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=-13.7 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED 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 58A89C433DB for ; Mon, 22 Feb 2021 09:26:15 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 065AC64EC3 for ; Mon, 22 Feb 2021 09:26:14 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230184AbhBVJZx (ORCPT ); Mon, 22 Feb 2021 04:25:53 -0500 Received: from foss.arm.com ([217.140.110.172]:36840 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230044AbhBVJZQ (ORCPT ); Mon, 22 Feb 2021 04:25:16 -0500 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 68B51D6E; Mon, 22 Feb 2021 01:24:28 -0800 (PST) Received: from e124901.cambridge.arm.com (usa-sjc-imap-foss1.foss.arm.com [10.121.207.14]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id DB5A43F73B; Mon, 22 Feb 2021 01:24:26 -0800 (PST) Date: Mon, 22 Feb 2021 09:24:26 +0000 From: Vincent Donnefort To: Vincent Guittot Cc: Peter Zijlstra , Dietmar Eggemann , linux-kernel , Patrick Bellasi , Valentin Schneider Subject: Re: [PATCH] sched/pelt: Fix task util_est update filtering Message-ID: <20210222092426.GA5716@e124901.cambridge.arm.com> References: <20210216163921.572228-1-vincent.donnefort@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Feb 19, 2021 at 11:48:28AM +0100, Vincent Guittot wrote: > On Tue, 16 Feb 2021 at 17:39, wrote: > > > > From: Vincent Donnefort > > > > Being called for each dequeue, util_est reduces the number of its updates > > by filtering out when the EWMA signal is different from the task util_avg > > by less than 1%. It is a problem for a sudden util_avg ramp-up. Due to the > > decay from a previous high util_avg, EWMA might now be close enough to > > the new util_avg. No update would then happen while it would leave > > ue.enqueued with an out-of-date value. > > > > Taking into consideration the two util_est members, EWMA and enqueued for > > the filtering, ensures, for both, an up-to-date value. > > > > This is for now an issue only for the trace probe that might return the > > stale value. Functional-wise, it isn't (yet) a problem, as the value is > > What do you mean by "it isn't (yet) a problem" ? How could this become > a problem ? I wrote "yet" as nothing prevents anyone from using the ue.enqueued signal. > > > always accessed through max(enqueued, ewma). > > > > This adds more tests and or update of struct avg.util_est. It would > be good to have an idea of the perf impact. Especially because this > only fixes a tracing problem I ran hackbench on the big cores of a SD845C board. After 100 iterations of 100 loops runs, the geometric mean of the hackbench test is 0.1% lower with this patch applied (2.0833s vs 2.0858s). The p-value, computed with the ks_2samp [1] is 0.37. We can't conclude that the two distributions are different. This patch, in this scenario seems completely harmless. Shall I include those results in the commit message? [1] https://docs.scipy.org/doc/scipy/reference/generated/scipy.stats.ks_2samp.html > > > > This problem has been observed using LISA's UtilConvergence:test_means on > > the sd845c board. > > > > Signed-off-by: Vincent Donnefort > > > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > > index 794c2cb945f8..9008e0c42def 100644 > > --- a/kernel/sched/fair.c > > +++ b/kernel/sched/fair.c > > @@ -3941,24 +3941,27 @@ static inline void util_est_dequeue(struct cfs_rq *cfs_rq, > > trace_sched_util_est_cfs_tp(cfs_rq); > > } > > > > +#define UTIL_EST_MARGIN (SCHED_CAPACITY_SCALE / 100) > > + > > /* > > - * Check if a (signed) value is within a specified (unsigned) margin, > > + * Check if a (signed) value is within the (unsigned) util_est margin, > > * based on the observation that: > > * > > * abs(x) < y := (unsigned)(x + y - 1) < (2 * y - 1) > > * > > - * NOTE: this only works when value + maring < INT_MAX. > > + * NOTE: this only works when value + UTIL_EST_MARGIN < INT_MAX. > > */ > > -static inline bool within_margin(int value, int margin) > > +static inline bool util_est_within_margin(int value) > > { > > - return ((unsigned int)(value + margin - 1) < (2 * margin - 1)); > > + return ((unsigned int)(value + UTIL_EST_MARGIN - 1) < > > + (2 * UTIL_EST_MARGIN - 1)); > > } > > > > static inline void util_est_update(struct cfs_rq *cfs_rq, > > struct task_struct *p, > > bool task_sleep) > > { > > - long last_ewma_diff; > > + long last_ewma_diff, last_enqueued_diff; > > struct util_est ue; > > > > if (!sched_feat(UTIL_EST)) > > @@ -3979,6 +3982,8 @@ static inline void util_est_update(struct cfs_rq *cfs_rq, > > if (ue.enqueued & UTIL_AVG_UNCHANGED) > > return; > > > > + last_enqueued_diff = ue.enqueued; > > + > > /* > > * Reset EWMA on utilization increases, the moving average is used only > > * to smooth utilization decreases. > > @@ -3992,12 +3997,19 @@ static inline void util_est_update(struct cfs_rq *cfs_rq, > > } > > > > /* > > - * Skip update of task's estimated utilization when its EWMA is > > + * Skip update of task's estimated utilization when its members are > > * already ~1% close to its last activation value. > > */ > > last_ewma_diff = ue.enqueued - ue.ewma; > > - if (within_margin(last_ewma_diff, (SCHED_CAPACITY_SCALE / 100))) > > + last_enqueued_diff -= ue.enqueued; > > + if (util_est_within_margin(last_ewma_diff)) { > > + if (!util_est_within_margin(last_enqueued_diff)) { > > + ue.ewma = ue.enqueued; > > + goto done; > > + } > > + > > return; > > + } > > > > /* > > * To avoid overestimation of actual task utilization, skip updates if > > -- > > 2.25.1 > >