From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757691AbZKSWvw (ORCPT ); Thu, 19 Nov 2009 17:51:52 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1756818AbZKSWvv (ORCPT ); Thu, 19 Nov 2009 17:51:51 -0500 Received: from bombadil.infradead.org ([18.85.46.34]:39142 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756250AbZKSWvu (ORCPT ); Thu, 19 Nov 2009 17:51:50 -0500 Subject: Re: [PATCH 1/1] sched: move update_curr() in check_preempt_wakeup() to avoid redundant call From: Peter Zijlstra To: Jupyung Lee Cc: LKML , Ingo Molnar In-Reply-To: <1258451500-6714-1-git-send-email-jupyung@gmail.com> References: <1258451500-6714-1-git-send-email-jupyung@gmail.com> Content-Type: text/plain; charset="UTF-8" Date: Thu, 19 Nov 2009 23:51:53 +0100 Message-ID: <1258671113.11284.299.camel@laptop> Mime-Version: 1.0 X-Mailer: Evolution 2.28.1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 2009-11-17 at 18:51 +0900, Jupyung Lee wrote: > Impact: micro-optimization > > If a RT task is woken up while a non-RT task is running, > check_preempt_wakeup() is called to check whether the new task can > preempt the old task. The function returns quickly without going deeper > because it is apparent that a RT task can always preempt a non-RT task. > > In this situation, check_preempt_wakeup() always calls update_curr() to > update vruntime value of the currently running task. However, the function > call is unnecessary and redundant at that moment because (1) a non-RT task can > always be preempted by a RT task regardless of its vruntime value, and > (2) update_curr() will be called shortly when the context switch between two occurs. > > By moving update_curr() in check_preempt_wakeup(), we can avoid redundant > call to update_curr(), slightly reducing the time taken to wake up RT tasks. So far so good, but then you let me figure out why you placed it where you did. Anyway, I think the patch is good and took the patch, thanks! > Signed-off-by: Jupyung Lee > --- > kernel/sched_fair.c | 4 ++-- > 1 files changed, 2 insertions(+), 2 deletions(-) > > diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c > index 37087a7..32256c0 100644 > --- a/kernel/sched_fair.c > +++ b/kernel/sched_fair.c > @@ -1596,8 +1596,6 @@ static void check_preempt_wakeup(struct rq *rq, struct task_struct *p, int wake_ > int sync = wake_flags & WF_SYNC; > int scale = cfs_rq->nr_running >= sched_nr_latency; > > - update_curr(cfs_rq); > - > if (unlikely(rt_prio(p->prio))) { > resched_task(curr); > return; > @@ -1655,6 +1653,8 @@ static void check_preempt_wakeup(struct rq *rq, struct task_struct *p, int wake_ > > BUG_ON(!pse); > > + update_curr(cfs_rq); > + > if (wakeup_preempt_entity(se, pse) == 1) { > resched_task(curr); > /*