linux-kernel - Re: [PATCH V2 0/6][RFC] futex: FUTEX_LOCK with optional adaptive spinning

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Message-ID: <4BBB5CEF.6060803@redhat.com>
Date:	Tue, 06 Apr 2010 19:10:23 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Darren Hart <dvhltc@...ibm.com>, linux-kernel@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>,
	Eric Dumazet <eric.dumazet@...il.com>,
	"Peter W. Morreale" <pmorreale@...ell.com>,
	Rik van Riel <riel@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Gregory Haskins <ghaskins@...ell.com>,
	Sven-Thorsten Dietrich <sdietrich@...ell.com>,
	Chris Mason <chris.mason@...cle.com>,
	John Cooper <john.cooper@...rd-harmonic.com>,
	Chris Wright <chrisw@...s-sol.org>
Subject: Re: [PATCH V2 0/6][RFC] futex: FUTEX_LOCK with optional adaptive
 spinning

On 04/06/2010 05:09 PM, Peter Zijlstra wrote:
> On Tue, 2010-04-06 at 16:41 +0300, Avi Kivity wrote:
>    
>> On 04/06/2010 04:35 PM, Peter Zijlstra wrote:
>>      
>>> On Tue, 2010-04-06 at 16:28 +0300, Avi Kivity wrote:
>>>
>>>        
>>>> Yes, but that's the best case for spinning.  You could simply use a
>>>> userspace spinlock in this case.
>>>>
>>>>          
>>> Userspace spinlocks are evil.. they should _never_ be used.
>>>
>>>        
>> But in this case they're fastest.  If we don't provide a non-evil
>> alternative, people will use them.
>>
>>      
> That's what FUTEX_LOCK is about.
>    

That works for the uncontended case.  For the contended case, the waiter 
and the owner have to go into the kernel and back out to transfer 
ownership.  In the non-adaptive case you have to switch to the idle task 
and back as well, and send an IPI.  That's a lot of latency if the 
unlock happened just after the waiter started the descent into the kernel.

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

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