From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from casper.infradead.org (casper.infradead.org [90.155.50.34]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 098A9296BD3 for ; Mon, 30 Mar 2026 14:07:00 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=90.155.50.34 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1774879623; cv=none; b=X+Ysl3fujKakmOqffsR+8SRyWw3fhWwT30EdOEeBoku1dp1RbPuRVMiugC1X8pbxPY6gt2fGzYr2MBqewP2xG4qnAa69UTETcRSRF1wd+JQP6JQ096YnYpbxJnOWDymHnbVyv2sSfc/VL6/7A9HnUmDM0WngddKfpJ6zMqaKxuw= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1774879623; c=relaxed/simple; bh=OC3XfzIGX21gOQyo0/WVC4wkT4mwzsTyXmyn6+p6aQk=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=dyWhm2LAJ4nIKXICels/ht48qpVmvJdJsByAgogZO+2nBFGIJqg6BhPd9BcGZ9wmhvsLWY39mxcWIGha7C9J3jwNYE68+qCaKJVV5/5J/mlEBu/5umfh2EKq6V6uW1e1PcPObtWFbf+MjssZSQjJO3bb3ZHzcsJWD84vO38JbkU= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=infradead.org; spf=none smtp.mailfrom=infradead.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b=l4DSXMQf; arc=none smtp.client-ip=90.155.50.34 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=infradead.org Authentication-Results: smtp.subspace.kernel.org; spf=none smtp.mailfrom=infradead.org Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="l4DSXMQf" DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=sm+YoMN636dc1II8tVVuIRV/lCH+ot049lLFK73jwhc=; b=l4DSXMQfmTMmLA4K476ntaQqI4 +/Agisxo86ZVyXkq9+p0Ph1O7jQuopGNiNaep/LMdCbezmcUm7PJ2J1s5fpkp7wEcdRB0fl8WoTim L9zMHKaEChI0LIdgSP4dhZ16zee/HoaY45K3zJJSK4RG/oCn6XaE9u66HVF2FWMkIhEq0QMrx1kk7 W/LzZZoQYrdBN91tkCKIES4PiXJgbL/goCy0tjEc1aLPV+QFOAEUFwGDNpXijVFAUj5nqIpJkJ1LY TNs6XTdHrsdtI7qyxPJihtHjJIy3Noe/7rnwGlH2OkbEO4QAGw/p8Vc4+Uq4NsEZr9OSMWcbZT/Q3 ECtDkaXQ==; Received: from 2001-1c00-8d85-4b00-266e-96ff-fe07-7dcc.cable.dynamic.v6.ziggo.nl ([2001:1c00:8d85:4b00:266e:96ff:fe07:7dcc] helo=noisy.programming.kicks-ass.net) by casper.infradead.org with esmtpsa (Exim 4.98.2 #2 (Red Hat Linux)) id 1w7DGY-00000006mDn-2xxw; Mon, 30 Mar 2026 14:06:54 +0000 Received: by noisy.programming.kicks-ass.net (Postfix, from userid 1000) id 4E110301BDE; Mon, 30 Mar 2026 16:06:53 +0200 (CEST) Date: Mon, 30 Mar 2026 16:06:53 +0200 From: Peter Zijlstra To: Vincent Guittot Cc: mingo@redhat.com, juri.lelli@redhat.com, dietmar.eggemann@arm.com, rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de, vschneid@redhat.com, kprateek.nayak@amd.com, linux-kernel@vger.kernel.org Subject: Re: [PATCH] sched: fair: Prevent negative lag increase during delayed dequeue Message-ID: <20260330140653.GO3738010@noisy.programming.kicks-ass.net> References: <20260327180219.2879905-1-vincent.guittot@linaro.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260327180219.2879905-1-vincent.guittot@linaro.org> On Fri, Mar 27, 2026 at 07:02:19PM +0100, Vincent Guittot wrote: > Delayed dequeue feature aims to reduce the negative lag of a dequeued task > while sleeping but it can happens that newly enqueued tasks will move > backward the avg vruntime and increase its negative lag. > When the delayed dequeued task wakes up, it has more neg lag compared to > being dequeued immediately or to other delayed or not tasks that have been > dequeued just before theses new enqueues. > > Ensure that the negative lag of a delayed dequeued task doesn't increase > during its delayed dequeued phase while waiting for its neg lag to > diseappear. Similarly, we remove any positive lag that the delayed > dequeued task could have gain during thsi period. *groan*, indeed! > Signed-off-by: Vincent Guittot > --- > kernel/sched/fair.c | 9 +++++++-- > 1 file changed, 7 insertions(+), 2 deletions(-) > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index 226509231e67..efa9dfa8c583 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -5595,6 +5595,7 @@ dequeue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags) > if (sched_feat(DELAY_DEQUEUE) && delay && > !entity_eligible(cfs_rq, se)) { > update_load_avg(cfs_rq, se, 0); > + update_entity_lag(cfs_rq, se); > set_delayed(se); > return false; > } > @@ -7089,12 +7090,16 @@ requeue_delayed_entity(struct sched_entity *se) > WARN_ON_ONCE(!se->on_rq); > > if (sched_feat(DELAY_ZERO)) { > + s64 vlag, prev_vlag = se->vlag; > update_entity_lag(cfs_rq, se); > - if (se->vlag > 0) { > + /* prev_vlag < 0 otherwise se would not be delayed */ > + vlag = clamp(se->vlag, prev_vlag, 0); > + > + if (vlag != se->vlag) { > cfs_rq->nr_queued--; > if (se != cfs_rq->curr) > __dequeue_entity(cfs_rq, se); > - se->vlag = 0; > + se->vlag = vlag; > place_entity(cfs_rq, se, 0); > if (se != cfs_rq->curr) > __enqueue_entity(cfs_rq, se); So I think this should apply irrespective of DELAY_ZERO -- although since we default to that, it is the most relevant. Also, I'm thinking we might want a comment with this. We're bound to forget the details at some point. Also, there is one other site that has DLAY_ZERO/clear_delayed(), does that want updating too?