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

Jason Low
2014-07-21T20:17:23Z

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.