linux-kernel - Re: Core isolation, cpu hotplug and stop

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Date:   Mon, 22 May 2017 21:02:12 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Fredrik Markström <fredrik.markstrom@...il.com>
cc:     linux-kernel@...r.kernel.org
Subject: Re: Core isolation, cpu hotplug and stop_machine

On Mon, 22 May 2017, Fredrik Markström wrote:
> So is there any ongoing work to solve this problem and what is the
> preferred way to solve it, is it:
> 
> 1) stop_machine() free hotplug. It seems to be quite complex but
> probably solves more problems
>     then the one above.

Yes, but it's a non-trivial problem. I looked into it not too long ago and
it just refuses to fall in place due to circular dependencies. It's on my
todo list, but certainly not on the top.

And aside of that cpu hotplug is a vehicle and not a solution.

> 2) Invent some new mechanism/interface separate from hotplug to
> migrate timers and hrtimers.

There is ongoing work to make this nonsensical timer placement stuff
sane. It's buggy atm, but we are working on a fix. We have a similar plan
for hrtimers.

  https://marc.info/?i=20170418111102.490432548%40linutronix.de

> 3) Don't, because this isn't a use-case that linux should support.

Well, we want full isolation to work, but we want proper solutions for it.

Thanks,

	tglx