linux-kernel - Re: [PATCH 8/5] llist: Remove cpu

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Message-ID: <4E6F6E0E.6070202@redhat.com>
Date:	Tue, 13 Sep 2011 17:51:58 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Huang Ying <ying.huang@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, Andi Kleen <andi@...stfloor.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Subject: Re: [PATCH 8/5] llist: Remove cpu_relax() usage in cmpxchg loops

On 09/13/2011 05:22 PM, Peter Zijlstra wrote:
> On Tue, 2011-09-13 at 14:43 +0300, Avi Kivity wrote:
>
> >  Another issue is that hypervisors use PAUSE to detect a spinning guest
> >  and issue a directed yield to another vcpu.  But for cmpxchg loops, the
> >  "spinner" would just commit on the next loop, no?  So I think there's no
> >  objection from that front.
>
> Right, we shouldn't ever spend a significant amount spinning on a
> cmpxchg. If we do we need to fix that instead.

I hate arguing while agreeing, but the issue isn't that we don't spend a 
significant time spinning, but that there is no owner.  Should the other 
cpu go away, we just pick up a new copy of oldval and complete the 
transaction.

With spinlocks, even if you hold it for just a single guest cycle, the 
situation is different.  If the vcpu that holds the spinlock is 
preempted, the spinner is forced to spin until the owner is rescheduled.

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

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