linux-kernel - RE: [PATCH][KVM][retry 1] Add support for Pause Filtering to AMDSVM

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Date:	Mon, 11 May 2009 10:58:24 -0500
From:	"Langsdorf, Mark" <mark.langsdorf@....com>
To:	"Ingo Molnar" <mingo@...e.hu>, "Avi Kivity" <avi@...hat.com>
CC:	"Peter Zijlstra" <a.p.zijlstra@...llo.nl>,
	"Roedel, Joerg" <Joerg.Roedel@....com>, <kvm@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>
Subject: RE: [PATCH][KVM][retry 1] Add support for Pause Filtering to AMDSVM

> >> Please dont even think about using yield for this though - 

Oops.  I'm glad I waited to get some benchmark results before
submitting that version.

> >> A gradual and linear back-off from the current timeline is 
> >> more of a fair negotiation process between vcpus and
> >> results in more or less  
> >> sane (and fair) scheduling, and no unnecessary looping.
> >>
> >> You could even do an exponential backoff up to a limit of 
> >> 1-10 msecs or so, starting at 100 usecs.
> >
> > Good idea, it eliminates another variable to be tuned.
> 
> It could be made fully self-tuning, if the filter threshold can be 
> tuned fast enough. (an MSR write? A VM context field update?)

VMCB field update.

So the suggestion is to add a function similar to set_task_cpu()
that increases the vmruntime with an exponential backoff?  Is
that sufficient to cause a new VCPU to step in?  I'm obviously
not very familiar with the scheduler code.
 
> I.e. the 3000 cycles value itself could be eliminated as well. (with 
> just a common-sense max of say 100,000 cycles enforced)

I don't understand what you're saying here.  There needs to be
some value in the pause filter counter to trigger the intercept.

-Mark Langsdorf
Operating System Research Center
AMD

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