linux-kernel - Re: [PATCH RFC V8 0/17] Paravirtualized ticket spinlocks

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Message-ID: <4FA8E272.5040307@redhat.com>
Date:	Tue, 08 May 2012 12:08:02 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
CC:	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>,
	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Marcelo Tosatti <mtosatti@...hat.com>, X86 <x86@...nel.org>,
	Gleb Natapov <gleb@...hat.com>, Ingo Molnar <mingo@...hat.com>,
	Attilio Rao <attilio.rao@...rix.com>,
	Virtualization <virtualization@...ts.linux-foundation.org>,
	Xen Devel <xen-devel@...ts.xensource.com>,
	linux-doc@...r.kernel.org, KVM <kvm@...r.kernel.org>,
	Andi Kleen <andi@...stfloor.org>,
	Stefano Stabellini <stefano.stabellini@...citrix.com>,
	Stephan Diestelhorst <stephan.diestelhorst@....com>,
	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH RFC V8 0/17] Paravirtualized ticket spinlocks

On 05/08/2012 02:15 AM, Jeremy Fitzhardinge wrote:
> On 05/07/2012 06:49 AM, Avi Kivity wrote:
> > On 05/07/2012 04:46 PM, Srivatsa Vaddagiri wrote:
> >> * Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com> [2012-05-07 19:08:51]:
> >>
> >>> I 'll get hold of a PLE mc  and come up with the numbers soon. but I
> >>> 'll expect the improvement around 1-3% as it was in last version.
> >> Deferring preemption (when vcpu is holding lock) may give us better than 1-3% 
> >> results on PLE hardware. Something worth trying IMHO.
> > Is the improvement so low, because PLE is interfering with the patch, or
> > because PLE already does a good job?
>
> How does PLE help with ticket scheduling on unlock?  I thought it would
> just help with the actual spin loops.

PLE yields to up a random vcpu, hoping it is the lock holder.  This
patchset wakes up the right vcpu.  For small vcpu counts the difference
is a few bad wakeups (and even a bad wakeup sometimes works, since it
can put the spinner to sleep for a bit).  I expect that large vcpu
counts would show a greater difference.

-- 
error compiling committee.c: too many arguments to function

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