From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752769Ab1GBCSl (ORCPT ); Fri, 1 Jul 2011 22:18:41 -0400 Received: from lo.gmane.org ([80.91.229.12]:38950 "EHLO lo.gmane.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751844Ab1GBCSk (ORCPT ); Fri, 1 Jul 2011 22:18:40 -0400 X-Injected-Via-Gmane: http://gmane.org/ To: linux-kernel@vger.kernel.org From: Paul Turner Subject: Re: [PATCH] sched: Check nr_running before calling pick_next_task in schedule(). Date: Fri, 01 Jul 2011 19:18:26 -0700 Message-ID: <4E0E7FF2.5030209@google.com> References: <1309545711.4303.2.camel@localhost.localdomain> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Complaints-To: usenet@dough.gmane.org Cc: Ingo Molnar , Peter Zijlstra , linux-kernel X-Gmane-NNTP-Posting-Host: 216-239-45-4.google.com User-Agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.2.17) Gecko/20110424 Thunderbird/3.1.10 In-Reply-To: <1309545711.4303.2.camel@localhost.localdomain> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Rakib, This doesn't strike me as a very good trade. It adds a branch to the case where we actually have work to save branches in the case when we're idle anyway? - Paul On 07/01/11 11:41, Rakib Mullick wrote: > Currently at schedule(), when we call pick_next_task we don't check whether current rq is empty or not. Since idle_balance can fail, > its nice to check whether we really have any task on rq or not. If not, we can call idle_sched_class.pick_next_task straight. > > Signed-off-by: Rakib Mullick > --- > > diff --git a/kernel/sched.c b/kernel/sched.c > index 5925275..a4f4f58 100644 > --- a/kernel/sched.c > +++ b/kernel/sched.c > @@ -4273,7 +4273,14 @@ need_resched: > idle_balance(cpu, rq); > > put_prev_task(rq, prev); > - next = pick_next_task(rq); > + /* Since idle_balance can fail, its better to check rq->nr_running. > + * Otherwise we can call idle_sched_class.pick_next_task straight, > + * cause we need to do some accounting. > + */ > + if (likely(rq->nr_running)) > + next = pick_next_task(rq); > + else > + next = idle_sched_class.pick_next_task(rq); > clear_tsk_need_resched(prev); > rq->skip_clock_update = 0; > > >