Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932544AbbDMS4D (ORCPT ); Mon, 13 Apr 2015 14:56:03 -0400 Received: from g4t3425.houston.hp.com ([15.201.208.53]:35981 "EHLO g4t3425.houston.hp.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751524AbbDMSz6 (ORCPT ); Mon, 13 Apr 2015 14:55:58 -0400 Message-ID: <1428951347.2605.12.camel@j-VirtualBox> Subject: Re: sched: Improve load balancing in the presence of idle CPUs From: Jason Low To: Srikar Dronamraju Cc: Preeti U Murthy , Morten Rasmussen , "peterz@infradead.org" , "mingo@kernel.org" , Daniel Lezcano , "riel@redhat.com" , "vincent.guittot@linaro.org" , "pjt@google.com" , "benh@kernel.crashing.org" , "efault@gmx.de" , "linux-kernel@vger.kernel.org" , "iamjoonsoo.kim@lge.com" , "svaidy@linux.vnet.ibm.com" , "tim.c.chen@linux.intel.com" , jason.low2@hp.com Date: Mon, 13 Apr 2015 11:55:47 -0700 In-Reply-To: <20150410083741.GB10331@linux.vnet.ibm.com> References: <551A5CCE.70008@linux.vnet.ibm.com> <1427828056.2492.24.camel@j-VirtualBox> <551B9514.80701@linux.vnet.ibm.com> <20150401170418.GX18994@e105550-lin.cambridge.arm.com> <1427954347.2556.43.camel@j-VirtualBox> <551FB5F5.5050906@linux.vnet.ibm.com> <1428449300.2556.79.camel@j-VirtualBox> <1428451666.2556.84.camel@j-VirtualBox> <20150408111216.GA24645@linux.vnet.ibm.com> <1428528169.3506.34.camel@j-VirtualBox> <20150410083741.GB10331@linux.vnet.ibm.com> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.2.3-0ubuntu6 Content-Transfer-Encoding: 7bit Mime-Version: 1.0 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1435 Lines: 36 On Fri, 2015-04-10 at 14:07 +0530, Srikar Dronamraju wrote: > > > > > > > > #ifdef CONFIG_NO_HZ_COMMON > > > > +static inline bool nohz_kick_needed(struct rq *rq); > > > > + > > > > +static inline void pass_nohz_balance(struct rq *this_rq, int this_cpu) > > > > +{ > > > > + clear_bit(NOHZ_BALANCE_KICK, nohz_flags(this_cpu)); > > > > + nohz.next_balance = jiffies; > > > > > > Why are we updating nohz.next_balance here? > > > > This was just to make sure that since we're continuing the balancing on > > another CPU that the nohz next_balance is guaranteed to be "now". > > > > Since we are in nohz_idle_balance(), nohz.next_balance is guaranteed be > less than now. We do check for time_before(now, nohz.next_balance) in > nohz_kick_needed(). So in effect we are incrementing the nohz.next_balance. Hi Srikar, If now is equal to nohz.next_balance, we may attempt nohz_balancer_kick(). After it does nohz.next_balance++ in nohz_balancer_kick(), now can be 1 less than the new nohz.next_balance value by the time nohz_idle_balance() is attempted(). Without updating nohz.next_balance, the time_before(now, nohz.next_balance) check in nohz_kick_needed() may cause it to return false. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/