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

[Waiman Long <waiman.long@hp.com>]

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