Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756041AbbFPMRx (ORCPT ); Tue, 16 Jun 2015 08:17:53 -0400 Received: from foss.arm.com ([217.140.101.70]:36817 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752817AbbFPMRr (ORCPT ); Tue, 16 Jun 2015 08:17:47 -0400 Date: Tue, 16 Jun 2015 13:17:43 +0100 From: Will Deacon To: Waiman Long Cc: Peter Zijlstra , Ingo Molnar , Arnd Bergmann , "linux-arch@vger.kernel.org" , "linux-kernel@vger.kernel.org" , Scott J Norton , Douglas Hatch Subject: Re: [PATCH v3 1/2] locking/qrwlock: Better optimization for interrupt context readers Message-ID: <20150616121742.GB30522@arm.com> References: <1434407043-18741-1-git-send-email-Waiman.Long@hp.com> <1434407043-18741-2-git-send-email-Waiman.Long@hp.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1434407043-18741-2-git-send-email-Waiman.Long@hp.com> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2854 Lines: 68 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. Will -- 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/