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: f0e1a0643a ("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 <jpiecuch@google.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

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 */

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);

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

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;