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

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Message-ID: <4BBB5C0D.8050602@redhat.com>
Date:	Tue, 06 Apr 2010 19:06:37 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Darren Hart <dvhltc@...ibm.com>
CC:	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Peter Zijlstra <peterz@...radead.org>,
	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 06:28 PM, Darren Hart wrote:
> Alan Cox wrote:
>> On Tue, 06 Apr 2010 15:35:31 +0200
>> Peter Zijlstra <peterz@...radead.org> 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.
>>
>> Thats a gross and inaccurate simplification. For the case Avi is talking
>> about spinning in userspace makes sense in a lot of environments. Once
>> you've got one thread pinned per cpu (or gang scheduling >-) ) there are
>> various environments where it makes complete and utter sense.
>
> Hi Alan,
>
> Do you feel some of these situations would also benefit from some 
> kernel assistance to stop spinning when the owner schedules out? Or 
> are you saying that there are situations where pure userspace 
> spinlocks will always be the best option?
>
> If the latter, I'd think that they would also be situations where 
> sched_yield() is not used as part of the spin loop. If so, then these 
> are not our target situations for FUTEX_LOCK_ADAPTIVE, which hopes to 
> provide a better informed mechanism for making spin or sleep 
> decisions. If sleeping isn't part of the locking construct 
> implementation, then FUTEX_LOCK_ADAPTIVE doesn't have much to offer.

IMO the best solution is to spin in userspace while the lock holder is 
running, fall into the kernel when it is scheduled out.

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

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