From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (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 65A9F25F7B9; Thu, 28 May 2026 20:32:43 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780000364; cv=none; b=DG74ciJ1t+qAvwd+LovY5d+4ipVi/AQv4ivGwbYUopJ1q73No3u8JPZeWGaHr+6IHVS9LDPhGAWv4su235m5hQibXYVFMbddVd89lNy/lo6Y3J4GHFppFSPt/WoQYvtXrKBmoZDT4K7s9LOTFOcJXuDcoxZOKgJLnXA4bQB8B0M= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780000364; c=relaxed/simple; bh=M+ufhVo6aSsOxP8AI4LV88Ac0eiDl/Ug1FNuEaxK+d4=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=gKH3V/ioSdcHq13o4YGlumznGtUiZkZw5FnRj3N2tzu55QPdD0MHTLThoew4/1GpiLcTaub8PeNx9zEs5zqfmXEYlwARuAFBb9OhSGpCC1zABJx6YmKP/1+K+oef2gYcA7aJgux9grhPy7c2nuCWRHoLFr9w5Prp8PBv7dYtDmY= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=loR3yyMr; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b="loR3yyMr" Received: by smtp.kernel.org (Postfix) with ESMTPSA id C37731F000E9; Thu, 28 May 2026 20:32:42 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linuxfoundation.org; s=korg; t=1780000363; bh=lYzNuu6WepgvM2LrtbD1hTcjAUtwatiUHPf8M3hr8G4=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=loR3yyMrE0jEff35M5dj4J57puwvmCYVLKgtL6mZ/6zCxyEbZWWCQikJN14homqXg e7SAyHFjUjutnsUTy4eUF5o9TL/ioI0G/uCYwv1RQqiiUddhIBagz7VccNgOrMQJdh H5Hg+wdPkno54axlEqUpqXHA3dLC0GeIMuxQbpow= From: Greg Kroah-Hartman To: stable@vger.kernel.org Cc: Greg Kroah-Hartman , patches@lists.linux.dev, John Stultz , "Peter Zijlstra (Intel)" , Lukas Beckmann , Sasha Levin Subject: [PATCH 6.12 019/272] sched/deadline: Fix dl_server behaviour Date: Thu, 28 May 2026 21:46:33 +0200 Message-ID: <20260528194629.918741234@linuxfoundation.org> X-Mailer: git-send-email 2.54.0 In-Reply-To: <20260528194629.379955525@linuxfoundation.org> References: <20260528194629.379955525@linuxfoundation.org> User-Agent: quilt/0.69 X-stable: review X-Patchwork-Hint: ignore Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 6.12-stable review patch. If anyone has any objections, please let me know. ------------------ From: Peter Zijlstra commit a3a70caf7906708bf9bbc80018752a6b36543808 upstream. John reported undesirable behaviour with the dl_server since commit: cccb45d7c4295 ("sched/deadline: Less agressive dl_server handling"). When starving fair tasks on purpose (starting spinning FIFO tasks), his fair workload, which often goes (briefly) idle, would delay fair invocations for a second, running one invocation per second was both unexpected and terribly slow. The reason this happens is that when dl_se->server_pick_task() returns NULL, indicating no runnable tasks, it would yield, pushing any later jobs out a whole period (1 second). Instead simply stop the server. This should restore behaviour in that a later wakeup (which restarts the server) will be able to continue running (subject to the CBS wakeup rules). Notably, this does not re-introduce the behaviour cccb45d7c4295 set out to solve, any start/stop cycle is naturally throttled by the timer period (no active cancel). Fixes: cccb45d7c4295 ("sched/deadline: Less agressive dl_server handling") Reported-by: John Stultz Signed-off-by: Peter Zijlstra (Intel) Signed-off-by: Peter Zijlstra (Intel) Tested-by: John Stultz Closes: https://lore.kernel.org/regressions/04657838-46d1-432d-95e1-eb73b930b032@mailbox.org Signed-off-by: Lukas Beckmann Signed-off-by: Sasha Levin --- include/linux/sched.h | 1 - kernel/sched/deadline.c | 23 ++--------------------- kernel/sched/sched.h | 33 +++++++++++++++++++++++++++++++-- 3 files changed, 33 insertions(+), 24 deletions(-) diff --git a/include/linux/sched.h b/include/linux/sched.h index 464d281aa2e49..f9ffe42cae171 100644 --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -674,7 +674,6 @@ 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; /* * Bandwidth enforcement timer. Each -deadline task has its diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c index 609783d7de290..a6c699e43111d 100644 --- a/kernel/sched/deadline.c +++ b/kernel/sched/deadline.c @@ -1621,10 +1621,8 @@ void dl_server_update_idle_time(struct rq *rq, struct task_struct *p) void dl_server_update(struct sched_dl_entity *dl_se, s64 delta_exec) { /* 0 runtime = fair server disabled */ - if (dl_se->dl_runtime) { - dl_se->dl_server_idle = 0; + if (dl_se->dl_runtime) update_curr_dl_se(dl_se->rq, dl_se, delta_exec); - } } /* @@ -1868,20 +1866,6 @@ void dl_server_stop(struct sched_dl_entity *dl_se) dl_se->dl_server_active = 0; } -static bool dl_server_stopped(struct sched_dl_entity *dl_se) -{ - if (!dl_se->dl_server_active) - return true; - - if (dl_se->dl_server_idle) { - dl_server_stop(dl_se); - return true; - } - - dl_se->dl_server_idle = 1; - return false; -} - void dl_server_init(struct sched_dl_entity *dl_se, struct rq *rq, dl_server_pick_f pick_task) { @@ -2637,10 +2621,7 @@ static struct task_struct *__pick_task_dl(struct rq *rq) if (dl_server(dl_se)) { p = dl_se->server_pick_task(dl_se); if (!p) { - if (!dl_server_stopped(dl_se)) { - dl_se->dl_yielded = 1; - update_curr_dl_se(rq, dl_se, 0); - } + dl_server_stop(dl_se); goto again; } rq->dl_server = dl_se; diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h index 9391ff62cdaaa..7956abeb9154e 100644 --- a/kernel/sched/sched.h +++ b/kernel/sched/sched.h @@ -377,10 +377,39 @@ extern s64 dl_scaled_delta_exec(struct rq *rq, struct sched_dl_entity *dl_se, s6 * dl_server_update() -- called from update_curr_common(), propagates runtime * to the server. * - * dl_server_start() - * dl_server_stop() -- start/stop the server when it has (no) tasks. + * dl_server_start() -- start the server when it has tasks; it will stop + * automatically when there are no more tasks, per + * dl_se::server_pick() returning NULL. + * + * dl_server_stop() -- (force) stop the server; use when updating + * parameters. * * dl_server_init() -- initializes the server. + * + * When started the dl_server will (per dl_defer) schedule a timer for its + * zero-laxity point -- that is, unlike regular EDF tasks which run ASAP, a + * server will run at the very end of its period. + * + * This is done such that any runtime from the target class can be accounted + * against the server -- through dl_server_update() above -- such that when it + * becomes time to run, it might already be out of runtime and get deferred + * until the next period. In this case dl_server_timer() will alternate + * between defer and replenish but never actually enqueue the server. + * + * Only when the target class does not manage to exhaust the server's runtime + * (there's actualy starvation in the given period), will the dl_server get on + * the runqueue. Once queued it will pick tasks from the target class and run + * them until either its runtime is exhaused, at which point its back to + * dl_server_timer, or until there are no more tasks to run, at which point + * the dl_server stops itself. + * + * By stopping at this point the dl_server retains bandwidth, which, if a new + * task wakes up imminently (starting the server again), can be used -- + * subject to CBS wakeup rules -- without having to wait for the next period. + * + * Additionally, because of the dl_defer behaviour the start/stop behaviour is + * naturally thottled to once per period, avoiding high context switch + * workloads from spamming the hrtimer program/cancel paths. */ extern void dl_server_update(struct sched_dl_entity *dl_se, s64 delta_exec); extern void dl_server_start(struct sched_dl_entity *dl_se); -- 2.53.0