From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932569AbaGWPMW (ORCPT ); Wed, 23 Jul 2014 11:12:22 -0400 Received: from mail-la0-f53.google.com ([209.85.215.53]:46409 "EHLO mail-la0-f53.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932535AbaGWPMR (ORCPT ); Wed, 23 Jul 2014 11:12:17 -0400 MIME-Version: 1.0 In-Reply-To: <20140723142314.GV11241@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> From: Pranith Kumar Date: Wed, 23 Jul 2014 11:11:45 -0400 Message-ID: Subject: Re: [PATCH 09/16] rcu: Remove redundant check for online cpu To: Paul McKenney Cc: Josh Triplett , Steven Rostedt , Mathieu Desnoyers , Lai Jiangshan , "open list:READ-COPY UPDATE..." Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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? 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