From nobody Fri Dec 19 14:21:08 2025 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 9AFBA265CAC for ; Tue, 20 May 2025 10:18:53 +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=1747736335; cv=none; b=bmxEesC4Ki80GuHHhmadCi4ip05YX+BsWxy4CRpav6QRVS3suWCWt8bIknMBHm4z941bPmFYCKYkNFO05mCtnzY9p5kmVUDw5NqoRW9dDhTURkdiGxLGa7gbFieKXNYfhK9KruSR9NrOmob/Su7029WMRwJcEpUX1N73MtAaaYw= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1747736335; c=relaxed/simple; bh=ZFZHPQB2gr7QrnWvcowqbVRfTF6upcz8CBZDjNdGW+w=; h=Message-ID:Date:From:To:Cc:Subject:References:MIME-Version: Content-Type; b=qtjEo9Ne5fHqLYffHQN98OVeovBvd6gKo5DKhcshHH7jX/02Dlig7b2IgVM2CPgKk5CeaFo6ebavhg91t66aOIYERPbq/zz6Q7hq+u5XqGSS+Y25Tb25qZiOu669AB+2Eoxza1HB8tfQ7/pNMO8PegBdZ4/Vzzs3ZCQ+DHT89CM= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=none (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=lVTkAkRM; arc=none smtp.client-ip=90.155.50.34 Authentication-Results: smtp.subspace.kernel.org; dmarc=none (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="lVTkAkRM" DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=Content-Type:MIME-Version:References: Subject:Cc:To:From:Date:Message-ID:Sender:Reply-To:Content-Transfer-Encoding: Content-ID:Content-Description:In-Reply-To; bh=dirTgPpnRB9/rGEebkVK3Wz8ypqGQFLT/f1SYZQ6aJs=; b=lVTkAkRMZQqgdbUrym6f8BdRNK nZDbeTn+1EZ111LfCdMraFR5B5Sxw/VbVH07P3HwlH1ZMPkw1RJdsXnYdDxOHiP1F6PTn/6baWjWb 6mG1OUPmBalFGg0+QD0S40fF7FAvzObdP4bLwyvH2J4Yr9tsdc9at2g5w2VVEae5HbvyGcTKWo5kE YrpDlXKX+HzveGc9xX8HXFSr+wtOhoP6UlHeIFTwbLtRBdQjXO2MY1aNua+gPu+AxWXIdeqw+37Cw 8nyiFx3Fy5lvPos8UZIpeJRje1yxbXRvPT0UTQX9r0kvlYSjs1X+Nb3P51sLyj0LrTN4VKVRfHoGg 5CjPtDzQ==; Received: from 77-249-17-252.cable.dynamic.v4.ziggo.nl ([77.249.17.252] helo=noisy.programming.kicks-ass.net) by casper.infradead.org with esmtpsa (Exim 4.98.2 #2 (Red Hat Linux)) id 1uHK3Z-000000038Hf-2SAU; Tue, 20 May 2025 10:18:45 +0000 Received: by noisy.programming.kicks-ass.net (Postfix, from userid 0) id B75FE300780; Tue, 20 May 2025 12:18:44 +0200 (CEST) Message-ID: <20250520101727.507378961@infradead.org> User-Agent: quilt/0.66 Date: Tue, 20 May 2025 11:45:39 +0200 From: Peter Zijlstra To: mingo@redhat.com, juri.lelli@redhat.com, vincent.guittot@linaro.org, dietmar.eggemann@arm.com, rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de, vschneid@redhat.com, clm@meta.com Cc: linux-kernel@vger.kernel.org, peterz@infradead.org Subject: [RFC][PATCH 1/5] sched/deadline: Less agressive dl_server handling References: <20250520094538.086709102@infradead.org> 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" Chris reported that commit 5f6bd380c7bd ("sched/rt: Remove default bandwidth control") caused a significant dip in his favourite benchmark of the day. Simply disabling dl_server cured things. His workload hammers the 0->1, 1->0 transitions, and the dl_server_{start,stop}() overhead kills it -- fairly obviously a bad idea in hind sight and all that. Change things around to only disable the dl_server when