From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756358Ab1G2LqT (ORCPT ); Fri, 29 Jul 2011 07:46:19 -0400 Received: from casper.infradead.org ([85.118.1.10]:59296 "EHLO casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755622Ab1G2LqS convert rfc822-to-8bit (ORCPT ); Fri, 29 Jul 2011 07:46:18 -0400 Subject: Re: [PATCH] sched: Remove WAKEUP_PREEMPT feature check in entity_tick From: Peter Zijlstra To: Yong Zhang Cc: Lin Ming , "mingo@elte.hu" , lkml , Mike Galbraith In-Reply-To: <20110729084637.GC12106@zhy> References: <1311846203.3938.1555.camel@minggr.sh.intel.com> <20110729062158.GA8971@zhy> <1311922180.3938.1573.camel@minggr.sh.intel.com> <20110729070356.GA10420@zhy> <1311924975.3938.1583.camel@minggr.sh.intel.com> <20110729074635.GB10420@zhy> <1311926218.5890.215.camel@twins> <20110729081800.GA12106@zhy> <1311927650.5890.217.camel@twins> <20110729084637.GC12106@zhy> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 8BIT Date: Fri, 29 Jul 2011 13:45:33 +0200 Message-ID: <1311939933.5890.341.camel@twins> Mime-Version: 1.0 X-Mailer: Evolution 2.30.3 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, 2011-07-29 at 16:46 +0800, Yong Zhang wrote: > Let's take UP for example, we have cpu-hug task A and threadirq B. > > n tick ---> n+1 tick > set_tsk_need_resched(A); > B comes in and > wake up thread-B; > > So for system on which we disable WAKEUP_PREEMPT, > if we don't have that check, thread-B will wait until n+1 tick comes > to get to run. > But if we have that check, thread-B will get to run after IRQ-B returns. But that's exactly what wakeup preemption is about, waking tasks don't get to preempt running tasks. So no doing that preemption is exactly right for !WAKEUP_PREEMPT. Anyway, I've queued the removal patch since that removes all confusion ;-)