From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 691F72367D9 for ; Wed, 8 Apr 2026 07:00:51 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1775631651; cv=none; b=Lmyj+foTFZVlxIxWx78eS6fRAdlgA8An3rb+CTE5uUJOR2Rqo7uaTRoPdxLxHnyOUGveruqaDa6w9tOjAWe9dn6O9z+VQWXY7qFkpGmEKJETvU8/PJZbqhxaglDdTcsPkH+RN7wYwqDZTb3c71YxJzcECJq+XkEiDnyyu2V33nc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1775631651; c=relaxed/simple; bh=vnJ2tS7dMzPvFmb6y+QMfDKx+KZJcONhQYufHgRnxU8=; h=Subject:To:Cc:From:Date:Message-ID:MIME-Version:Content-Type; b=j40hzE3JgSfHNW35X0xyD9hT11yzlA6axDiM4g6+NCk50/6GK8LxjfgQVAM1gBBLiWOKtVMABHELYeyeI5SQwoJZgy82QaoXDv62XmcKi0edAeOj/vb2c01hbLotSA4IHeZQKKsxr88a835I4D0KH9yi/hT4qnL+35z4mnBFTiQ= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=SDAjnqv2; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b="SDAjnqv2" Received: by smtp.kernel.org (Postfix) with ESMTPSA id CD5C0C19425; Wed, 8 Apr 2026 07:00:50 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1775631651; bh=vnJ2tS7dMzPvFmb6y+QMfDKx+KZJcONhQYufHgRnxU8=; h=Subject:To:Cc:From:Date:From; b=SDAjnqv2Eo2fx4uigPwoVtVIiWk2SsaHzuI5oQyhM3KLrgI8R3liJ2OoEuS5vNXdc mCYhkfDOOlZGr1TMeBAEgjQxcSghNPljHNBdKJfbkmoYOQxxd9QukWdJXyJ7mpZjeB wEdPxfIaFFY7OkpAHJ1X5cCUIrw12adT1ILxVEjU= Subject: FAILED: patch "[PATCH] sched_ext: Fix stale direct dispatch state in ddsp_dsq_id" failed to apply to 6.12-stable tree To: arighi@nvidia.com,hodgesd@meta.com,patsomaru@meta.com,tj@kernel.org Cc: From: Date: Wed, 08 Apr 2026 09:00:40 +0200 Message-ID: <2026040840-slightly-linked-0149@gregkh> Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: 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 7e0ffb72de8aa3b25989c2d980e81b829c577010 # git commit -s git send-email --to '' --in-reply-to '2026040840-slightly-linked-0149@gregkh' --subject-prefix 'PATCH 6.12.y' HEAD^.. Possible dependencies: thanks, greg k-h ------------------ original commit in Linus's tree ------------------ >From 7e0ffb72de8aa3b25989c2d980e81b829c577010 Mon Sep 17 00:00:00 2001 From: Andrea Righi Date: Fri, 3 Apr 2026 08:57:20 +0200 Subject: [PATCH] sched_ext: Fix stale direct dispatch state in ddsp_dsq_id @p->scx.ddsp_dsq_id can be left set (non-SCX_DSQ_INVALID) triggering a spurious warning in mark_direct_dispatch() when the next wakeup's ops.select_cpu() calls scx_bpf_dsq_insert(), such as: WARNING: kernel/sched/ext.c:1273 at scx_dsq_insert_commit+0xcd/0x140 The root cause is that ddsp_dsq_id was only cleared in dispatch_enqueue(), which is not reached in all paths that consume or cancel a direct dispatch verdict. Fix it by clearing it at the right places: - direct_dispatch(): cache the direct dispatch state in local variables and clear it before dispatch_enqueue() on the synchronous path. For the deferred path, the direct dispatch state must remain set until process_ddsp_deferred_locals() consumes them. - process_ddsp_deferred_locals(): cache the dispatch state in local variables and clear it before calling dispatch_to_local_dsq(), which may migrate the task to another rq. - do_enqueue_task(): clear the dispatch state on the enqueue path (local/global/bypass fallbacks), where the direct dispatch verdict is ignored. - dequeue_task_scx(): clear the dispatch state after dispatch_dequeue() to handle both the deferred dispatch cancellation and the holding_cpu race, covering all cases where a pending direct dispatch is cancelled. - scx_disable_task(): clear the direct dispatch state when transitioning a task out of the current scheduler. Waking tasks may have had the direct dispatch state set by the outgoing scheduler's ops.select_cpu() and then been queued on a wake_list via ttwu_queue_wakelist(), when SCX_OPS_ALLOW_QUEUED_WAKEUP is set. Such tasks are not on the runqueue and are not iterated by scx_bypass(), so their direct dispatch state won't be cleared. Without this clear, any subsequent SCX scheduler that tries to direct dispatch the task will trigger