From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754877Ab2ILWR2 (ORCPT ); Wed, 12 Sep 2012 18:17:28 -0400 Received: from mga09.intel.com ([134.134.136.24]:37137 "EHLO mga09.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753673Ab2ILWR1 (ORCPT ); Wed, 12 Sep 2012 18:17:27 -0400 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="4.80,413,1344236400"; d="scan'208";a="202519559" Subject: Re: [PATCH 2/2] sched: trigger_load_balance clean up From: Suresh Siddha Reply-To: Suresh Siddha To: Alex Shi Cc: mingo@redhat.com, peterz@infradead.org, tglx@linutronix.de, linux-kernel@vger.kernel.org, venki@google.com, "Paul E. McKenney" Date: Wed, 12 Sep 2012 15:16:44 -0700 In-Reply-To: <505040A6.7020608@intel.com> References: <1347261059-24747-1-git-send-email-alex.shi@intel.com> <1347261059-24747-2-git-send-email-alex.shi@intel.com> <1347388574.26695.135.camel@sbsiddha-desk.sc.intel.com> <505040A6.7020608@intel.com> Organization: Intel Corp Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.0.3 (3.0.3-1.fc15) Content-Transfer-Encoding: 7bit Message-ID: <1347488204.26695.230.camel@sbsiddha-desk.sc.intel.com> Mime-Version: 1.0 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 2012-09-12 at 15:58 +0800, Alex Shi wrote: > On 09/12/2012 02:36 AM, Suresh Siddha wrote: > > > On Mon, 2012-09-10 at 15:10 +0800, Alex Shi wrote: > >> Remove a redundant check for on_null_domain(cpu), and rerange the code > >> that make it more readable. > > > > hmm, but we are now doing the on_null_domain() check always, > > irrespective of whether we need the load balance or not. > > > > do we really need the on_null_domain() check there? What happens if we > > just remove it? > > > A very very simple try can not show removing causes crash. But as to > RCU details, I don't know. :( > So the right way to approach the issue is to do couple of "git blame" and see who added that code. My check shows commit 8a0be9ef8225638d26b455788f988c8f84ce9e75 Author: Frederic Weisbecker Date: Thu Mar 5 01:27:02 2009 +0100 sched: don't rebalance if attached on NULL domain And the changelog explains that it has to do with avoiding unnecessary loadbalances (triggered by softirq) for a SMP kernel on a UP system and actually has nothing to do with RCU per-say (as the load balancer already takes the rcu locks etc before traversing the scheduler-domain). And the second on_null_domain() in that function is added by Venki and me, probably by mistake. As the nohz_kick_needed() will never return true for the SMP kernel running on a UP system. So please just remove the second on_null_domain() check in that function instead of consolidating both the usages and include the above info in the changelog. thanks, suresh > > > > thanks, > > suresh > > > >> > >> Signed-off-by: Alex Shi > >> --- > >> kernel/sched/fair.c | 8 +++++--- > >> 1 files changed, 5 insertions(+), 3 deletions(-) > >> > >> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > >> index 5bbc4bf..529092d 100644 > >> --- a/kernel/sched/fair.c > >> +++ b/kernel/sched/fair.c > >> @@ -4934,11 +4934,13 @@ static inline int on_null_domain(int cpu) > >> void trigger_load_balance(struct rq *rq, int cpu) > >> { > >> /* Don't need to rebalance while attached to NULL domain */ > >> - if (time_after_eq(jiffies, rq->next_balance) && > >> - likely(!on_null_domain(cpu))) > >> + if (unlikely(on_null_domain(cpu))) > >> + return; > >> + > >> + if (time_after_eq(jiffies, rq->next_balance)) > >> raise_softirq(SCHED_SOFTIRQ); > >> #ifdef CONFIG_NO_HZ > >> - if (nohz_kick_needed(rq, cpu) && likely(!on_null_domain(cpu))) > >> + if (nohz_kick_needed(rq, cpu)) > >> nohz_balancer_kick(cpu); > >> #endif > >> } > > > > > >