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=-13.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=ham 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 A5083C11F64 for ; Thu, 1 Jul 2021 11:32:38 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 7EA6361481 for ; Thu, 1 Jul 2021 11:32:38 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S236277AbhGALfH (ORCPT ); Thu, 1 Jul 2021 07:35:07 -0400 Received: from foss.arm.com ([217.140.110.172]:51996 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S236130AbhGALfG (ORCPT ); Thu, 1 Jul 2021 07:35:06 -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 289E66D; Thu, 1 Jul 2021 04:32:36 -0700 (PDT) Received: from e107158-lin.cambridge.arm.com (unknown [10.1.195.57]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 5E9EE3F718; Thu, 1 Jul 2021 04:32:34 -0700 (PDT) Date: Thu, 1 Jul 2021 12:32:32 +0100 From: Qais Yousef To: Xuewen Yan Cc: valentin.schneider@arm.com, mingo@redhat.com, peterz@infradead.org, juri.lelli@redhat.com, vincent.guittot@linaro.org, dietmar.eggemann@arm.com, rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de, bristot@redhat.com, linux-kernel@vger.kernel.org, patrick.bellasi@matbug.net, qperret@google.com Subject: Re: [PATCH v2] sched/uclamp: Avoid getting unreasonable ucalmp value when rq is idle Message-ID: <20210701113232.quqmxjy22udr2hfb@e107158-lin.cambridge.arm.com> References: <20210630141204.8197-1-xuewen.yan94@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20210630141204.8197-1-xuewen.yan94@gmail.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 06/30/21 22:12, Xuewen Yan wrote: > From: Xuewen Yan > > Now in uclamp_rq_util_with(), when the task != NULL, the uclamp_max as following: > uc_rq_max = rq->uclamp[UCLAMP_MAX].value; > uc_eff_max = uclamp_eff_value(p, UCLAMP_MAX); > uclamp_max = max{uc_rq_max, uc_eff_max}; > > Consider the following scenario: > (1)the rq is idle, the uc_rq_max is last runnable task's UCLAMP_MAX; > (2)the p's uc_eff_max < uc_rq_max. > > As a result, the uclamp_max = uc_rq_max instead of uc_eff_max, it is unreasonable. > > The scenario often happens in find_energy_efficient_cpu(), when the task has smaller UCLAMP_MAX. > > When rq has UCLAMP_FLAG_IDLE flag, enqueuing the task will lift UCLAMP_FLAG_IDLE > and set the rq clamp as the task's via uclamp_idle_reset(). It doesn't need > to read the rq clamp. And it can also avoid the problems described above. > > Fixes: 9d20ad7dfc9a ("sched/uclamp: Add uclamp_util_with()") > > Signed-off-by: Xuewen Yan > > --- > change v2: > *add Fixes(Valentin Schneider); > *ignore all rq clamp when idle (Valentin Schneider) > --- > kernel/sched/sched.h | 21 ++++++++++++++------- > 1 file changed, 14 insertions(+), 7 deletions(-) > > diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h > index c80d42e9589b..14a41a243f7b 100644 > --- a/kernel/sched/sched.h > +++ b/kernel/sched/sched.h > @@ -2818,20 +2818,27 @@ static __always_inline > unsigned long uclamp_rq_util_with(struct rq *rq, unsigned long util, > struct task_struct *p) > { > - unsigned long min_util; > - unsigned long max_util; > + unsigned long min_util = 0; > + unsigned long max_util = 0; > > if (!static_branch_likely(&sched_uclamp_used)) > return util; > > - min_util = READ_ONCE(rq->uclamp[UCLAMP_MIN].value); > - max_util = READ_ONCE(rq->uclamp[UCLAMP_MAX].value); > - > if (p) { > - min_util = max(min_util, uclamp_eff_value(p, UCLAMP_MIN)); > - max_util = max(max_util, uclamp_eff_value(p, UCLAMP_MAX)); > + min_util = uclamp_eff_value(p, UCLAMP_MIN); > + max_util = uclamp_eff_value(p, UCLAMP_MAX); > + > + /* > + * Ignore last runnable task's max clamp, as this task will > + * reset it. Similarly, no need to read the rq's min clamp. > + */ > + if (rq->uclamp_flags & UCLAMP_FLAG_IDLE) > + goto out; We read rq->uclamp_flags without locks here. Me thinks this needs READ_ONCE(). But since we care only about a single bit, I can't see any risk for inconsistency, so we're fine. Reviewed-by: Qais Yousef Thanks! -- Qais Yousef > } > > + min_util = max_t(unsigned long, min_util, READ_ONCE(rq->uclamp[UCLAMP_MIN].value)); > + max_util = max_t(unsigned long, max_util, READ_ONCE(rq->uclamp[UCLAMP_MAX].value)); > +out: > /* > * Since CPU's {min,max}_util clamps are MAX aggregated considering > * RUNNABLE tasks with _different_ clamps, we can end up with an > -- > 2.25.1 >