linux-kernel - Re: Possible sandybridge livelock issue

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Message-ID: <m262pezhfe.fsf@firstfloor.org>
Date:	Fri, 13 May 2011 09:36:21 -0700
From:	Andi Kleen <andi@...stfloor.org>
To:	James Bottomley <James.Bottomley@...senPartnership.com>
Cc:	x86@...nel.org, linux-mm <linux-mm@...ck.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Mel Gorman <mgorman@...e.de>
Subject: Re: Possible sandybridge livelock issue

James Bottomley <James.Bottomley@...senPartnership.com> writes:
>
> When the hang occurred, kswapd basically pegged one core in 100% system
> time.  This looks like there's something specific to sandybridge that
> causes this type of bad interaction.  I was wondering if it could be
> something to to with a scheduling problem in turbo mode?  Once kswapd
> goes flat out, the core its on will kick into turbo mode, which causes
> it to get preferentially scheduled there, leading to the live lock.

Sounds unlikely to me.

Turbo mode does not affect the scheduler and the cores are (reasonably) 
independent.


> The only evidence I have to support this theory is that when I reproduce
> the problem with PREEMPT, the core pegs at 100% system time and stays
> there even if I turn off the load.  However, if I can execute work that
> causes kswapd to be kicked off the core it's running on, it will calm
> back down and go to sleep.

Turbo mode just makes the CPU faster, but it should not change 
the scheduler decisions.

-Andi

-- 
ak@...ux.intel.com -- Speaking for myself only
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