From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=100.103.45.18 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1792594934; b=BUgXgehSAvvN6VwQIMls4zSlsHJyhwPyaiQcAxyrOKBnzfSuoseu5Uia3tDDPClQvIc+ wA3ijDxlDHtdtCQLUb2KHWHMPXDcZnBn46sBVagIpyQy+LucnUvGgkXnKE4uFUyRECEwE FNWD354XpySzvncRgeMNIKCU8cyaM+gJ0SSMV+8Ymq28/aXk9E78huYg4GG/FUOWb23La /euNY9ThwkKtuImsbfcQbk0DmC38fCrRhOd62pRjoGm2eA0Zdm4XTj8oSMcE3bP0ywEJl RIoSVKNyKN4fwVlYpD+L6AtgChTB3yK+DwAfuYEzqWFPuqOboNOSg29sgzOARHpPldw== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1792594934; h=Received:DKIM-Signature:Subject:To:Cc:From:Date:Message-ID: MIME-Version:Content-Type:Content-Transfer-Encoding; bh=rT0lcj78m3hKLq1gI4Qh637rM2DKuQ8cAxTfZ+P58Po=; b=SzjfDHiBYrsLAJSL+oEVbKKCussw2PIQei2GafIworh0hSjgS1DEaHVxw2/EE49AGSZ6 SlUCGRMO2hpFUXgSE5o2P0FQ0d7qEIaX307jSlXATqk1Ndx2ZSkdoQrz5jQretujv2Ljj JCM+ypDlF9u2RyyepZRfe7mBrCyDUR0DrqTbshOCZidrFSytRfmpPg8J4lj30esoQsMTP cdRuAmF6UVF+sLCFl/hBf4rOj5PwGNTKFebJDLcoXqDG2CSTPo1ZWZrL8i6vHQP29ROEM IBhjBDaxlWcozna7UFsFxXTtvtRFYhk3veFf996xXoXoDoFlkeYmscZwwqvTIQVRaGw== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass header.d=linuxfoundation.org header.i=@linuxfoundation.org header.a=rsa-sha256 header.s=korg header.b="dUh/9HPU"; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id D2CCF606A319 for ; Wed, 21 Oct 2026 15:02:13 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key, unprotected) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.a=rsa-sha256 header.s=korg header.b=dUh/9HPU Received: by smtp.kernel.org (Postfix) with ESMTPSA id E94C51F000C1; Wed, 21 Oct 2026 15:02:12 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linuxfoundation.org; s=korg; t=1792594933; bh=rT0lcj78m3hKLq1gI4Qh637rM2DKuQ8cAxTfZ+P58Po=; h=Subject:To:Cc:From:Date; b=dUh/9HPU1jTd1sqKvtLfeJsqecpZpilhmQwGRl8u7luLv+v7V1/GcEu3i9jyHKaWW L0mWC8TlXCcYrQWkDcIkFnvx/MImexFkP5jQzx4g/IhHZNJN3jvJ0AfnDPFCq3ZTRg k2zvqhmXSDY7/N6t55K1pDUZ6qVdnFBjxt4CixSw= Subject: FAILED: patch "[PATCH] sched_ext: Fix stall when a task enqueued with SCX_ENQ_LAST" failed to apply to 6.12-stable tree To: tj@kernel.org,schatzberg.dan@gmail.com,xbu@meta.com Cc: From: Date: Wed, 21 Oct 2026 16:50:01 +0200 Message-ID: <2026102101-kilogram-periscope-454b@gregkh> X-Mailing-List: stable@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=ANSI_X3.4-1968 Content-Transfer-Encoding: 8bit The patch below does not apply to the 6.12-stable tree. If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to . To reproduce the conflict and resubmit, you may use the following commands: git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.12.y git checkout FETCH_HEAD git cherry-pick -x 7a9df76679633cf26e0fea882a684012c6ec245a # git commit -s git send-email --to '' --in-reply-to '2026102101-kilogram-periscope-454b@gregkh' --subject-prefix 'PATCH 6.12.y' 'HEAD^..' Possible dependencies: thanks, greg k-h ------------------ original commit in Linus's tree ------------------ >From 7a9df76679633cf26e0fea882a684012c6ec245a Mon Sep 17 00:00:00 2001 From: Tejun Heo Date: Wed, 7 Oct 2026 06:26:47 -1000 Subject: [PATCH] sched_ext: Fix stall when a task enqueued with SCX_ENQ_LAST lands back on its