linux-kernel - RE: [PATCH][KVM][retry 2] Add support for Pause Filtering to AMD SVM

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Date:	Fri, 8 May 2009 13:47:04 -0500
From:	"Langsdorf, Mark" <mark.langsdorf@....com>
To:	"Avi Kivity" <avi@...hat.com>
CC:	"Roedel, Joerg" <Joerg.Roedel@....com>, <kvm@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>
Subject: RE: [PATCH][KVM][retry 2] Add support for Pause Filtering to AMD SVM

> What's the differences wrt retry 1?

I'm using git format-patch as you requested.
 
> > 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.
> 
> 3000.

Thanks, I keep missing that.
 
> > 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 1%.  Further performance
> > improvement may be possible with a more sophisticated
> > yield algorithm.
> >   
> 
> Like I mentioned earlier, I don't think schedule() does 
> anything on CFS.
> 
> Try sched_yield(), but set /proc/sys/kernel/sched_compat_yield.

Will do.

> > +static int pause_interception(struct vcpu_svm *svm, struct 
> kvm_run *kvm_run)
> > +{
> > +	/* Simple yield */
> > +	vcpu_put(&svm->vcpu);
> > +	schedule();
> > +	vcpu_load(&svm->vcpu);
> > +	return 1;
> > +}
> > +
> >   
> 
> You don't need to vcpu_put() and vcpu_load().  The scheduler 
> will call them for you if/when it switches tasks.

I was waiting for feedback from Ingo on that issue, but I'll
try sched_yield() instead.

-Mark Langsdorf
Operating System Research Center
AMD

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