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

[Darren Hart <dvhltc@us.ibm.com>]

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

Thanks,

-- 
Darren Hart
IBM Linux Technology Center
Real-Time Linux Team