From mboxrd@z Thu Jan 1 00:00:00 1970 From: Waiman Long Subject: Re: [PATCH v3 1/2] locking/qrwlock: Better optimization for interrupt context readers Date: Wed, 17 Jun 2015 21:30:10 -0400 Message-ID: <55821F22.9040902@hp.com> References: <1434407043-18741-1-git-send-email-Waiman.Long@hp.com> <1434407043-18741-2-git-send-email-Waiman.Long@hp.com> <20150616121742.GB30522@arm.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: Received: from g4t3427.houston.hp.com ([15.201.208.55]:43153 "EHLO g4t3427.houston.hp.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753281AbbFRBaQ (ORCPT ); Wed, 17 Jun 2015 21:30:16 -0400 In-Reply-To: <20150616121742.GB30522@arm.com> Sender: linux-arch-owner@vger.kernel.org List-ID: To: Will Deacon Cc: Peter Zijlstra , Ingo Molnar , Arnd Bergmann , "linux-arch@vger.kernel.org" , "linux-kernel@vger.kernel.org" , Scott J Norton , Douglas Hatch On 06/16/2015 08:17 AM, Will Deacon wrote: > Hi Waiman, > > On Mon, Jun 15, 2015 at 11:24:02PM +0100, Waiman Long wrote: >> The qrwlock is fair in the process context, but becoming unfair when >> in the interrupt context to support use cases like the tasklist_lock. >> >> The current code isn't that well-documented on what happens when >> in the interrupt context. The rspin_until_writer_unlock() will only >> spin if the writer has gotten the lock. If the writer is still in the >> waiting state, the increment in the reader count will cause the writer >> to remain in the waiting state and the new interrupt context reader >> will get the lock and return immediately. The current code, however, >> do an additional read of the lock value which is not necessary as the >> information have already been there in the fast path. This may sometime >> cause an additional cacheline load when the lock is highly contended. >> >> This patch passes the lock value information gotten in the fast path >> to the slow path to eliminate the additional read. It also clarify the >> action for the interrupt context readers more explicitly. >> >> Signed-off-by: Waiman Long >> --- >> include/asm-generic/qrwlock.h | 4 ++-- >> kernel/locking/qrwlock.c | 14 ++++++++------ >> 2 files changed, 10 insertions(+), 8 deletions(-) > [...] > >> diff --git a/kernel/locking/qrwlock.c b/kernel/locking/qrwlock.c >> index 00c12bb..d7d7557 100644 >> --- a/kernel/locking/qrwlock.c >> +++ b/kernel/locking/qrwlock.c >> @@ -43,22 +43,24 @@ rspin_until_writer_unlock(struct qrwlock *lock, u32 cnts) >> * queue_read_lock_slowpath - acquire read lock of a queue rwlock >> * @lock: Pointer to queue rwlock structure >> */ >> -void queue_read_lock_slowpath(struct qrwlock *lock) >> +void queue_read_lock_slowpath(struct qrwlock *lock, u32 cnts) >> { >> - u32 cnts; >> - >> /* >> * Readers come here when they cannot get the lock without waiting >> */ >> if (unlikely(in_interrupt())) { >> /* >> - * Readers in interrupt context will spin until the lock is >> - * available without waiting in the queue. >> + * Readers in interrupt context will get the lock immediately >> + * if the writer is just waiting (not holding the lock yet) >> + * or they will spin until the lock is available without >> + * waiting in the queue. >> */ >> - cnts = smp_load_acquire((u32 *)&lock->cnts); >> + if ((cnts& _QW_WMASK) != _QW_LOCKED) >> + return; > I really doubt the check here is gaining you any performance, given > rspin_until_write_unlock does the same check immediately and should be > inlined. Just dropping the acquire and passing cnts through should be > sufficient. Yes, you are right. I can just pass the cnt to rspin_until_write_unlock() and be done with it. Cheers, Longman