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From: Avi Kivity <avi@redhat.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Huang Ying <ying.huang@intel.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	linux-kernel@vger.kernel.org, Andi Kleen <andi@firstfloor.org>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Subject: Re: [PATCH 8/5] llist: Remove cpu_relax() usage in cmpxchg loops
Date: Tue, 13 Sep 2011 17:51:58 +0300	[thread overview]
Message-ID: <4E6F6E0E.6070202@redhat.com> (raw)
In-Reply-To: <1315923724.5977.15.camel@twins>

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


  reply	other threads:[~2011-09-13 14:52 UTC|newest]

Thread overview: 23+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2011-09-08  6:00 [PATCH -mm -v2 0/5] irq_work, Use llist in irq_work Huang Ying
2011-09-08  6:00 ` [PATCH -mm -v2 1/5] llist, Make all llist functions inline Huang Ying
2011-09-08  6:00 ` [PATCH -mm -v2 2/5] llist, Define macro to check NMI safe cmpxchg Huang Ying
2011-09-08  6:00 ` [PATCH -mm -v2 3/5] llist, Move cpu_relax after cmpxchg Huang Ying
2011-09-08  6:00 ` [PATCH -mm -v2 4/5] llist, Return whether list is empty before adding in llist_add Huang Ying
2011-09-08  6:00 ` [PATCH -mm -v2 5/5] irq_work, Use llist in irq_work Huang Ying
2011-09-12 14:05 ` [PATCH 6/5] llist: Add llist_next() Peter Zijlstra
2011-09-12 14:05 ` [PATCH 7/5] sched: Convert to use llist Peter Zijlstra
2011-09-12 14:05 ` [PATCH 8/5] llist: Remove cpu_relax() usage in cmpxchg loops Peter Zijlstra
2011-09-12 14:23   ` Andi Kleen
2011-09-12 14:23     ` Peter Zijlstra
2011-09-12 14:47     ` Mathieu Desnoyers
2011-09-12 15:09       ` Peter Zijlstra
2011-09-12 15:24       ` Peter Zijlstra
2011-09-12 16:38       ` Andi Kleen
2011-09-12 18:53         ` Peter Zijlstra
2011-09-12 14:26   ` Avi Kivity
2011-09-12 14:32     ` Peter Zijlstra
2011-09-13 11:43       ` Avi Kivity
2011-09-13 14:22         ` Peter Zijlstra
2011-09-13 14:51           ` Avi Kivity [this message]
2011-09-13 14:53             ` Peter Zijlstra
2011-09-12 14:06 ` [PATCH -mm -v2 0/5] irq_work, Use llist in irq_work Peter Zijlstra

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