From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755313AbaDXKAz (ORCPT ); Thu, 24 Apr 2014 06:00:55 -0400 Received: from bombadil.infradead.org ([198.137.202.9]:47926 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753975AbaDXKAv (ORCPT ); Thu, 24 Apr 2014 06:00:51 -0400 Date: Thu, 24 Apr 2014 12:00:47 +0200 From: Peter Zijlstra To: Tim Chen Cc: Ingo Molnar , linux-kernel@vger.kernel.org, Andi Kleen , Len Brown Subject: Re: [PATCH] sched: Skip double execution of pick_next_task_fair Message-ID: <20140424100047.GP11096@twins.programming.kicks-ass.net> References: <1398292317.2970.63.camel@schen9-DESK> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1398292317.2970.63.camel@schen9-DESK> User-Agent: Mutt/1.5.21 (2012-12-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Apr 23, 2014 at 03:31:57PM -0700, Tim Chen wrote: > The current code will call pick_next_task_fair a second time > in the slow path if we did not pull any task in our first try. > This is really unnecessary as we already know no task can > be pulled and it doubles the delay for the cpu to enter idle. > > We instrumented some network workloads and that saw that > pick_next_task_fair is frequently called twice before a cpu enters idle. > The call to pick_next_task_fair can add > non trivial latency as it calls load_balance which runs find_busiest_group > on an hierarchy of sched domains spanning the cpus for a large system. For > some 4 socket systems, we saw almost 0.25 msec spent per call > of pick_next_task_fair before a cpu can be idled. > > This patch skips pick_next_task_fair in the slow path if it > has already been invoked. How about something like so? Its a little more contained. --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -2636,8 +2636,14 @@ pick_next_task(struct rq *rq, struct tas if (likely(prev->sched_class == class && rq->nr_running == rq->cfs.h_nr_running)) { p = fair_sched_class.pick_next_task(rq, prev); - if (likely(p && p != RETRY_TASK)) - return p; + if (unlikely(p == RETRY_TASK)) + goto again; + + /* assumes fair_sched_class->next == idle_sched_class */ + if (unlikely(!p)) + p = pick_next_task_idle(rq, prev); + + return p; } again: