Re: [RFC PATCH 2/5] futex: add optimistic spinning to FUTEX_SPIN_LOCK

[Waiman Long]

On 07/21/2014 04:17 PM, Jason Low wrote:
> On Mon, 2014-07-21 at 11:24 -0400, Waiman Long wrote:
> > This patch adds code to do optimistic spinning for the FUTEX_SPIN_LOCK
> > primitive on the futex value when the lock owner is running. It is
> > the same optimistic spinning technique that is done in the mutex and
> > rw semaphore code to improve their performance especially on large
> > systems with large number of CPUs. When the lock owner is not running,
> > the spinning tasks will go to sleep.
> Perhaps we could introduce a "CONFIG_FUTEX_SPIN_ON_OWNER" that depends
> on SMP and ARCH_SUPPORTS_ATOMIC_RMW?

The new futex opcode depends on the ability to do cmpxchg() in the futex 
context. The code will be disabled if futex cmpxchg is not supported. I 
guess that should be enough to limit it to just a handful of architectures.

> > There is 2 major advantages of doing optimistic spinning here:
> >   1) It eliminates the context switching latency and overhead (at
> >      least a few us) associated with sleeping and wakeup.
> >   2) It eliminates most of the need to call futex(2) with
> >      FUTEX_SPIN_UNLOCK as spinning is done without the need to set
> >      the FUTEX_WAITERS bit.
> >   struct futex_q_head {
> >   	struct list_head	      hnode;
> >   	struct list_head	      waitq;
> >   	union futex_key		      key;
> > +	struct optimistic_spin_queue *osq;
> And this would have to be updated to
>
> +	struct optimistic_spin_queue osq;
>
> given the recent changes to the osq lock.

Yes, I will make the change in the next iteration of the patch.

-Longman

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