linux-kernel - Re: [RFC PATCH 0/4] Gang scheduling in CFS

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Message-ID: <20120220120159.GA2350@linux.vnet.ibm.com>
Date:	Mon, 20 Feb 2012 17:32:00 +0530
From:	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>
To:	Nikunj A Dadhania <nikunj@...ux.vnet.ibm.com>
Cc:	Peter Zijlstra <peterz@...radead.org>, Avi Kivity <avi@...hat.com>,
	Ingo Molnar <mingo@...e.hu>, Rik van Riel <riel@...hat.com>,
	linux-kernel@...r.kernel.org, bharata@...ux.vnet.ibm.com
Subject: Re: [RFC PATCH 0/4] Gang scheduling in CFS

* Nikunj A Dadhania <nikunj@...ux.vnet.ibm.com> [2012-02-20 17:23:16]:

> On Mon, 20 Feb 2012 11:51:13 +0100, Peter Zijlstra <peterz@...radead.org> wrote:
> > On Mon, 2012-02-20 at 13:38 +0530, Nikunj A Dadhania wrote:
> > > +#ifdef CONFIG_PARAVIRT_FLUSH_TLB
> > > +               while (!cpumask_empty(to_cpumask(f->flush_cpumask)) && --loop)
> > > +                       cpu_relax();
> > > +               if (!loop && !cpumask_empty(to_cpumask(f->flush_cpumask)))
> > > +                       halt();
> > 
> > 
> > That's just vile, you don't need to wait for it, all you need to make
> > sure is that when that vcpu wakes up it does the flush.
> >
> Yes, but we are not sure the vcpu will be sleeping or running. In the
> case where vcpus are running, it might be beneficial to wait a while.

I guess one possibility is for host scheduler to export run/preempt
information to guest OS, as was discussed in the context of this thread
as well:

http://lkml.org/lkml/2010/4/6/223

Essentially guest OS will know (using such exported information) which of its 
vcpus are currently running and thus do a busy-wait for them.

- vatsa

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