Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755389AbbGFTti (ORCPT ); Mon, 6 Jul 2015 15:49:38 -0400 Received: from g4t3426.houston.hp.com ([15.201.208.54]:43683 "EHLO g4t3426.houston.hp.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754469AbbGFTth (ORCPT ); Mon, 6 Jul 2015 15:49:37 -0400 Message-ID: <559ADBCD.6020803@hp.com> Date: Mon, 06 Jul 2015 15:49:33 -0400 From: Waiman Long User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:10.0.12) Gecko/20130109 Thunderbird/10.0.12 MIME-Version: 1.0 To: Will Deacon CC: Peter Zijlstra , Ingo Molnar , Arnd Bergmann , Thomas Gleixner , "linux-arch@vger.kernel.org" , "linux-kernel@vger.kernel.org" , Scott J Norton , Douglas Hatch Subject: Re: [PATCH 2/4] locking/qrwlock: Reduce reader/writer to reader lock transfer latency References: <1436197386-58635-1-git-send-email-Waiman.Long@hp.com> <1436197386-58635-3-git-send-email-Waiman.Long@hp.com> <20150706182353.GC1607@arm.com> In-Reply-To: <20150706182353.GC1607@arm.com> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3163 Lines: 71 On 07/06/2015 02:23 PM, Will Deacon wrote: > Hi Waiman, > > On Mon, Jul 06, 2015 at 04:43:04PM +0100, Waiman Long wrote: >> Currently, a reader will check first to make sure that the writer mode >> byte is cleared before incrementing the reader count. That waiting is >> not really necessary. It increases the latency in the reader/writer >> to reader transition and reduces readers performance. >> >> This patch eliminates that waiting. It also has the side effect >> of reducing the chance of writer lock stealing and improving the >> fairness of the lock. Using a locking microbenchmark, a 10-threads 5M >> locking loop of mostly readers (RW ratio = 10,000:1) has the following >> performance numbers in a Haswell-EX box: >> >> Kernel Locking Rate (Kops/s) >> ------ --------------------- >> 4.1.1 15,063,081 >> Patched 4.1.1 17,241,552 >> >> Signed-off-by: Waiman Long > I've just finished rebasing my arm64 qrwlock stuff, but I think it will > conflict with these patches. Do you mind if I post them for review anyway, > so we can at least co-ordinate our efforts? Yes, sure. I would also like to coordinate my changes with yours to minimize conflict. BTW, I just got 2 tip-bot messages about the commits: locking/qrwlock: Better optimization for interrupt context readers locking/qrwlock: Rename functions to queued_*() So I need to rebase my patches also. >> --- >> kernel/locking/qrwlock.c | 12 ++++-------- >> 1 files changed, 4 insertions(+), 8 deletions(-) >> >> diff --git a/kernel/locking/qrwlock.c b/kernel/locking/qrwlock.c >> index 81bae99..ecd2d19 100644 >> --- a/kernel/locking/qrwlock.c >> +++ b/kernel/locking/qrwlock.c >> @@ -88,15 +88,11 @@ void queue_read_lock_slowpath(struct qrwlock *lock, u32 cnts) >> arch_spin_lock(&lock->lock); >> >> /* >> - * At the head of the wait queue now, wait until the writer state >> - * goes to 0 and then try to increment the reader count and get >> - * the lock. It is possible that an incoming writer may steal the >> - * lock in the interim, so it is necessary to check the writer byte >> - * to make sure that the write lock isn't taken. >> + * At the head of the wait queue now, increment the reader count >> + * and wait until the writer, if it has the lock, has gone away. >> + * At ths stage, it is not possible for a writer to remain in the >> + * waiting state (_QW_WAITING). So there won't be any deadlock. >> */ >> - while (atomic_read(&lock->cnts)& _QW_WMASK) >> - cpu_relax_lowlatency(); > Thinking about it, can we kill _QW_WAITING altogether and set (cmpxchg > from 0) wmode to _QW_LOCKED in the write_lock slowpath, polling (acquire) > rmode until it hits zero? No, this is how we make the lock fair so that an incoming streams of later readers won't block a writer from getting the lock. Cheers, Longman -- 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/