From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mout-p-202.mailbox.org (mout-p-202.mailbox.org [80.241.56.172]) (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 9EFC138A299 for ; Fri, 22 May 2026 21:31:35 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=80.241.56.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1779485497; cv=none; b=T6cRyQ+tL4G/EwzjLJLX5vumlZozWIW18sElgFoeSu4Y+PhqKyCgMs0YKoNXMZ8kDYhblhHe4B3udUjAu9gdgmugj6sVvc25L7hqnzW2HHXu6syub4awE4qWCcJS4bHzS8PXN4K8E5RdufRSsKfmsMWcZbCdGt4Swy/1PN/+RK8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1779485497; c=relaxed/simple; bh=vcMNDE+hOT51Fh5tW0+/q5lQ+FknIWM2KMavnMOpM+M=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=Yt3rjobmx00c7XfpbzH7y7Iv6Vv4viT2sX/MsCdJhMYFRMSSeBvOinY2m5w8VbatfP3nAVGa+PabXM+XWLgsdR7b4kVN1ZBLDzwlVFg4jX7CMAodUbqS9rEQArYSWX0fTXh0P/cGb8JhWJhHDmTxCV1ftqTUNxaqXMOG8FRNpRg= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=mailbox.org; spf=pass smtp.mailfrom=mailbox.org; dkim=pass (2048-bit key) header.d=mailbox.org header.i=@mailbox.org header.b=GX2gdHxK; arc=none smtp.client-ip=80.241.56.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=mailbox.org Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=mailbox.org Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=mailbox.org header.i=@mailbox.org header.b="GX2gdHxK" Received: from smtp2.mailbox.org (smtp2.mailbox.org [IPv6:2001:67c:2050:b231:465::2]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (4096 bits) server-digest SHA256) (No client certificate requested) by mout-p-202.mailbox.org (Postfix) with ESMTPS id 4gMdkt0jNJz9v5j; Fri, 22 May 2026 23:31:34 +0200 (CEST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=mailbox.org; s=mail20150812; t=1779485494; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=Ga6yOivf1BItHTEhFihWVIi+//+QToeV8i2rNnNMJZo=; b=GX2gdHxKLfhlpD1ffSz3VUszrQGlD/AcTDi3plZ7JasBy9ZvSZwQuCKMeuN/HT9ra4XPN2 Z9f8z74l3joXxpTL9s1SYHAI7ABGRs4zvrwv6cTo5zFqp0a1ru7akFT9u/cyU9KFkF9v5u 9UOAJd61pX0PvKcZwTx5WO/BNdUC+nCkUa99gcj1ZZNrhsvhdPF9F28Xu+GXFiLnQAUeLo wqWRZuMIzv0fuxb4UnFgFpkVIgVVfz4jFWpABWUT1s4/RMqERHFOWKJCLxEAgxVbp8oqZ/ 83ldIrHRziBGe/ltHLuj1B/NkFWJ1CaUk4gQwCeKNZ2Rbv0ElLNU1XAOPPOMjg== From: Lukas Beckmann To: Sasha Levin Cc: stable@vger.kernel.org, regressions@lists.linux.dev, Mike Galbraith , Peter Zijlstra , Juri Lelli Subject: [PATCH 6.12.y 2/3] sched/deadline: Fix dl_server getting stuck Date: Fri, 22 May 2026 23:31:19 +0200 Message-ID: <20260522213120.1205100-3-lbckmnn@mailbox.org> In-Reply-To: <20260522213120.1205100-1-lbckmnn@mailbox.org> References: <20260522213120.1205100-1-lbckmnn@mailbox.org> Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-MBO-RS-ID: c71a87632b9704e5746 X-MBO-RS-META: gq1c5zginjropqhjymmbbxn5qtdspeeb From: Peter Zijlstra commit 4ae8d9aa9f9dc7137ea5e564d79c5aa5af1bc45c upstream. John found it was easy to hit lockup warnings when running locktorture on a 2 CPU VM, which he bisected down to: commit cccb45d7c429 ("sched/deadline: Less agressive dl_server handling"). While debugging it seems there is a chance where we end up with the dl_server dequeued, with dl_se->dl_server_active. This causes dl_server_start() to return without enqueueing the dl_server, thus it fails to run when RT tasks starve the cpu. When this happens, dl_server_timer() catches the '!dl_se->server_has_tasks(dl_se)' case, which then calls replenish_dl_entity() and dl_server_stopped() and finally return HRTIMER_NO_RESTART. This ends in no new timer and also no enqueue, leaving the dl_server 'dead', allowing starvation. What should have happened is for the bandwidth timer to start the zero-laxity timer, which in turn would enqueue the dl_server and cause dl_se->server_pick_task() to be called -- which will stop the dl_server if no fair tasks are observed for a whole period. IOW, it is totally irrelevant if there are fair tasks at the moment of bandwidth refresh. This removes all dl_se->server_has_tasks() users, so remove the whole thing. 