linux-kernel - Re: [PATCH][KVM][retry 1] Add support for Pause Filtering to AMD SVM

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Date:	Mon, 11 May 2009 17:01:49 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Avi Kivity <avi@...hat.com>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Mark Langsdorf <mark.langsdorf@....com>,
	Joerg Roedel <joerg.roedel@....com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH][KVM][retry 1] Add support for Pause Filtering to AMD
 SVM

On Mon, 2009-05-11 at 17:51 +0300, Avi Kivity wrote:
> Ingo Molnar wrote:
> > * Avi Kivity <avi@...hat.com> wrote:
> >
> >   
> >>> I.e. this is a somewhat poor solution as far as scheduling goes. 
> >>> But i'm wondering what the CPU side does. Can REP-NOP really take 
> >>> thousands of cycles? If yes, under what circumstances?
> >>>       
> >> The guest is running rep-nop in a loop while trying to acquire a 
> >> spinlock.  The hardware detects this (most likely, repeated 
> >> rep-nop with the same rip) and exits.  We can program the loop 
> >> count; obviously if we're spinning for only a short while it's 
> >> better to keep spinning while hoping the lock will be released 
> >> soon.
> >>
> >> The idea is to detect that the guest is not making forward 
> >> progress and yield.  If I could tell the scheduler, you may charge 
> >> me a couple of milliseconds, I promise not to sue, that would be 
> >> ideal. [...]
> >>     
> >
> > Ok, with such a waiver, who could refuse?
> >
> > This really needs a new kernel-internal scheduler API though, which 
> > does a lot of fancy things to do:
> >
> >         se->vruntime += 1000000;
> >
> > i.e. add 1 msec worth of nanoseconds to the task's timeline. (first 
> > remove it from the rbtree, then add it back, and nice-weight it as 
> > well) 
> 
> I suspected it would be as simple as this.

Is that thread guaranteed to run as SCHED_OTHER?

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