From mboxrd@z Thu Jan 1 00:00:00 1970 From: Waiman Long Subject: Re: [RFC PATCH 2/5] futex: add optimistic spinning to FUTEX_SPIN_LOCK Date: Tue, 22 Jul 2014 15:34:51 -0400 Message-ID: <53CEBCDB.8060106@hp.com> References: <1405956271-34339-1-git-send-email-Waiman.Long@hp.com> <1405956271-34339-3-git-send-email-Waiman.Long@hp.com> <1405973838.4677.11.camel@j-VirtualBox> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <1405973838.4677.11.camel@j-VirtualBox> Sender: linux-doc-owner@vger.kernel.org To: Jason Low Cc: Thomas Gleixner , Ingo Molnar , Peter Zijlstra , Darren Hart , Davidlohr Bueso , Heiko Carstens , linux-kernel@vger.kernel.org, linux-api@vger.kernel.org, linux-doc@vger.kernel.org, Scott J Norton List-Id: linux-api@vger.kernel.org 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