From nobody Wed Oct 8 05:39:11 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 1B198274668 for ; Wed, 2 Jul 2025 12:13:01 +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=1751458382; cv=none; b=r6o2KKn6M2jaQ+ZBT3oRTuTD5CB99QXIzd7ZYun/4QN3nZaFiOxNpMWi3SKsj+kGaNn4LAPMfY1A702iu6Qx+blCnWwUKKUcjM7NfZgJeAod2yz8FzkCl6oyzHXxU8reVfPOcaBMUn0ziIWcjaQZ2Rl6SfrjCz6aUeb/U9ySao8= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1751458382; c=relaxed/simple; bh=98o4EUwZvkaJiNLHs0TNmxNMeC//8w5lyaohIp/i0gc=; h=Message-ID:Date:From:To:Cc:Subject:References:MIME-Version: Content-Type; b=reJHuaEoV95pyn95bSTdBF4ZMAguQMOmUGnl+eu0ki+hofgB5+RAYQ08F5zpjus0cMiVtS8ldh98xcpi/citpbfCIM5nQmnu9M9BKRrdHiMR5V76OIx7g3GwQ9qYaCCeMWis0viCIkl3lqMh6KvRvl8wEZ+xCmnq5HJ+EUFE4Ys= 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=lpIgoQbS; 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="lpIgoQbS" 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=QQTEg01HMGHcuECpwoqe+vJNXoQ2Sir/0YX71f4+jJQ=; b=lpIgoQbS7GXrdriJDYOyAFWJ6j 07riU9eKQpNScyeKeEC46zoeabwfcpKGP+sf95E2DxPnwjxyooJtAUwpa9hV2oH9q+n9O2cSBAeAZ 9rtPDG9ognst698Hdf31ckB4zwoQJh99bZeS8eNaEcYiA4WJtrvzsKJczi4NBG8XtQESTsOm4+8Zf /kbmeNydesD5NpOEk3euXA5dJBXCcwhGYnSQLBzdCcf4mD+morqgqFX117k54oOuVrR9JQvT5OF9q e+J42+MijhuNVLsiCPv4iXU9GbZlL3oqdFoIfbUxQHT9CVifY1IYAUxmpA7MALsHbrjqT/JbUfvGP 8gKJXY/g==; 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 1uWwKV-00000009lF1-3VB2; Wed, 02 Jul 2025 12:12:47 +0000 Received: by noisy.programming.kicks-ass.net (Postfix, from userid 0) id BF363300158; Wed, 02 Jul 2025 14:12:46 +0200 (CEST) Message-ID: <20250702121158.465086194@infradead.org> User-Agent: quilt/0.68 Date: Wed, 02 Jul 2025 13:49:26 +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: [PATCH v2 02/12] sched/deadline: Less agressive dl_server handling References: <20250702114924.091581796@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) Link: https://lkml.kernel.org/r/20250520101727.507378961@infradead.org Acked-by: Juri Lelli Acked-by: Mel Gorman Reviewed-by: Juri Lelli --- include/linux/sched.h | 1 + kernel/sched/deadline.c | 25 ++++++++++++++++++++++--- kernel/sched/fair.c | 9 --------- 3 files changed, 23 insertions(+), 12 deletions(-) --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -701,6 +701,7 @@ 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; @@ -1684,6 +1689,20 @@ 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_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 +2454,7 @@ 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); } --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -5879,7 +5879,6 @@ static bool throttle_cfs_rq(struct cfs_r struct cfs_bandwidth *cfs_b =3D tg_cfs_bandwidth(cfs_rq->tg); struct sched_entity *se; long queued_delta, runnable_delta, idle_delta, dequeue =3D 1; - long rq_h_nr_queued =3D rq->cfs.h_nr_queued; =20 raw_spin_lock(&cfs_b->lock); /* This will start the period timer if necessary */ @@ -5963,10 +5962,6 @@ static bool throttle_cfs_rq(struct cfs_r =20 /* At this point se is NULL and we are at root level*/ sub_nr_running(rq, queued_delta); - - /* Stop the fair server if throttling resulted in no runnable tasks */ - if (rq_h_nr_queued && !rq->cfs.h_nr_queued) - dl_server_stop(&rq->fair_server); done: /* * Note: distribution will already see us throttled via the @@ -7060,7 +7055,6 @@ static void set_next_buddy(struct sched_ static int dequeue_entities(struct rq *rq, struct sched_entity *se, int fl= ags) { bool was_sched_idle =3D sched_idle_rq(rq); - int rq_h_nr_queued =3D rq->cfs.h_nr_queued; bool task_sleep =3D flags & DEQUEUE_SLEEP; bool task_delayed =3D flags & DEQUEUE_DELAYED; struct task_struct *p =3D NULL; @@ -7144,9 +7138,6 @@ static int dequeue_entities(struct rq *r =20 sub_nr_running(rq, h_nr_queued); =20 - if (rq_h_nr_queued && !rq->cfs.h_nr_queued) - dl_server_stop(&rq->fair_server); - /* balance early to pull high priority tasks */ if (unlikely(!was_sched_idle && sched_idle_rq(rq))) rq->next_balance =3D jiffies;