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

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.

And this would have to be updated to

+	struct optimistic_spin_queue osq;

given the recent changes to the osq lock.