CPU's local DSQ With SCX_OPS_ENQ_LAST, the last runnable task on a CPU is passed to ops.enqueue() instead of being kept running. Exiting tasks without SCX_OPS_ENQ_EXITING, migration disabled tasks without SCX_OPS_ENQ_MIGRATION_DISABLED and tasks on an offline rq skip ops.enqueue() and go straight to the local DSQ. This happens in put_prev_task_scx() after the pick has settled on idle, so nothing reschedules the CPU, which idles with the task queued until an unrelated wakeup lands on it or the watchdog fires. scx_mitosis, which sets SCX_OPS_ENQ_LAST without SCX_OPS_ENQ_EXITING, hit this during mass cgroup teardown: exiting tasks stuck in exit_mmap() for over 40 seconds. A task put on the local DSQ runs, as anywhere else. If the SCX_ENQ_LAST enqueue left the task on this CPU's local DSQ, set need_resched on the task being switched in, as proxy_resched_idle() does. resched_curr() would flag the task being put instead, and __schedule() clears that right after the pick. Fixes: f0e1a0643a59 ("sched_ext: Implement BPF extensible scheduler class") Cc: stable@vger.kernel.org # v6.12+ Reported-by: Xiangyu Bu Link: https://github.com/sched-ext/scx/pull/3870 Signed-off-by: Tejun Heo Cc: Dan Schatzberg diff --git a/kernel/sched/ext/ext.c b/kernel/sched/ext/ext.c index ac8fe70a9fb6..64d937fb2b24 100644 --- a/kernel/sched/ext/ext.c +++ b/kernel/sched/ext/ext.c @@ -3213,8 +3213,7 @@ static void put_prev_task_scx(struct rq *rq, struct task_struct *p, /* * If @p is runnable but we're about to enter a lower * sched_class, %SCX_OPS_ENQ_LAST must be set. Tell - * ops.enqueue() that @p is the only one available for this cpu, - * which should trigger an explicit follow-up scheduling event. + * ops.enqueue() that @p is the only one available for this cpu. * This doesn't apply if the baseline access on the CPU is lost. * * Under core scheduling, a pick dispatches only when nothing is @@ -3226,6 +3225,16 @@ static void put_prev_task_scx(struct rq *rq, struct task_struct *p, WARN_ON_ONCE(!sched_core_enabled(rq) && !(sch->ops.flags & SCX_OPS_ENQ_LAST)); scx_do_enqueue_task(rq, p, SCX_ENQ_LAST, -1); + + /* + * A task put on the local DSQ runs, as anywhere else. + * Here the insert can't reschedule on its own: the pick + * has settled on @next and @p is still curr, so + * resched_curr() would flag @p and __schedule() clears + * that right after the pick. Flag @next instead. + */ + if (p->scx.dsq == &rq->scx.local_dsq) + set_tsk_need_resched(next); } else { scx_do_enqueue_task(rq, p, 0, -1); } diff --git a/kernel/sched/ext/internal.h b/kernel/sched/ext/internal.h index 5b37faa532d6..85579fcabf52 100644 --- a/kernel/sched/ext/internal.h +++ b/kernel/sched/ext/internal.h @@ -1774,8 +1774,9 @@ enum scx_enq_flags { * %SCX_OPS_ENQ_LAST is specified, they're ops.enqueue()'d with the * %SCX_ENQ_LAST flag set. * - * The BPF scheduler is responsible for triggering a follow-up - * scheduling event. Otherwise, Execution may stall. + * If the task is queued on the local DSQ of the CPU it was running on, + * it continues to run. Otherwise, the CPU goes idle. A scheduler that + * wants a full dispatch cycle on the CPU should kick it. */ SCX_ENQ_LAST = 1LLU << 41,