From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932494Ab3AJCbO (ORCPT ); Wed, 9 Jan 2013 21:31:14 -0500 Received: from mx1.redhat.com ([209.132.183.28]:3862 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751399Ab3AJCbN (ORCPT ); Wed, 9 Jan 2013 21:31:13 -0500 Date: Thu, 10 Jan 2013 00:30:05 -0200 From: Rafael Aquini To: Rik van Riel Cc: linux-kernel@vger.kernel.org, walken@google.com, eric.dumazet@gmail.com, lwoodman@redhat.com, jeremy@goop.org, Jan Beulich , knoel@redhat.com, chegu_vinod@hp.com, raghavendra.kt@linux.vnet.ibm.com, mingo@redhat.com Subject: Re: [PATCH 2/5] x86,smp: proportional backoff for ticket spinlocks Message-ID: <20130110023004.GB1636@x61.redhat.com> References: <20130108172632.1126898a@annuminas.surriel.com> <20130108173241.3e1b1d2d@annuminas.surriel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20130108173241.3e1b1d2d@annuminas.surriel.com> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jan 08, 2013 at 05:32:41PM -0500, Rik van Riel wrote: > Subject: x86,smp: proportional backoff for ticket spinlocks > > Simple fixed value proportional backoff for ticket spinlocks. > By pounding on the cacheline with the spin lock less often, > bus traffic is reduced. In cases of a data structure with > embedded spinlock, the lock holder has a better chance of > making progress. > > If we are next in line behind the current holder of the > lock, we do a fast spin, so as not to waste any time when > the lock is released. > > The number 50 is likely to be wrong for many setups, and > this patch is mostly to illustrate the concept of proportional > backup. The next patch automatically tunes the delay value. > > Signed-off-by: Rik van Riel > Signed-off-by: Michel Lespinasse > --- Acked-by: Rafael Aquini > arch/x86/kernel/smp.c | 23 ++++++++++++++++++++--- > 1 files changed, 20 insertions(+), 3 deletions(-) > > diff --git a/arch/x86/kernel/smp.c b/arch/x86/kernel/smp.c > index 20da354..aa743e9 100644 > --- a/arch/x86/kernel/smp.c > +++ b/arch/x86/kernel/smp.c > @@ -117,11 +117,28 @@ static bool smp_no_nmi_ipi = false; > */ > void ticket_spin_lock_wait(arch_spinlock_t *lock, struct __raw_tickets inc) > { > + __ticket_t head = inc.head, ticket = inc.tail; > + __ticket_t waiters_ahead; > + unsigned loops; > + > for (;;) { > - cpu_relax(); > - inc.head = ACCESS_ONCE(lock->tickets.head); > + waiters_ahead = ticket - head - 1; > + /* > + * We are next after the current lock holder. Check often > + * to avoid wasting time when the lock is released. > + */ > + if (!waiters_ahead) { > + do { > + cpu_relax(); > + } while (ACCESS_ONCE(lock->tickets.head) != ticket); > + break; > + } > + loops = 50 * waiters_ahead; > + while (loops--) > + cpu_relax(); > > - if (inc.head == inc.tail) > + head = ACCESS_ONCE(lock->tickets.head); > + if (head == ticket) > break; > } > } >