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

Waiman Long
2014-07-22T19:34:57Z

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