From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758333AbaGWPah (ORCPT ); Wed, 23 Jul 2014 11:30:37 -0400 Received: from e37.co.us.ibm.com ([32.97.110.158]:32892 "EHLO e37.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757971AbaGWPaY (ORCPT ); Wed, 23 Jul 2014 11:30:24 -0400 Date: Wed, 23 Jul 2014 08:30:18 -0700 From: "Paul E. McKenney" To: Pranith Kumar Cc: Josh Triplett , Steven Rostedt , Mathieu Desnoyers , Lai Jiangshan , "open list:READ-COPY UPDATE..." Subject: Re: [PATCH 09/16] rcu: Remove redundant check for online cpu Message-ID: <20140723153018.GY11241@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <1406092194-13004-1-git-send-email-bobby.prani@gmail.com> <1406092194-13004-10-git-send-email-bobby.prani@gmail.com> <20140723122112.GJ11241@linux.vnet.ibm.com> <20140723135024.GR11241@linux.vnet.ibm.com> <20140723142314.GV11241@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 14072315-7164-0000-0000-00000354B561 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jul 23, 2014 at 11:11:45AM -0400, Pranith Kumar wrote: > On Wed, Jul 23, 2014 at 10:23 AM, Paul E. McKenney > wrote: > > On Wed, Jul 23, 2014 at 10:12:54AM -0400, Pranith Kumar wrote: > >> On Wed, Jul 23, 2014 at 9:50 AM, Paul E. McKenney > >> wrote: > >> > On Wed, Jul 23, 2014 at 08:59:06AM -0400, Pranith Kumar wrote: > >> >> On Wed, Jul 23, 2014 at 8:21 AM, Paul E. McKenney > >> >> wrote: > >> >> > On Wed, Jul 23, 2014 at 01:09:46AM -0400, Pranith Kumar wrote: > >> >> >> There are two checks for an online CPU if two if() conditions. This commit > >> >> >> simplies this by replacing it with only one check for the online CPU. > >> >> >> > >> >> >> Signed-off-by: Pranith Kumar > >> >> > > >> >> > I admit that it is very early in the morning my time, but I don't see > >> >> > this change as preserving the semantics in all cases. Please recheck > >> >> > your changes to the second check. > >> >> > > >> >> > Thanx, Paul > >> >> > >> >> I guess you must be thrown off by the complementary checks, the first > >> >> check is for cpu_online() and second is for cpu_is_offline(). :) > >> >> > >> >> Previously, if a cpu is offline, the first condition is false and the > >> >> second condition is true, so we return from the second if() condition. > >> >> The same semantics are being preserved. > >> > > >> > Fair enough! > >> > > >> > Nevertheless, I am not seeing this as a simplification. > >> > >> I am not sure what you mean here, do you mean that both the checks are > >> actually required? > > > > I mean that the current compound tests each mean something. Pulling out > > the offline test adds lines of code and obscures that meaning. This means > > that it is easier (for me, anyway) to see why the current code is correct > > than it is to see why your suggested change is correct. > > > > That is a valid point. I did not mean to reduce readability of the > code. Just trying to avoid the overhead of smp_processor_id(). > > Not sure if you would prefer this, but how about the following? If you change the "awake" to something like "am_online", I could get behind this one. Thanx, Paul > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > index f1ba773..3a26008 100644 > --- a/kernel/rcu/tree.c > +++ b/kernel/rcu/tree.c > @@ -2516,15 +2516,16 @@ static void __call_rcu_core(struct rcu_state > *rsp, struct rcu_data *rdp, > { > bool needwake; > > + bool awake = cpu_online(smp_processor_id); > /* > * If called from an extended quiescent state, invoke the RCU > * core in order to force a re-evaluation of RCU's idleness. > */ > - if (!rcu_is_watching() && cpu_online(smp_processor_id())) > + if (!rcu_is_watching() && awake) > invoke_rcu_core(); > > /* If interrupts were disabled or CPU offline, don't invoke RCU core. */ > - if (irqs_disabled_flags(flags) || cpu_is_offline(smp_processor_id())) > + if (irqs_disabled_flags(flags) || !awake) > return; > > /* > > > > > >> >> -- > >> >> Pranith. > >> >> > >> >> > >> >> > >> >> > > >> >> >> --- > >> >> >> kernel/rcu/tree.c | 9 ++++++--- > >> >> >> 1 file changed, 6 insertions(+), 3 deletions(-) > >> >> >> > >> >> >> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > >> >> >> index 5dcbf36..8d598a2 100644 > >> >> >> --- a/kernel/rcu/tree.c > >> >> >> +++ b/kernel/rcu/tree.c > >> >> >> @@ -2602,15 +2602,18 @@ static void __call_rcu_core(struct rcu_state *rsp, struct rcu_data *rdp, > >> >> >> { > >> >> >> bool needwake; > >> >> >> > >> >> >> + if (!cpu_online(smp_processor_id())) > >> >> >> + return; > >> >> >> + > >> >> >> /* > >> >> >> * If called from an extended quiescent state, invoke the RCU > >> >> >> * core in order to force a re-evaluation of RCU's idleness. > >> >> >> */ > >> >> >> - if (!rcu_is_watching() && cpu_online(smp_processor_id())) > >> >> >> + if (!rcu_is_watching()) > >> >> >> invoke_rcu_core(); > >> >> >> > >> >> >> - /* If interrupts were disabled or CPU offline, don't invoke RCU core. */ > >> >> >> - if (irqs_disabled_flags(flags) || cpu_is_offline(smp_processor_id())) > >> >> >> + /* If interrupts were disabled, don't invoke RCU core. */ > >> >> >> + if (irqs_disabled_flags(flags)) > >> >> >> return; > >> >> >> > >> >> >> /* > >> >> >> -- > >> >> >> 2.0.0.rc2 > >> >> >> > >> >> > > >> >> > >> >> > >> >> > >> >> -- > >> >> Pranith > >> >> > >> > > >> > >> > >> > >> -- > >> Pranith > >> > > > > > > -- > Pranith >