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 [ adjust renamed variable in fair_server_has_tasks (which this patch removes) ] Signed-off-by: Lukas Beckmann --- include/linux/sched.h | 1 - kernel/sched/deadline.c | 12 +----------- kernel/sched/fair.c | 7 +------ kernel/sched/sched.h | 4 ---- 4 files changed, 2 insertions(+), 22 deletions(-) diff --git a/include/linux/sched.h b/include/linux/sched.h index 299a65a92d2e..464d281aa2e4 100644 --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -701,7 +701,6 @@ struct sched_dl_entity { * runnable task. */ struct rq *rq; - dl_server_has_tasks_f server_has_tasks; dl_server_pick_f server_pick_task; #ifdef CONFIG_RT_MUTEXES diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c index 9c5fa95b345a..7a2ea2183975 100644 --- a/kernel/sched/deadline.c +++ b/kernel/sched/deadline.c @@ -916,7 +916,7 @@ static void replenish_dl_entity(struct sched_dl_entity *dl_se) */ if (dl_se->dl_defer && !dl_se->dl_defer_running && dl_time_before(rq_clock(dl_se->rq), dl_se->deadline - dl_se->runtime)) { - if (!is_dl_boosted(dl_se) && dl_se->server_has_tasks(dl_se)) { + if (!is_dl_boosted(dl_se)) { /* * Set dl_se->dl_defer_armed and dl_throttled variables to @@ -1201,8 +1201,6 @@ static void __push_dl_task(struct rq *rq, struct rq_flags *rf) /* a defer timer will not be reset if the runtime consumed was < dl_server_min_res */ static const u64 dl_server_min_res = 1 * NSEC_PER_MSEC; -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 = rq_of_dl_se(dl_se); @@ -1220,12 +1218,6 @@ static enum hrtimer_restart dl_server_timer(struct hrtimer *timer, struct sched_ if (!dl_se->dl_runtime) return HRTIMER_NORESTART; - if (!dl_se->server_has_tasks(dl_se)) { - replenish_dl_entity(dl_se); - dl_server_stopped(dl_se); - return HRTIMER_NORESTART; - } - if (dl_se->dl_defer_armed) { /* * First check if the server could consume runtime in background. @@ -1891,11 +1883,9 @@ static bool dl_server_stopped(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) { dl_se->rq = rq; - dl_se->server_has_tasks = has_tasks; dl_se->server_pick_task = pick_task; } diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index d26e078d0623..f36512892adf 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -9058,11 +9058,6 @@ static struct task_struct *__pick_next_task_fair(struct rq *rq, struct task_stru return pick_next_task_fair(rq, prev, NULL); } -static bool fair_server_has_tasks(struct sched_dl_entity *dl_se) -{ - return !!dl_se->rq->cfs.nr_running; -} - static struct task_struct *fair_server_pick_task(struct sched_dl_entity *dl_se) { return pick_task_fair(dl_se->rq); @@ -9074,7 +9069,7 @@ void fair_server_init(struct rq *rq) init_dl_entity(dl_se); - dl_server_init(dl_se, rq, fair_server_has_tasks, fair_server_pick_task); + dl_server_init(dl_se, rq, fair_server_pick_task); } /* diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h index a09e2d25edd5..9391ff62cdaa 100644 --- a/kernel/sched/sched.h +++ b/kernel/sched/sched.h @@ -371,9 +371,6 @@ extern s64 dl_scaled_delta_exec(struct rq *rq, struct sched_dl_entity *dl_se, s6 * * dl_se::rq -- runqueue we belong to. * - * dl_se::server_has_tasks() -- used on bandwidth enforcement; we 'stop' the - * server when it runs out of tasks to run. - * * dl_se::server_pick() -- nested pick_next_task(); we yield the period if this * returns NULL. * @@ -389,7 +386,6 @@ 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); extern void dl_server_stop(struct sched_dl_entity *dl_se); extern 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); extern void dl_server_update_idle_time(struct rq *rq, -- 2.54.0