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 327BD49D5BD for ; Mon, 12 Oct 2026 15:25:06 +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=1791818709; cv=none; b=YG9JqWpc4jHiY7p5zkPbDoXOMx9EBYtKmIr3gr0ju9GJ61MdmMZtkheMc5Jlgx8XlhZGA5cZxGDXfN8Bws3JG804jb3Y+PWCnrlE3AR+LqDxSxD9bqbdnMoM3ym47BIheplvG/Ol7s6ZzpZSGPYblNAztVbjGmMXEESoQBHMqZQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1791818709; c=relaxed/simple; bh=OogQrJ0LjwI3lcyWjzvYOyOsIT8HSnL7WPqNzn37rns=; h=Subject:To:Cc:From:Date:Message-ID:MIME-Version:Content-Type; b=ohtPICvMeqNGYA2hjv7Pe/IutIHJZAkWEPQy8xoqEG75Z8eafbBErDfvS8LX6TLGkVF5pmO8Mpc9CsgRsJajQWrxiFk2sHsgwkYMWZGRRmAki97L39RgK5PqhUjyOSlqz2UNWiVnJQvDnd5JL6mSTNoMskOyYEE87ZlQj/wOLhw= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=X0zBDybU; 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="X0zBDybU" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 1E39F1F0089B; Mon, 12 Oct 2026 15:25:05 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linuxfoundation.org; s=korg; t=1791818706; bh=Bk8gAJzDSB8Zolwa5y+NLeUXO6KzVvtoiZHMaB0yAQ4=; h=Subject:To:Cc:From:Date; b=X0zBDybUORIVkWYeuZ4u6YCY/xoi0csD3TQbLs0xJWUfHlbU2a/s39Ekfa1D2X8vF KF1TNew5hqdN07KwvDtCawXkHGel3dG4EZbWGmxE50fy9p9NXw+dLdGpK3JxS/vAZS x0ZrqXOXxHu/ZZRDIHDhkdP3sIJQivULMmsHGPAI= Subject: FAILED: patch "[PATCH] sched_ext: Generate qseq from a per-task counter" failed to apply to 7.2-stable tree To: jpiecuch@google.com,tj@kernel.org Cc: From: Date: Mon, 12 Oct 2026 17:25:12 +0200 Message-ID: <2026101211-seismic-mutable-0582@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 7.2-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-7.2.y git checkout FETCH_HEAD git cherry-pick -x e6ac89b8b1c12e9104df45a14a26e2dfa96ded06 # git commit -s git send-email --to '' --in-reply-to '2026101211-seismic-mutable-0582@gregkh' --subject-prefix 'PATCH 7.2.y' 'HEAD^..' Possible dependencies: thanks, greg k-h ------------------ original commit in Linus's tree ------------------ >From e6ac89b8b1c12e9104df45a14a26e2dfa96ded06 Mon Sep 17 00:00:00 2001 From: Kuba Piecuch Date: Sat, 3 Oct 2026 11:53:17 +0000 Subject: [PATCH] sched_ext: Generate qseq from a per-task counter finish_dispatch() uses the qseq embedded in p->scx.ops_state to tell whether the QUEUED instance of a task it's about to claim is the one scx_bpf_dsq_insert() saw. qseq is generated from rq->scx.ops_qseq, but the counters of different rqs are independent, so if a task is dequeued and re-enqueued on a different rq between scx_bpf_dsq_insert() and finish_dispatch(), the new QUEUED instance can end up with the same qseq as the old one: CPU X CPU Z ----- ----- enqueue p on rq A, qseq = N ops.dispatch() scx_bpf_dsq_insert(p) records qseq N sched_setaffinity(p) dequeue p from rq A enqueue p on rq B, qseq = N finish_dispatch(p, N) qseq matches, p is claimed The claim itself is still atomic so the core stays consistent, but an insert issued for a previous QUEUED instance gets applied to a new one which the BPF scheduler has just received through ops.enqueue(). This breaks the guarantee that dispatches targeting a stale instance are ignored. Generate qseq from a per-task counter, p->scx.ops_qseq, instead so that consecutive QUEUED instances of a task never share a qseq regardless of which rq they're on. The counter is only updated in scx_do_enqueue_task() with the task's rq locked, so no additional synchronization is needed, and it fits in an existing hole in struct sched_ext_entity on 64bit. Remove the now unused rq->scx.ops_qseq. Never generate qseq 0. NONE and DISPATCHING don't carry a qseq, so scx_bpf_dsq_insert() on a task in either state records 0. With a per-task counter, every