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=1792678751; b=aafnu18kh6SqBv1bQbUhAmSlcrA2/5T2bZKzpkitXsda9I0BqrJHJF+Oc1cLaeCzG3UE +X1Z5lVuSQ+J2/jgXtnoCjVBSY1hXMFt3ZuWJ0VR5Da+4fm0WJbvb934aHOcHIC89itwp q4ZCVF8jMQrSJLxrMxnHrWiCgikvWV4rAc0vaKL6cEMjnEunexOhv3+MSYMm2qnn7veG/ pPMCGN0xBteEdh4poPw5woe00g7G7a2Rje6ypn//+bOS148EHMsDCETjSaFZjk7gfeuK2 Yi6f9o7dqoo6WGaeRvhZUkA7ID5e8kd6XN7CnTjvhXfOo6IHQ1+ycYdfniBirLh/ycQ== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1792678751; h=Received:DKIM-Signature:From:To:Cc:Subject:Date:Message-ID:X-Mailer: In-Reply-To:References:MIME-Version:Content-Transfer-Encoding; bh=XVLJl6/F2raWBUV0gl06fxTC6SmLwzEIOWKACsZ0Rwc=; b=KMSD7gVIz8PgSJi9BIKwjwNOmvVdM/PXNG90QeT02DRf9+NsCGNaGYUMGw0IAL2uJvZe w4DF768IeixBT/bxC9smHB/EXJlagUDTkFMa37bcNG5aTzDB0vqdbek6JVvXNJbHloQXv L2aMHo+0wWPeGVR0yMoDa0XG3vyAb+rJlm8B02wEtL0gSNtorfdH9IepaI1evDA7MR9DB QnhfJZWmG/WztlTM99DQvp/BXSYZYWVHBIsSOjHsyXSpfiBEvyjF8iOUhdaIzkhZaAspj XIeCzR7jA5/D2Loin/Z6aQn5ssgvtEUWfLINGLe7X4xGBnjr1KKBLoPEZkhsRhJ/6Mg== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=PXiMuZKi; 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 C693B606A319 for ; Thu, 22 Oct 2026 14:19:10 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key, unprotected) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=PXiMuZKi Received: by smtp.kernel.org (Postfix) with ESMTPSA id CFB641F000C2; Thu, 22 Oct 2026 14:19:09 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1792678750; bh=XVLJl6/F2raWBUV0gl06fxTC6SmLwzEIOWKACsZ0Rwc=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=PXiMuZKiGwc6/xm+QsowHqPw8BhKSHCKRP/z96XuXYst0VmREMWkK7EvoR3MbJoHS coYDvv5bStXNy7yuS+1efQRr/VJTR5XxyX3iXu8ziHiR0z/uyG7T+e1CzcXx+W+1Wg 8sG08yFpImqY9+1WvQG7TJIYqLuMR23KNUrAhubwnyPDr9tVB3szRgB/MKkO6v+qwL HuFgyLdpAzL0be6dfgYmnSd7yLQSZrlEyqsDVmfy7oYIYNvK1j0qCdxnJ18xRJnM7S hJLon2LexYx6cidqUP/iVY2ohb6F9v19vUb0k/nXQTicaL9n9Cd5x3MVa6agQBduRD KhaSSseioTglw== From: Sasha Levin To: stable@vger.kernel.org Cc: Tejun Heo , Xiangyu Bu , Dan Schatzberg , Sasha Levin Subject: [PATCH 6.12.y] sched_ext: Fix stall when a task enqueued with SCX_ENQ_LAST lands back on its CPU's local DSQ Date: Thu, 22 Oct 2026 10:19:08 -0400 Message-ID: <20261022140850.1455884-1-sashal@kernel.org> X-Mailer: git-send-email 2.53.0 In-Reply-To: <2026102101-kilogram-periscope-454b@gregkh> References: <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-Transfer-Encoding: 8bit From: Tejun Heo [ Upstream commit 7a9df76679633cf26e0fea882a684012c6ec245a ] 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 [ sashal: Relocated the hunks from separate source and header files into the monolithic sched_ext source file. ] Signed-off-by: Sasha Levin --- kernel/sched/ext.c | 18 ++++++++++++++---- 1 file changed, 14 insertions(+), 4 deletions(-) diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c index 876adf5568ed1..ded7a0dd2e533 100644 --- a/kernel/sched/ext.c +++ b/kernel/sched/ext.c @@ -730,8 +730,9 @@ 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, @@ -3083,8 +3084,7 @@ 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. * * Core scheduling can force this CPU idle while @p stays * runnable. @p's cookie then won't match the core's, so skip @@ -3094,6 +3094,16 @@ static void put_prev_task_scx(struct rq *rq, struct task_struct *p, WARN_ON_ONCE(sched_cpu_cookie_match(rq, p) && !static_branch_unlikely(&scx_ops_enq_last)); 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 { do_enqueue_task(rq, p, 0, -1); } -- 2.53.0