From nobody Wed Dec 17 17:45:09 2025 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id 1ABA013AA2C for ; Mon, 24 Jun 2024 10:24:25 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1719224667; cv=none; b=KJfbEt6RekzdfA7gYcgjEFF9inSNrepPWR0btuQScpe7897Wb9VJS47dClnqhzDOHMMN+I/aSD5wJiDTanSufBaaNvP2hOTLKQhPF3RptxFjshnirwJ9OGiWJrAg/hhwSlvOmSb3W7jnEs2mFZsflEErw/wOnRWvozV05xqUiTI= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1719224667; c=relaxed/simple; bh=cQws8ortMF+x3AGFx4Uqqj8FnUiGqPelfHlVhi+4xhI=; h=From:To:Cc:Subject:Date:Message-Id:In-Reply-To:References: MIME-Version; b=BzCcis+qZwpV0szsvG9DoxT8QnZvOG+vfhvgK8ZseLaVrNTjKr5fHKhNAb9sNTWBSRGr/tJIxRYzuveRyQLXmDOhVcBvJMabSAK6W3RUjY2Etq1tenFVF4YDG9mggIkfn19B4lECNpCqlCSmSgAPuRNBJtMWo8BpFS63YWzQjiI= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass smtp.mailfrom=arm.com; arc=none smtp.client-ip=217.140.110.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arm.com 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 5188FDA7; Mon, 24 Jun 2024 03:24:50 -0700 (PDT) Received: from e130256.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 7122E3F766; Mon, 24 Jun 2024 03:24:23 -0700 (PDT) From: Hongyan Xia To: Ingo Molnar , Peter Zijlstra , Vincent Guittot , Dietmar Eggemann , Juri Lelli , Steven Rostedt , Ben Segall , Mel Gorman , Daniel Bristot de Oliveira , Valentin Schneider Cc: Qais Yousef , Morten Rasmussen , Lukasz Luba , Christian Loehle , Pierre Gondois , Youssef Esmat , linux-kernel@vger.kernel.org Subject: [PATCH 7/7] Propagate negative bias Date: Mon, 24 Jun 2024 11:23:56 +0100 Message-Id: <60985d07acd8a2daf4f3adf31ce4bf3be2982306.1719223916.git.hongyan.xia2@arm.com> X-Mailer: git-send-email 2.34.1 In-Reply-To: References: Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" Negative bias is interesting, because dequeuing such a task will actually increase utilization. Solve by applying PELT decay to negative biases as well. This in fact can be implemented easily with some math tricks. Signed-off-by: Hongyan Xia --- kernel/sched/fair.c | 40 ++++++++++++++++++++++++++++++++++++++++ kernel/sched/sched.h | 4 ++++ 2 files changed, 44 insertions(+) diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 3bb077df52ae..d09af6abf464 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -4878,6 +4878,45 @@ static inline unsigned long root_cfs_util_uclamp(str= uct rq *rq) =20 return max(ret, 0L); } + +/* + * Negative biases are tricky. If we remove them right away then dequeuing= a + * uclamp_max task has the interesting effect that dequeuing results in a = higher + * rq utilization. Solve this by applying PELT decay to the bias itself. + * + * Keeping track of a PELT-decayed negative bias is extra overhead. Howeve= r, we + * observe this interesting math property, where y is the decay factor and= p is + * the number of periods elapsed: + * + * util_new =3D util_old * y^p - neg_bias * y^p + * =3D (util_old - neg_bias) * y^p + * + * Therefore, we simply subtract the negative bias from util_avg the momen= t we + * dequeue, then the PELT signal itself is the total of util_avg and the d= ecayed + * negative bias, and we no longer need to track the decayed bias separate= ly. + */ +static void propagate_negative_bias(struct task_struct *p) +{ + if (task_util_bias(p) < 0 && !task_on_rq_migrating(p)) { + unsigned long neg_bias =3D -task_util_bias(p); + struct sched_entity *se =3D &p->se; + struct cfs_rq *cfs_rq; + + p->se.avg.util_avg_bias =3D 0; + + for_each_sched_entity(se) { + u32 divider, neg_sum; + + cfs_rq =3D cfs_rq_of(se); + divider =3D get_pelt_divider(&cfs_rq->avg); + neg_sum =3D neg_bias * divider; + sub_positive(&se->avg.util_avg, neg_bias); + sub_positive(&se->avg.util_sum, neg_sum); + sub_positive(&cfs_rq->avg.util_avg, neg_bias); + sub_positive(&cfs_rq->avg.util_sum, neg_sum); + } + } +} #else static inline long task_util_bias(struct task_struct *p) { @@ -6869,6 +6908,7 @@ static void dequeue_task_fair(struct rq *rq, struct t= ask_struct *p, int flags) /* At this point se is NULL and we are at root level*/ sub_nr_running(rq, 1); util_bias_dequeue(rq, p); + propagate_negative_bias(p); =20 /* balance early to pull high priority tasks */ if (unlikely(!was_sched_idle && sched_idle_rq(rq))) diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h index 59e5ea421a4c..9d14ef9c717e 100644 --- a/kernel/sched/sched.h +++ b/kernel/sched/sched.h @@ -3140,6 +3140,10 @@ static inline void util_bias_dequeue(struct rq *rq, = struct task_struct *p) { } =20 +static inline void propagate_negative_bias(struct task_struct *p) +{ +} + #endif /* !CONFIG_UCLAMP_TASK */ =20 #ifdef CONFIG_HAVE_SCHED_AVG_IRQ --=20 2.34.1