From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751568AbdBWR4l (ORCPT ); Thu, 23 Feb 2017 12:56:41 -0500 Received: from merlin.infradead.org ([205.233.59.134]:48674 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751125AbdBWR4O (ORCPT ); Thu, 23 Feb 2017 12:56:14 -0500 Date: Thu, 23 Feb 2017 18:54:38 +0100 From: Peter Zijlstra To: Pavan Kondeti Cc: Steven Rostedt , LKML , Ingo Molnar , Andrew Morton Subject: Re: [PATCH] sched: Optimize pick_next_task for idle_sched_class too Message-ID: <20170223175438.GX6536@twins.programming.kicks-ass.net> References: <20170119101703.2abeaeb6@gandalf.local.home> <20170119174408.GN6485@twins.programming.kicks-ass.net> <20170223135458.GC6515@twins.programming.kicks-ass.net> <20170223152533.GG6515@twins.programming.kicks-ass.net> <20170223163737.GW6536@twins.programming.kicks-ass.net> <20170223174505.GI6515@twins.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170223174505.GI6515@twins.programming.kicks-ass.net> User-Agent: Mutt/1.5.23.1 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Feb 23, 2017 at 06:45:05PM +0100, Peter Zijlstra wrote: > Hurm.. maybe we should do what Steve initially suggested. The > alternative is link order trickery, and I'm not sure we want to do that. That is, given: kernel/sched/Makefile: obj-y += idle_task.o fair.o rt.o deadline.o stop_task.o results in: readelf -s defconfig-build/vmlinux | awk '/sched_class/ {print $2 " " $8}' | sort -n 00000000602c93c0 idle_sched_class 00000000602c9480 fair_sched_class 00000000602c9580 rt_sched_class 00000000602c96c0 dl_sched_class 00000000602c97c0 stop_sched_class we can do this, but yuck! --- kernel/sched/core.c | 12 +++++++++--- 1 file changed, 9 insertions(+), 3 deletions(-) diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 8f972df76eb2..eebe6729ceb7 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -3285,10 +3285,16 @@ pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf) struct task_struct *p; /* - * Optimization: we know that if all tasks are in - * the fair class we can call that function directly: + * Optimization: we know that if all tasks are in the fair class we can + * call that function directly, but only if the @prev task wasn't of a + * higher scheduling class, because otherwise those loose the + * opportinity to pull in more work from other CPUs. + * + * Depends on link order in kernel/sched/Makefile. */ - if (likely(rq->nr_running == rq->cfs.h_nr_running)) { + if (likely(rq->nr_running == rq->cfs.h_nr_running && + prev->sched_class <= &fair_sched_class)) { + p = fair_sched_class.pick_next_task(rq, prev, rf); if (unlikely(p == RETRY_TASK)) goto again;