From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-5.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id AA221C433DF for ; Wed, 3 Jun 2020 16:52:16 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 8C4EF20679 for ; Wed, 3 Jun 2020 16:52:16 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726151AbgFCQwH (ORCPT ); Wed, 3 Jun 2020 12:52:07 -0400 Received: from foss.arm.com ([217.140.110.172]:35692 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725961AbgFCQwH (ORCPT ); Wed, 3 Jun 2020 12:52:07 -0400 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 20F3A31B; Wed, 3 Jun 2020 09:52:06 -0700 (PDT) Received: from e107158-lin.cambridge.arm.com (e107158-lin.cambridge.arm.com [10.1.195.21]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 6A6C83F305; Wed, 3 Jun 2020 09:52:03 -0700 (PDT) Date: Wed, 3 Jun 2020 17:52:00 +0100 From: Qais Yousef To: Vincent Guittot Cc: Mel Gorman , Patrick Bellasi , Dietmar Eggemann , Peter Zijlstra , Ingo Molnar , Randy Dunlap , Jonathan Corbet , Juri Lelli , Steven Rostedt , Ben Segall , Luis Chamberlain , Kees Cook , Iurii Zaikin , Quentin Perret , Valentin Schneider , Pavan Kondeti , linux-doc@vger.kernel.org, linux-kernel , linux-fs Subject: Re: [PATCH 1/2] sched/uclamp: Add a new sysctl to control RT default boost value Message-ID: <20200603165200.v2ypeagziht7kxdw@e107158-lin.cambridge.arm.com> References: <20200511154053.7822-1-qais.yousef@arm.com> <20200528132327.GB706460@hirez.programming.kicks-ass.net> <20200528155800.yjrmx3hj72xreryh@e107158-lin.cambridge.arm.com> <20200528161112.GI2483@worktop.programming.kicks-ass.net> <20200529100806.GA3070@suse.de> <87v9k84knx.derkling@matbug.net> <20200603101022.GG3070@suse.de> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: User-Agent: NeoMutt/20171215 Sender: linux-fsdevel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On 06/03/20 16:59, Vincent Guittot wrote: > When I want to stress the fast path i usually use "perf bench sched pipe -T " > The tip/sched/core on my arm octo core gives the following results for > 20 iterations of perf bench sched pipe -T -l 50000 > > all uclamp config disabled 50035.4(+/- 0.334%) > all uclamp config enabled 48749.8(+/- 0.339%) -2.64% > > It's quite easy to reproduce and probably easier to study the impact Thanks Vincent. This is very useful! I could reproduce that on my Juno. One of the codepath I was suspecting seems to affect it. diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 0464569f26a7..9f48090eb926 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -1063,10 +1063,12 @@ static inline void uclamp_rq_dec_id(struct rq *rq, struct task_struct *p, * e.g. due to future modification, warn and fixup the expected value. */ SCHED_WARN_ON(bucket->value > rq_clamp); +#if 0 if (bucket->value >= rq_clamp) { bkt_clamp = uclamp_rq_max_value(rq, clamp_id, uc_se->value); WRITE_ONCE(uc_rq->value, bkt_clamp); } +#endif } static inline void uclamp_rq_inc(struct rq *rq, struct task_struct *p) uclamp_rq_max_value() could be expensive as it loops over all buckets. Commenting this whole path out strangely doesn't just 'fix' it, but produces better results to no-uclamp kernel :-/ # ./perf bench -r 20 sched pipe -T -l 50000 Without uclamp: 5039 With uclamp: 4832 With uclamp+patch: 5729 It might be because schedutil gets biased differently by uclamp..? If I move to performance governor these numbers almost double. I don't know. But this promoted me to look closer and I think I spotted a bug where in the if condition we check for '>=' instead of '>', causing us to take the supposedly impossible fail safe path. Mind trying with the below patch please? diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 0464569f26a7..50d66d4016ff 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -1063,7 +1063,7 @@ static inline void uclamp_rq_dec_id(struct rq *rq, struct task_struct *p, * e.g. due to future modification, warn and fixup the expected value. */ SCHED_WARN_ON(bucket->value > rq_clamp); - if (bucket->value >= rq_clamp) { + if (bucket->value > rq_clamp) { bkt_clamp = uclamp_rq_max_value(rq, clamp_id, uc_se->value); WRITE_ONCE(uc_rq->value, bkt_clamp); } Thanks -- Qais Yousef