Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752738Ab3FQTFi (ORCPT ); Mon, 17 Jun 2013 15:05:38 -0400 Received: from g1t0027.austin.hp.com ([15.216.28.34]:5381 "EHLO g1t0027.austin.hp.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752538Ab3FQTFh (ORCPT ); Mon, 17 Jun 2013 15:05:37 -0400 Message-ID: <1371495933.1778.29.camel@buesod1.americas.hpqcorp.net> Subject: Re: Performance regression from switching lock to rw-sem for anon-vma tree From: Davidlohr Bueso To: Tim Chen Cc: Alex Shi , Ingo Molnar , Rik van Riel , Peter Zijlstra , Andrea Arcangeli , Mel Gorman , Andi Kleen , Andrew Morton , Michel Lespinasse , "Wilcox, Matthew R" , Dave Hansen , "linux-kernel@vger.kernel.org" , "linux-mm@kvack.org" Date: Mon, 17 Jun 2013 12:05:33 -0700 In-Reply-To: <1371494746.27102.633.camel@schen9-DESK> References: <1371165333.27102.568.camel@schen9-DESK> <1371167015.1754.14.camel@buesod1.americas.hpqcorp.net> <51BD8A77.2080201@intel.com> <1371486122.1778.14.camel@buesod1.americas.hpqcorp.net> <1371494746.27102.633.camel@schen9-DESK> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.4.4 (3.4.4-2.fc17) Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 8078 Lines: 176 On Mon, 2013-06-17 at 11:45 -0700, Tim Chen wrote: > On Mon, 2013-06-17 at 09:22 -0700, Davidlohr Bueso wrote: > > On Sun, 2013-06-16 at 17:50 +0800, Alex Shi wrote: > > > On 06/14/2013 07:43 AM, Davidlohr Bueso wrote: > > > > I was hoping that the lack of spin on owner was the main difference with > > > > rwsems and am/was in the middle of implementing it. Could you send your > > > > patch so I can give it a try on my workloads? > > > > > > > > Note that there have been a few recent (3.10) changes to mutexes that > > > > give a nice performance boost, specially on large systems, most > > > > noticeably: > > > > > > > > commit 2bd2c92c (mutex: Make more scalable by doing less atomic > > > > operations) > > > > > > > > commit 0dc8c730 (mutex: Queue mutex spinners with MCS lock to reduce > > > > cacheline contention) > > > > > > > > It might be worth looking into doing something similar to commit > > > > 0dc8c730, in addition to the optimistic spinning. > > > > > > It is a good tunning for large machine. I just following what the commit > > > 0dc8c730 done, give a RFC patch here. I tried it on my NHM EP machine. seems no > > > clear help on aim7. but maybe it is helpful on large machine. :) > > > > After a lot of benchmarking, I finally got the ideal results for aim7, > > so far: this patch + optimistic spinning with preemption disabled. Just > > like optimistic spinning, this patch by itself makes little to no > > difference, yet combined is where we actually outperform 3.10-rc5. In > > addition, I noticed extra throughput when disabling preemption in > > try_optimistic_spin(). > > > > With i_mmap as a rwsem and these changes I could see performance > > benefits for alltests (+14.5%), custom (+17%), disk (+11%), high_systime > > (+5%), shared (+15%) and short (+4%), most of them after around 500 > > users, for fewer users, it made little to no difference. > > > > Thanks. Those are encouraging numbers. On my exim workload I didn't > get a boost when I added in the preempt disable in optimistic spin and > put Alex's changes in. Can you send me your combined patch to see if > there may be something you did that I've missed. I have a tweak to > Alex's patch below to simplify things a bit. > I'm using: int rwsem_optimistic_spin(struct rw_semaphore *sem) { struct task_struct *owner; /* sem->wait_lock should not be held when attempting optimistic spinning */ if (!rwsem_can_spin_on_owner(sem)) return 0; preempt_disable(); for (;;) { owner = ACCESS_ONCE(sem->owner); if (owner && !rwsem_spin_on_owner(sem, owner)) break; /* wait_lock will be acquired if write_lock is obtained */ if (rwsem_try_write_lock(sem->count, true, sem)) { preempt_enable(); return 1; } /* * When there's no owner, we might have preempted between the * owner acquiring the lock and setting the owner field. If * we're an RT task that will live-lock because we won't let * the owner complete. */ if (!owner && (need_resched() || rt_task(current))) break; /* * The cpu_relax() call is a compiler barrier which forces * everything in this loop to be re-loaded. We don't need * memory barriers as we'll eventually observe the right * values at the cost of a few extra spins. */ arch_mutex_cpu_relax(); } preempt_enable(); return 0; } > > > > > > > > > > > diff --git a/include/asm-generic/rwsem.h b/include/asm-generic/rwsem.h > > > index bb1e2cd..240729a 100644 > > > --- a/include/asm-generic/rwsem.h > > > +++ b/include/asm-generic/rwsem.h > > > @@ -70,11 +70,11 @@ static inline void __down_write(struct rw_semaphore *sem) > > > > > > static inline int __down_write_trylock(struct rw_semaphore *sem) > > > { > > > - long tmp; > > > + if (unlikely(&sem->count != RWSEM_UNLOCKED_VALUE)) > > > + return 0; > > > > > > - tmp = cmpxchg(&sem->count, RWSEM_UNLOCKED_VALUE, > > > - RWSEM_ACTIVE_WRITE_BIAS); > > > - return tmp == RWSEM_UNLOCKED_VALUE; > > > + return cmpxchg(&sem->count, RWSEM_UNLOCKED_VALUE, > > > + RWSEM_ACTIVE_WRITE_BIAS) == RWSEM_UNLOCKED_VALUE; > > > } > > > > > > /* > > > diff --git a/lib/rwsem.c b/lib/rwsem.c > > > index 19c5fa9..9e54e20 100644 > > > --- a/lib/rwsem.c > > > +++ b/lib/rwsem.c > > > @@ -64,7 +64,7 @@ __rwsem_do_wake(struct rw_semaphore *sem, enum rwsem_wake_type wake_type) > > > struct rwsem_waiter *waiter; > > > struct task_struct *tsk; > > > struct list_head *next; > > > - long oldcount, woken, loop, adjustment; > > > + long woken, loop, adjustment; > > > > > > waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list); > > > if (waiter->type == RWSEM_WAITING_FOR_WRITE) { > > > @@ -75,7 +75,7 @@ __rwsem_do_wake(struct rw_semaphore *sem, enum rwsem_wake_type wake_type) > > > * will block as they will notice the queued writer. > > > */ > > > wake_up_process(waiter->task); > > > - goto out; > > > + return sem; > > > } > > > > > > /* Writers might steal the lock before we grant it to the next reader. > > > @@ -85,15 +85,28 @@ __rwsem_do_wake(struct rw_semaphore *sem, enum rwsem_wake_type wake_type) > > > adjustment = 0; > > > if (wake_type != RWSEM_WAKE_READ_OWNED) { > > > adjustment = RWSEM_ACTIVE_READ_BIAS; > > > - try_reader_grant: > > > - oldcount = rwsem_atomic_update(adjustment, sem) - adjustment; > > > - if (unlikely(oldcount < RWSEM_WAITING_BIAS)) { > > > - /* A writer stole the lock. Undo our reader grant. */ > > > + while (1) { > > > + long oldcount; > > > + > > > + /* A writer stole the lock. */ > > > + if (unlikely(sem->count & RWSEM_ACTIVE_MASK)) > > > + return sem; > > > + > > > + if (unlikely(sem->count < RWSEM_WAITING_BIAS)) { > > > + cpu_relax(); > > > + continue; > > > + } > > The above two if statements could be cleaned up as a single check: > > if (unlikely(sem->count < RWSEM_WAITING_BIAS)) > return sem; > > This one statement is sufficient to check that we don't have a writer > stolen the lock before we attempt to acquire the read lock by modifying > sem->count. We probably still want to keep the cpu relaxation if the statement doesn't comply. Thanks, Davidlohr -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/