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Thu, 10 Dec 2020 09:32:47 +0000 Received: from outbound-smtp24.blacknight.com ([81.17.249.192]) by merlin.infradead.org with esmtps (Exim 4.92.3 #3 (Red Hat Linux)) id 1knIJY-00069c-6B for linux-arm-kernel@lists.infradead.org; Thu, 10 Dec 2020 09:32:45 +0000 Received: from mail.blacknight.com (pemlinmail03.blacknight.ie [81.17.254.16]) by outbound-smtp24.blacknight.com (Postfix) with ESMTPS id AE913C0B16 for ; Thu, 10 Dec 2020 09:32:41 +0000 (GMT) Received: (qmail 30451 invoked from network); 10 Dec 2020 09:32:41 -0000 Received: from unknown (HELO techsingularity.net) (mgorman@techsingularity.net@[84.203.22.4]) by 81.17.254.9 with ESMTPSA (AES256-SHA encrypted, authenticated); 10 Dec 2020 09:32:41 -0000 Date: Thu, 10 Dec 2020 09:32:40 +0000 From: Mel Gorman To: "Li, Aubrey" Subject: Re: [PATCH 2/4] sched/fair: Move avg_scan_cost calculations under SIS_PROP Message-ID: <20201210093240.GQ3371@techsingularity.net> References: <20201208153501.1467-1-mgorman@techsingularity.net> <20201208153501.1467-3-mgorman@techsingularity.net> <1963d0ca-054c-19f9-94e0-d019a2e8e259@linux.intel.com> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <1963d0ca-054c-19f9-94e0-d019a2e8e259@linux.intel.com> User-Agent: Mutt/1.10.1 (2018-07-13) X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20201210_043244_369291_2AC01114 X-CRM114-Status: GOOD ( 18.24 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Barry Song , Juri Lelli , Vincent Guittot , Peter Ziljstra , LKML , Ingo Molnar , Valentin Schneider , Linux-ARM Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Thu, Dec 10, 2020 at 01:18:05PM +0800, Li, Aubrey wrote: > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > > index ac7b34e7372b..5c41875aec23 100644 > > --- a/kernel/sched/fair.c > > +++ b/kernel/sched/fair.c > > @@ -6153,6 +6153,8 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t > > if (!this_sd) > > return -1; > > > > + cpumask_and(cpus, sched_domain_span(sd), p->cpus_ptr); > > + > > if (sched_feat(SIS_PROP)) { > > u64 avg_cost, avg_idle, span_avg; > > > > @@ -6168,11 +6170,9 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t > > nr = div_u64(span_avg, avg_cost); > > else > > nr = 4; > > - } > > - > > - time = cpu_clock(this); > > > > - cpumask_and(cpus, sched_domain_span(sd), p->cpus_ptr); > > + time = cpu_clock(this); > > + } > > > > for_each_cpu_wrap(cpu, cpus, target) { > > if (!--nr) > > return -1; > > I thought about this again and here seems not to be consistent: > - even if nr reduces to 0, shouldn't avg_scan_cost be updated as well before return -1? You're right, but it's outside the scope of this patch. I noted that this was a problem in lore.kernel.org/r/lore.kernel.org/r/20201203141124.7391-8-mgorman@techsingularity.net It's neither a consistent win or loss to always account for it and so was dropped for this series to keep the number of controversial patches to a minimum. > - if avg_scan_cost is not updated because nr is throttled, the first > time = cpu_clock(this); > can be optimized. As nr is calculated and we already know which of the weight of cpumask and nr is greater. > That is also outside the scope of this patch. To do that, cpumask_weight() would have to be calculated but it's likely to be a net loss. Even under light load, nr will be smaller than the domain weight incurring both the cost of cpumask_weight and the clock read in the common case. -- Mel Gorman SUSE Labs _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel