman@...hat.com>
Subject: Re: [PATCH] timers: Exclude isolated cpus from timer migation

Le Thu, Apr 10, 2025 at 03:56:02PM +0200, Gabriele Monaco a écrit :
> On Thu, 2025-04-10 at 15:27 +0200, Frederic Weisbecker wrote:
> > But how do we handle global timers that have been initialized and
> > queued from
> > isolated CPUs?
> 
> I need to sketch a bit more the solution but the rough idea is:
> 1. isolated CPUs don't pull remote timers

That's the "easy" part.

> 2. isolated CPUs ignore their global timers and let others pull them
>   perhaps with some more logic to avoid it expiring

This will always involve added overhead because you may need to wake up
a CPU upon enqueueing a global timer to make sure it will be handled.
At least when all other CPUs are idle.

> Wouldn't that be sufficient?
> 
> Also, I would definitely do 1. for any kind of isolation, but I'm not
> sure about 2.
> Strictly speaking domain isolated cores don't claim to be free of
> kernel noise, even if they initiate it (but nohz_full ones do).
> What would be the expectation there?

I don't know, I haven't heard complains about isolcpus handling global
timers so far...

I wouldn't pay much attention to 2) until anybody complains. Does 1) even
matter to anybody outside nohz_full ?

Thanks.

-- 
Frederic Weisbecker
SUSE Labs