linux-kernel - Re: Missing recalculation of scheduler tunables in case of cpu hot add/remove

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Message-ID: <20091203091206.GA1478@ucw.cz>
Date:	Thu, 3 Dec 2009 10:12:06 +0100
From:	Pavel Machek <pavel@....cz>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Christian Ehrhardt <ehrhardt@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Holger.Wolf@...ibm.com, epasch@...ibm.com,
	Martin Schwidefsky <schwidefsky@...ibm.com>
Subject: Re: Missing recalculation of scheduler tunables in case of cpu hot
	add/remove

Hi!

> > >>> Aside from that, we probably should put an upper limit in place, as I
> > >>> guess large cpu count machines get silly large values
> > >>>       
> > >> I agree to that, but in the code is already an upper limit of 
> > >> 200.000.000 - well we might discuss if that is too low/high.
> > >>     
> > >
> > > Yeah, I think we should cap it around the 8-16 CPUs.
> > >
> > >   
> > ok for me, driven by that finding I think I have to measure different 
> > kind of scalings anyway, but as usually that takes some time :-/
> > At least too time much for the discussion & solution of that bug I guess.
> > 
> > The question for now is what we do on cpu hot add/remove?
> > Would hooking somewhere in kernel/cpu.c be the right approach - I'm not 
> > quite sure about my own suggestion yet :-).
> 
> Something like the below might work I suppose, just needs a cleanup and
> such.

I see a rather fundamental problem: what if user wants to override
those values, and wants them stay that way?

If you do this, suspend/resume will put the old values back AFAICT.

									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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