Messages in this thread | | | Date | Tue, 22 Jul 2014 15:34:51 -0400 | From | Waiman Long <> | Subject | Re: [RFC PATCH 2/5] futex: add optimistic spinning to FUTEX_SPIN_LOCK |
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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
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