From mboxrd@z Thu Jan 1 00:00:00 1970 From: Peter Zijlstra Subject: Re: [PATCH 1/3] sched: enable interrupts and drop rq-lock duringnewidle balancing Date: Tue, 24 Jun 2008 14:24:59 +0200 Message-ID: <1214310299.4351.27.camel@twins> References: <20080623225645.31515.36393.stgit@lsg.lsg.lab.novell.com> <20080623230440.31515.97377.stgit@lsg.lsg.lab.novell.com> <1214302405.4351.21.camel@twins> <4860BB14.BA47.005A.0@novell.com> Mime-Version: 1.0 Content-Type: text/plain Content-Transfer-Encoding: 7bit Cc: mingo@elte.hu, rostedt@goodmis.org, tglx@linutronix.de, David Bahi , linux-kernel@vger.kernel.org, linux-rt-users@vger.kernel.org To: Gregory Haskins Return-path: Received: from bombadil.infradead.org ([18.85.46.34]:46844 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750766AbYFXMZR (ORCPT ); Tue, 24 Jun 2008 08:25:17 -0400 In-Reply-To: <4860BB14.BA47.005A.0@novell.com> Sender: linux-rt-users-owner@vger.kernel.org List-ID: On Tue, 2008-06-24 at 07:15 -0600, Gregory Haskins wrote: > >>> On Tue, Jun 24, 2008 at 6:13 AM, in message <1214302405.4351.21.camel@twins>, > Peter Zijlstra wrote: > > On Mon, 2008-06-23 at 17:04 -0600, Gregory Haskins wrote: > >> We do find_busiest_groups() et. al. without locks held for normal balancing, > >> so lets do it for newidle as well. It will allow other cpus to make > >> forward progress (against our RQ) while we try to balance and allow > >> some interrupts to occur. > > > > Is running f_b_g really that expensive? > > According to our oprofile data, yes. I speculate that it works out that way because most newidle > attempts result in "no imbalance". But we were spending ~60%+ time in find_busiest_groups() > because of all the heavy-context switching that goes on in PREEMPT_RT. So while f_b_g() is > probably cheaper than double-lock/move_tasks(), the ratio of occurrence is off the charts in > comparison. Prior to this patch, those occurrences were preempt-disabled/irq-disabled/rq->lock critical > sections. > > So while it is not clear if f_b_g() is the actual cost, it is a convenient (and legal, afaict) place to > deterministically reduce the rq->lock scope. Additionally, doing so measurably helps > performance, so I think its a win. Without this patch you have to hope the double_lock releases > this_rq, and even so were not checking for the NEEDS_RESCHED. See, having had this information in the changelog to begin with would have helped ;-)