linux-kernel - Core isolation, cpu hotplug and stop

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Message-ID: <CAKdL+dTvGK_JLUKQsBZ3EgyXtBHo9ggo0E+-7BjUVvmnYvO5HA@mail.gmail.com>
Date:   Mon, 22 May 2017 15:07:12 +0200
From:   Fredrik Markström <fredrik.markstrom@...il.com>
To:     linux-kernel@...r.kernel.org
Cc:     Thomas Gleixner <tglx@...utronix.de>
Subject: Core isolation, cpu hotplug and stop_machine

Hello,

As I understand it the recommended way to force pending timers of a
core when trying to isolate it (runtime) is to use cpu hotplug. The
current implementation of cpu hotplug has a serious drawback in that
it uses stop_machine() which disturbs already isolated cores. This is
a serious problem in our usecase.

I would guess we are not the only ones with this problem, but as far
as I can see there is currently no solution for this in the latest
next. There was a patchset back in 2013 that implemented
stop_machine-free hotplug, but that was never accepted.
(https://lwn.net/Articles/538819/)

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.
2) Invent some new mechanism/interface separate from hotplug to
migrate timers and hrtimers.
3) Don't, because this isn't a use-case that linux should support.

/Fredrik