Re: [PATCH V2 0/6][RFC] futex: FUTEX_LOCK with optional adaptivespinning

From: Darren Hart
Date: Tue Apr 06 2010 - 11:28:54 EST

Alan Cox wrote:
On Tue, 06 Apr 2010 15:35:31 +0200
Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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.


Darren Hart
IBM Linux Technology Center
Real-Time Linux Team
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