linux-kernel - Re: [PATCH 0/6] Optimize the cpu hotplug locking -v2

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Date:	Thu, 10 Oct 2013 18:54:12 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Ingo Molnar <mingo@...nel.org>,
	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/6] Optimize the cpu hotplug locking -v2

On 10/10, Andrew Morton wrote:
>
> On Thu, 10 Oct 2013 17:26:12 +0200 Oleg Nesterov <oleg@...hat.com> wrote:
>
> > On 10/10, Ingo Molnar wrote:
> > >
> > > So ... why not make it _really_ cheap, i.e. the read lock costing nothing,
> > > and tie CPU hotplug to freezing all tasks in the system?
> > >
> > > Actual CPU hot unplugging and repluggin is _ridiculously_ rare in a
> > > system, I don't understand how we tolerate _any_ overhead from this utter
> > > slowpath.
> >
> > Well, iirc Srivatsa (cc'ed) pointed out that some systems do cpu_down/up
> > quite often to save the power.
>
> cpu hotremove already uses stop_machine,

And Srivatsa wants to remove it from cpu_down().

> so such an approach shouldn't
> actually worsen things (a lot) for them?

this depends on what this "freezing all tasks" actually means.
I understood it as try_to_freeze_tasks/etc, looks too heavy...

But my only point was, I am not sure we can assume that cpu_down/up
is extremly rare and its cost does not matter.

> use stop_machine() on the add/remove
> (ie, "writer") side and nothing at all on the "reader" side.  Is there
> anything which fundamentally prevents cpu hotplug from doing the same?

Well, then we actually need to park all tasks in system, I guess.
IOW, freezer.

Oleg.

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