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

[Jason Low <jason.low2@hp.com>]

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?

> 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.