the WARN_ON_ONCE() in mark_direct_dispatch(). Fixes: 5b26f7b920f7 ("sched_ext: Allow SCX_DSQ_LOCAL_ON for direct dispatches") Cc: stable@vger.kernel.org # v6.12+ Cc: Daniel Hodges Cc: Patrick Somaru Signed-off-by: Andrea Righi Signed-off-by: Tejun Heo diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c index d5bdcdb3f700..064eaa76be4b 100644 --- a/kernel/sched/ext.c +++ b/kernel/sched/ext.c @@ -1109,15 +1109,6 @@ static void dispatch_enqueue(struct scx_sched *sch, struct scx_dispatch_q *dsq, dsq_mod_nr(dsq, 1); p->scx.dsq = dsq; - /* - * scx.ddsp_dsq_id and scx.ddsp_enq_flags are only relevant on the - * direct dispatch path, but we clear them here because the direct - * dispatch verdict may be overridden on the enqueue path during e.g. - * bypass. - */ - p->scx.ddsp_dsq_id = SCX_DSQ_INVALID; - p->scx.ddsp_enq_flags = 0; - /* * We're transitioning out of QUEUEING or DISPATCHING. store_release to * match waiters' load_acquire. @@ -1283,12 +1274,34 @@ static void mark_direct_dispatch(struct scx_sched *sch, p->scx.ddsp_enq_flags = enq_flags; } +/* + * Clear @p direct dispatch state when leaving the scheduler. + * + * Direct dispatch state must be cleared in the following cases: + * - direct_dispatch(): cleared on the synchronous enqueue path, deferred + * dispatch keeps the state until consumed + * - process_ddsp_deferred_locals(): cleared after consuming deferred state, + * - do_enqueue_task(): cleared on enqueue fallbacks where the dispatch + * verdict is ignored (local/global/bypass) + * - dequeue_task_scx(): cleared after dispatch_dequeue(), covering deferred + * cancellation and holding_cpu races + * - scx_disable_task(): cleared for queued wakeup tasks, which are excluded by + * the scx_bypass() loop, so that stale state is not reused by a subsequent + * scheduler instance + */ +static inline void clear_direct_dispatch(struct task_struct *p) +{ + p->scx.ddsp_dsq_id = SCX_DSQ_INVALID; + p->scx.ddsp_enq_flags = 0; +} + static void direct_dispatch(struct scx_sched *sch, struct task_struct *p, u64 enq_flags) { struct rq *rq = task_rq(p); struct scx_dispatch_q *dsq = find_dsq_for_dispatch(sch, rq, p->scx.ddsp_dsq_id, p); + u64 ddsp_enq_flags; touch_core_sched_dispatch(rq, p); @@ -1329,8 +1342,10 @@ static void direct_dispatch(struct scx_sched *sch, struct task_struct *p, return; } - dispatch_enqueue(sch, dsq, p, - p->scx.ddsp_enq_flags | SCX_ENQ_CLEAR_OPSS); + ddsp_enq_flags = p->scx.ddsp_enq_flags; + clear_direct_dispatch(p); + + dispatch_enqueue(sch, dsq, p, ddsp_enq_flags | SCX_ENQ_CLEAR_OPSS); } static bool scx_rq_online(struct rq *rq) @@ -1439,6 +1454,7 @@ static void do_enqueue_task(struct rq *rq, struct task_struct *p, u64 enq_flags, */ touch_core_sched(rq, p); refill_task_slice_dfl(sch, p); + clear_direct_dispatch(p); dispatch_enqueue(sch, dsq, p, enq_flags); } @@ -1610,6 +1626,7 @@ static bool dequeue_task_scx(struct rq *rq, struct task_struct *p, int deq_flags sub_nr_running(rq, 1); dispatch_dequeue(rq, p); + clear_direct_dispatch(p); return true; } @@ -2293,13 +2310,15 @@ static void process_ddsp_deferred_locals(struct rq *rq) struct task_struct, scx.dsq_list.node))) { struct scx_sched *sch = scx_root; struct scx_dispatch_q *dsq; + u64 dsq_id = p->scx.ddsp_dsq_id; + u64 enq_flags = p->scx.ddsp_enq_flags; list_del_init(&p->scx.dsq_list.node); + clear_direct_dispatch(p); - dsq = find_dsq_for_dispatch(sch, rq, p->scx.ddsp_dsq_id, p); + dsq = find_dsq_for_dispatch(sch, rq, dsq_id, p); if (!WARN_ON_ONCE(dsq->id != SCX_DSQ_LOCAL)) - dispatch_to_local_dsq(sch, rq, dsq, p, - p->scx.ddsp_enq_flags); + dispatch_to_local_dsq(sch, rq, dsq, p, enq_flags); } } @@ -3015,6 +3034,8 @@ static void scx_disable_task(struct task_struct *p) lockdep_assert_rq_held(rq); WARN_ON_ONCE(scx_get_task_state(p) != SCX_TASK_ENABLED); + clear_direct_dispatch(p); + if (SCX_HAS_OP(sch, disable)) SCX_CALL_OP_TASK(sch, SCX_KF_REST, disable, rq, p); scx_set_task_state(p, SCX_TASK_READY);