linux-kernel - Re: [PATCH][retry 3] Support Pause Filter in AMD processors

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Date:	Mon, 12 Oct 2009 16:17:53 -0300
From:	Marcelo Tosatti <mtosatti@...hat.com>
To:	Mark Langsdorf <mark.langsdorf@....com>
Cc:	linux-kernel@...r.kernel.org, avi@...hat.com, kvm@...r.kernel.org
Subject: Re: [PATCH][retry 3] Support Pause Filter in AMD processors

On Tue, Oct 06, 2009 at 02:25:02PM -0500, Mark Langsdorf wrote:
> >From 8ec340648103510095ff339b914706c81e9d815d Mon Sep 17 00:00:00 2001
> From: Mark Langsdorf <mark.langsdorf@....com>
> Date: Wed, 9 Sep 2009 22:12:51 -0500
> Subject: [PATCH] [PATCH] Support Pause Filter in AMD processors
> 
> New AMD processors (Family 0x10 models 8+) support the Pause
> Filter Feature.  This feature creates a new field in the VMCB
> called Pause Filter Count.  If Pause Filter Count is greater
> than 0 and intercepting PAUSEs is enabled, the processor will
> increment an internal counter when a PAUSE instruction occurs
> instead of intercepting.  When the internal counter reaches the
> Pause Filter Count value, a PAUSE intercept will occur.
> 
> This feature can be used to detect contended spinlocks,
> especially when the lock holding VCPU is not scheduled.
> Rescheduling another VCPU prevents the VCPU seeking the
> lock from wasting its quantum by spinning idly.
> 
> Experimental results show that most spinlocks are held
> for less than 1000 PAUSE cycles or more than a few
> thousand.  Default the Pause Filter Counter to 3000 to
> detect the contended spinlocks.
> 
> Processor support for this feature is indicated by a CPUID
> bit.
> 
> On a 24 core system running 4 guests each with 16 VCPUs,
> this patch improved overall performance of each guest's
> 32 job kernbench by approximately 3-5% when combined
> with a scheduler algorithm thati caused the VCPU to
> sleep for a brief period. Further performance improvement
> may be possible with a more sophisticated yield algorithm.
> 
> This patch depends on the changes to the kvm code from
> "KVM:VMX: Add support for Pause Loop Exiting"
> http://www.mail-archive.com/kvm@vger.kernel.org/msg23089.html
> 
> -Mark Langsdorf
> Operating System Research Center
> AMD
> 
> Signed-of-by: Mark Langsdorf <mark.langsdorf@....com>

Applied, thanks.

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