From nobody Sat Feb 7 21:15:52 2026 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 989F5EB64D9 for ; Tue, 27 Jun 2023 19:12:55 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230003AbjF0TMy (ORCPT ); Tue, 27 Jun 2023 15:12:54 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:44548 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229813AbjF0TMw (ORCPT ); Tue, 27 Jun 2023 15:12:52 -0400 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id C813F11B for ; Tue, 27 Jun 2023 12:12:07 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1687893126; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding; bh=HuK+Pfp2LtRqUdprChnENfF2XD5swfpqbrkFMD6+4eo=; b=g8EWavW7OwkWJI2e0bzv4THcBnVTgREpE1UmxuW5YLbOuiLTUkBBSSYcLtOCUUbXoYu42L YQmYLb/RljPKSA2IdD/43QlsvUEyHgYYuURaTc3xsPXZc0r+yvGgw28Gb8O+2c1d1+R3rg uLEPgZ/PHI2Ym+AT2U71X7qyF8soLmQ= Received: from mimecast-mx02.redhat.com (mx3-rdu2.redhat.com [66.187.233.73]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-251-KnU2fPdfNpSGjrK6aEt_9g-1; Tue, 27 Jun 2023 15:12:04 -0400 X-MC-Unique: KnU2fPdfNpSGjrK6aEt_9g-1 Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.rdu2.redhat.com [10.11.54.5]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 4D0A6381495C; Tue, 27 Jun 2023 19:12:03 +0000 (UTC) Received: from pauld.bos.com (dhcp-17-165.bos.redhat.com [10.18.17.165]) by smtp.corp.redhat.com (Postfix) with ESMTP id D5772F5CDB; Tue, 27 Jun 2023 19:12:02 +0000 (UTC) From: Phil Auld To: linux-kernel@vger.kernel.org Cc: Juri Lelli , Ingo Molnar , Daniel Bristot de Oliveira , Peter Zijlstra , Vincent Guittot , Dietmar Eggemann , Valentin Schneider , Ben Segall , Steven Rostedt , Mel Gorman , Phil Auld Subject: [PATCH v2] Sched/fair: Block nohz tick_stop when cfs bandwidth in use Date: Tue, 27 Jun 2023 15:12:01 -0400 Message-Id: <20230627191201.344110-1-pauld@redhat.com> MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable X-Scanned-By: MIMEDefang 3.1 on 10.11.54.5 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Type: text/plain; charset="utf-8" CFS bandwidth limits and NOHZ full don't play well together. Tasks can easily run well past their quotas before a remote tick does accounting. This leads to long, multi-period stalls before such tasks can run again. Currentlyi, when presented with these conflicting requirements the scheduler is favoring nohz_full and letting the tick be stopped. However, nohz tick stopping is already best-effort, there are a number of conditions that can prevent it, whereas cfs runtime bandwidth is expected to be enforced. Make the scheduler favor bandwidth over stopping the tick by setting TICK_DEP_BIT_SCHED when the only running task is a cfs task with runtime limit enabled. Add sched_feat HZ_BW (off by default) to control this behavior. Signed-off-by: Phil Auld Cc: Ingo Molnar Cc: Peter Zijlstra Cc: Vincent Guittot Cc: Juri Lelli Cc: Dietmar Eggemann Cc: Valentin Schneider Cc: Ben Segall --- v2: Ben pointed out that the bit could get cleared in the dequeue path if we migrate a newly enqueued task without preempting curr. Added a=20 check for that edge case to sched_can_stop_tick. Removed the call to=20 sched_can_stop_tick from sched_fair_update_stop_tick since it was=20 redundant. kernel/sched/core.c | 12 +++++++++++ kernel/sched/fair.c | 45 +++++++++++++++++++++++++++++++++++++++++ kernel/sched/features.h | 2 ++ 3 files changed, 59 insertions(+) diff --git a/kernel/sched/core.c b/kernel/sched/core.c index a68d1276bab0..646f60bfc7e7 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -1194,6 +1194,8 @@ static void nohz_csd_func(void *info) #endif /* CONFIG_NO_HZ_COMMON */ =20 #ifdef CONFIG_NO_HZ_FULL +extern bool sched_cfs_bandwidth_active(struct cfs_rq *cfs_rq); + bool sched_can_stop_tick(struct rq *rq) { int fifo_nr_running; @@ -1229,6 +1231,16 @@ bool sched_can_stop_tick(struct rq *rq) if (rq->nr_running > 1) return false; =20 + /* + * If there is one task and it has CFS runtime bandwidth constraints + * and it's on the cpu now we don't want to stop the tick. + */ + if (sched_feat(HZ_BW) && rq->nr_running =3D=3D 1 && rq->curr + && rq->curr->sched_class =3D=3D &fair_sched_class && task_on_rq_queue= d(rq->curr)) { + if (sched_cfs_bandwidth_active(task_cfs_rq(rq->curr))) + return false; + } + return true; } #endif /* CONFIG_NO_HZ_FULL */ diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 373ff5f55884..a05af33b8da9 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -6139,6 +6139,42 @@ static void __maybe_unused unthrottle_offline_cfs_rq= s(struct rq *rq) rcu_read_unlock(); } =20 +#ifdef CONFIG_NO_HZ_FULL + +bool sched_cfs_bandwidth_active(struct cfs_rq *cfs_rq) +{ + if (cfs_bandwidth_used() && cfs_rq->runtime_enabled) + return true; + + return false; +} + +/* called from pick_next_task_fair() */ +static void sched_fair_update_stop_tick(struct rq *rq, struct task_struct = *p) +{ + struct cfs_rq *cfs_rq =3D task_cfs_rq(p); + int cpu =3D cpu_of(rq); + + if (!sched_feat(HZ_BW) || !cfs_bandwidth_used()) + return; + + if (!tick_nohz_full_cpu(cpu)) + return; + + if (rq->nr_running !=3D 1) + return; + + /* + * We know there is only one task runnable and we've just picked it. The + * normal enqueue path will have cleared TICK_DEP_BIT_SCHED if we will + * be otherwise able to stop the tick. Just need to check if we are using + * bandwidth control. + */ + if (cfs_rq->runtime_enabled) + tick_nohz_dep_set_cpu(cpu, TICK_DEP_BIT_SCHED); +} +#endif + #else /* CONFIG_CFS_BANDWIDTH */ =20 static inline bool cfs_bandwidth_used(void) @@ -6181,9 +6217,17 @@ static inline struct cfs_bandwidth *tg_cfs_bandwidth= (struct task_group *tg) static inline void destroy_cfs_bandwidth(struct cfs_bandwidth *cfs_b) {} static inline void update_runtime_enabled(struct rq *rq) {} static inline void unthrottle_offline_cfs_rqs(struct rq *rq) {} +bool sched_cfs_bandwidth_active(struct cfs_rq *cfs_rq) +{ + return false; +} =20 #endif /* CONFIG_CFS_BANDWIDTH */ =20 +#if !defined(CONFIG_CFS_BANDWIDTH) || !defined(CONFIG_NO_HZ_FULL) +static inline void sched_fair_update_stop_tick(struct rq *rq, struct task_= struct *p) {} +#endif + /************************************************** * CFS operations on tasks: */ @@ -8097,6 +8141,7 @@ done: __maybe_unused; hrtick_start_fair(rq, p); =20 update_misfit_status(p, rq); + sched_fair_update_stop_tick(rq, p); =20 return p; =20 diff --git a/kernel/sched/features.h b/kernel/sched/features.h index ee7f23c76bd3..6fdf1fdf6b17 100644 --- a/kernel/sched/features.h +++ b/kernel/sched/features.h @@ -101,3 +101,5 @@ SCHED_FEAT(LATENCY_WARN, false) =20 SCHED_FEAT(ALT_PERIOD, true) SCHED_FEAT(BASE_SLICE, true) + +SCHED_FEAT(HZ_BW, false) --=20 2.31.1