Re: [PATCH V2 0/6][RFC] futex: FUTEX_LOCK with optional adaptivespinning
From: Thomas Gleixner
Date: Tue Apr 06 2010 - 12:16:38 EST
On Tue, 6 Apr 2010, Avi Kivity wrote:
> 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@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.
> IMO the best solution is to spin in userspace while the lock holder is
> running, fall into the kernel when it is scheduled out.
That's just not realistic as user space has no idea whether the lock
holder is running or not and when it's scheduled out without a syscall :)
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