there has not been a fair task around for a whole period. Since the default period is 1 second, this ensures the benchmark never trips this, overhead gone. Fixes: 557a6bfc662c ("sched/fair: Add trivial fair server") Reported-by: Chris Mason Signed-off-by: Peter Zijlstra (Intel) --- include/linux/sched.h | 1 + kernel/sched/deadline.c | 31 +++++++++++++++++++++++++++---- 2 files changed, 28 insertions(+), 4 deletions(-) --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -702,6 +702,7 @@ struct sched_dl_entity { unsigned int dl_defer : 1; unsigned int dl_defer_armed : 1; unsigned int dl_defer_running : 1; + unsigned int dl_server_idle : 1; =20 /* * Bandwidth enforcement timer. Each -deadline task has its --- a/kernel/sched/deadline.c +++ b/kernel/sched/deadline.c @@ -1215,6 +1215,8 @@ static void __push_dl_task(struct rq *rq /* a defer timer will not be reset if the runtime consumed was < dl_server= _min_res */ static const u64 dl_server_min_res =3D 1 * NSEC_PER_MSEC; =20 +static bool dl_server_stopped(struct sched_dl_entity *dl_se); + static enum hrtimer_restart dl_server_timer(struct hrtimer *timer, struct = sched_dl_entity *dl_se) { struct rq *rq =3D rq_of_dl_se(dl_se); @@ -1234,6 +1236,7 @@ static enum hrtimer_restart dl_server_ti =20 if (!dl_se->server_has_tasks(dl_se)) { replenish_dl_entity(dl_se); + dl_server_stopped(dl_se); return HRTIMER_NORESTART; } =20 @@ -1639,8 +1642,10 @@ void dl_server_update_idle_time(struct r void dl_server_update(struct sched_dl_entity *dl_se, s64 delta_exec) { /* 0 runtime =3D fair server disabled */ - if (dl_se->dl_runtime) + if (dl_se->dl_runtime) { + dl_se->dl_server_idle =3D 0; update_curr_dl_se(dl_se->rq, dl_se, delta_exec); + } } =20 void dl_server_start(struct sched_dl_entity *dl_se) @@ -1663,7 +1668,7 @@ void dl_server_start(struct sched_dl_ent setup_new_dl_entity(dl_se); } =20 - if (!dl_se->dl_runtime) + if (!dl_se->dl_runtime || dl_se->dl_server_active) return; =20 dl_se->dl_server_active =3D 1; @@ -1672,7 +1677,7 @@ void dl_server_start(struct sched_dl_ent resched_curr(dl_se->rq); } =20 -void dl_server_stop(struct sched_dl_entity *dl_se) +static void __dl_server_stop(struct sched_dl_entity *dl_se) { if (!dl_se->dl_runtime) return; @@ -1684,6 +1689,24 @@ void dl_server_stop(struct sched_dl_enti dl_se->dl_server_active =3D 0; } =20 +static bool dl_server_stopped(struct sched_dl_entity *dl_se) +{ + if (!dl_se->dl_server_active) + return false; + + if (dl_se->dl_server_idle) { + __dl_server_stop(dl_se); + return true; + } + + dl_se->dl_server_idle =3D 1; + return false; +} + +void dl_server_stop(struct sched_dl_entity *dl_se) +{ +} + void dl_server_init(struct sched_dl_entity *dl_se, struct rq *rq, dl_server_has_tasks_f has_tasks, dl_server_pick_f pick_task) @@ -2435,7 +2458,7 @@ static struct task_struct *__pick_task_d if (dl_server(dl_se)) { p =3D dl_se->server_pick_task(dl_se); if (!p) { - if (dl_server_active(dl_se)) { + if (!dl_server_stopped(dl_se)) { dl_se->dl_yielded =3D 1; update_curr_dl_se(rq, dl_se, 0); }