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

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Message-ID: <f10fca10b106652e1bad2dc75ce474aa5bb7907e.camel@redhat.com>
Date: Mon, 14 Apr 2025 10:06:06 +0200
From: Gabriele Monaco <gmonaco@...hat.com>
To: Frederic Weisbecker <frederic@...nel.org>
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

On Sat, 2025-04-12 at 00:57 +0200, Frederic Weisbecker wrote:
> 
> If you want housekeeping CPUs to pull timers from isolated ones, then
> you
> need isolated CPUs to eventually be part of the hierarchy (be
> "available"),
> because they would need to propagate their global timers up to the
> top.
> 
> If they propagate their global timers then they must play the whole
> hierarchy
> game and be ready to pull the timers from any other CPUs.
> 
> Because if they propagate their timers, they must also propagate
> their
> idle state to synchronize with the whole hierarchy and know if there
> is
> a non-idle housekeeping CPU to take care of their timers. And if they
> propagate
> their (non) idle state, the isolated CPUs also declare themselves as
> available
> to handle other's timers.
> 
> And working around that would be very nasty.
> 

Well, that's exactly what I was trying to do, thanks for pointing this
out. I guess I won't go down that rabbit hole then.

Thanks,
Gabriele