task's first QUEUED instance would otherwise get qseq 0 and could be claimed by such an insert. Wrap the counter where the QSEQ field wraps so that it can't reach a value that shifts to 0 on 32bit either. Fixes: f0e1a0643a59 ("sched_ext: Implement BPF extensible scheduler class") Cc: stable@vger.kernel.org # v6.12+ Assisted-by: Claude:claude-opus-5.5 Signed-off-by: Kuba Piecuch Signed-off-by: Tejun Heo diff --git a/include/linux/sched/ext.h b/include/linux/sched/ext.h index 582d7cd4a983..36c04797436c 100644 --- a/include/linux/sched/ext.h +++ b/include/linux/sched/ext.h @@ -207,6 +207,7 @@ struct sched_ext_entity { s32 holding_cpu; s32 selected_cpu; s32 runnable_cpu; /* cpu @p is runnable on, -1 if not */ + u32 ops_qseq; /* protected by rq lock */ struct task_struct *kf_tasks[2]; /* see SCX_CALL_OP_TASK() */ struct list_head runnable_node; /* rq->scx.runnable_list */ diff --git a/kernel/sched/ext/ext.c b/kernel/sched/ext/ext.c index 5209510d6a5e..ac8fe70a9fb6 100644 --- a/kernel/sched/ext/ext.c +++ b/kernel/sched/ext/ext.c @@ -2057,8 +2057,14 @@ void scx_do_enqueue_task(struct rq *rq, struct task_struct *p, u64 enq_flags, if (unlikely(!SCX_HAS_OP(sch, enqueue))) goto global; - /* DSQ bypass didn't trigger, enqueue on the BPF scheduler */ - qseq = rq->scx.ops_qseq++ << SCX_OPSS_QSEQ_SHIFT; + /* + * DSQ bypass didn't trigger, enqueue on the BPF scheduler. Wrap the + * per-task qseq counter where the QSEQ field wraps and skip 0, which is + * what scx_bpf_dsq_insert() records for a task in NONE or DISPATCHING. + */ + p->scx.ops_qseq = ((p->scx.ops_qseq + 1) & + (SCX_OPSS_QSEQ_MASK >> SCX_OPSS_QSEQ_SHIFT)) ?: 1; + qseq = (unsigned long)p->scx.ops_qseq << SCX_OPSS_QSEQ_SHIFT; WARN_ON_ONCE(atomic_long_read(&p->scx.ops_state) != SCX_OPSS_NONE); atomic_long_set(&p->scx.ops_state, SCX_OPSS_QUEUEING | qseq); @@ -6969,9 +6975,9 @@ static void scx_dump_cpu(struct scx_sched *sch, struct seq_buf *s, seq_buf_init(&ns, buf, avail); dump_newline(&ns); - scx_dump_line(&ns, "CPU %-4d: nr_run=%u flags=0x%x cpu_rel=%d ops_qseq=%lu ksync=%lu", + scx_dump_line(&ns, "CPU %-4d: nr_run=%u flags=0x%x cpu_rel=%d ksync=%lu", cpu, rq->scx.nr_running, rq->scx.flags, rq->scx.cpu_released, - rq->scx.ops_qseq, rq->scx.kick_sync); + rq->scx.kick_sync); scx_rescue_dump(&ns, rq); scx_dump_line(&ns, " curr=%s[%d] class=%ps", rq->curr->comm, rq->curr->pid, rq->curr->sched_class); diff --git a/kernel/sched/ext/internal.h b/kernel/sched/ext/internal.h index 1df8f583b0ec..5b37faa532d6 100644 --- a/kernel/sched/ext/internal.h +++ b/kernel/sched/ext/internal.h @@ -1998,6 +1998,11 @@ enum scx_ops_state { * dequeue/requeue, the dispatcher can tell whether it still has a claim * on the task being dispatched. * + * QSEQ is generated from the per-task p->scx.ops_qseq counter so that + * it doesn't repeat across QUEUED instances of the same task even if + * the task moves between rqs. 0 is never used as a valid QSEQ since + * NONE and DISPATCHING map to this value. + * * As some 32bit archs can't do 64bit store_release/load_acquire, * p->scx.ops_state is atomic_long_t which leaves 30 bits for QSEQ on * 32bit machines. The dispatch race window QSEQ protects is very narrow diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h index 742cce9da4e9..9ed0cb605c33 100644 --- a/kernel/sched/sched.h +++ b/kernel/sched/sched.h @@ -813,7 +813,6 @@ struct scx_rq { #endif struct list_head runnable_list; /* runnable tasks on this rq */ struct list_head ddsp_deferred_locals; /* deferred ddsps from enq */ - unsigned long ops_qseq; /* both stashed across the activate_task() in move_remote_task_to_local_dsq() */ u64 remote_activate_enq_flags; struct scx_sched *remote_activate_sch;