sched/cache: Keep nr_pref_llc_running in the runnable domain, to fix LLC mis-scheduling buggrafted

alb_break_llc() decides whether to break LLC preference during active
load balance. It does so by testing that every runnable fair task on the
source rq prefers its LLC:

	env->src_rq->nr_pref_llc_running == env->src_rq->cfs.h_nr_runnable

But the two counters cover different sets. nr_pref_llc_running is updated
in account_llc_enqueue()/account_llc_dequeue(), next to cfs_rq->nr_queued,
so it follows queued tasks. h_nr_runnable is updated in set_delayed()/
clear_delayed() and drops delay-dequeued tasks.

So under DELAY_DEQUEUE, a preferring task that goes to sleep stays counted
in nr_pref_llc_running while h_nr_runnable falls. The equality then breaks,
alb_break_llc() returns false, and active balance is free to pull a task
off its preferred LLC. Active balance only moves runnable tasks, and this
is the only LLC check it consults: once the stopper runs, LBF_ACTIVE_LB
skips the per-task test in can_migrate_task(). The runnable set is the one
we want.

Fix it on the counter side. A task should be counted in
nr_pref_llc_running exactly while it is both queued on its preferred LLC
(pref_llc_queued) and runnable (!sched_delayed). Define that membership
once in task_pref_llc_runnable(), and adjust the counter only through
pref_llc_running_inc()/pref_llc_running_dec() from the four sites that
change either input: account_llc_enqueue(), account_llc_dequeue(),
set_delayed() and clear_delayed(). Gating every update on the same
predicate keeps the delay, wake and dequeue paths from double-counting
or underflowing; see the comments at those sites for the ordering.

nr_llc_running and sd->llc_counts are not touched and stay on queued
semantics.

Fixes: 714059f79ff0 ("sched/cache: Handle moving single tasks to/from their preferred LLC")
Closes: https://lore.kernel.org/lkml/20260827135000.735138-1-zhanxusheng@xiaomi.com/
Reported-by: Zhan Xusheng <zhanxusheng@xiaomi.com>
Suggested-by: Chen Yu <yu.c.chen@intel.com>
Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Kayra Cizmeci <kayracizmeci@gmail.com>
Cc: <stable@kernel.org> # v7.2.x
Link: https://patch.msgid.link/06af61afedac32e6477f57feb4d658f6c411c3af.1790035273.git.tim.c.chen@linux.intel.com

1 files changed, 134 insertions, 0 deletions

diff --git a/io_uring/mpscq.h b/io_uring/mpscq.h
new file mode 100644
index 000000000..f91052676
--- /dev/null
+++ b/io_uring/mpscq.h
@@ -0,0 +1,134 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef IOU_MPSCQ_H
+#define IOU_MPSCQ_H
+
+#include <linux/io_uring_types.h>
+
+/*
+ * mpscq - lockless multi-producer, single-consumer FIFO queue
+ *
+ * Unlike llist, which is LIFO ordered and hence needs an O(n)
+ * llist_reverse_order() pass before entries can be processed in queue order,
+ * this queue hands out nodes in the order they were pushed.
+ *
+ * The consumer cursor is held by the caller rather than in the queue struct
+ * (see below), and with the stub reinsertion done as a single cmpxchg attempt
+ * instead of an unconditional push, keeping tail == stub a reliable empty test
+ * while a producer is in the middle of a push.
+ *
+ * Producers may run in any context (task, softirq, hardirq) and are wait-free:
+ * a push is one xchg() plus one store, with no retry loops. FIFO order between
+ * producers is the order in which the xchg() on ->tail serializes them.
+ *
+ * The price for linked-list FIFO is that a push publishes the node in two
+ * steps: the xchg() makes it the new tail, and the subsequent store links it to
+ * its predecessor. In between, the tail end of the queue is not yet reachable
+ * from the head. mpscq_pop() detects this and returns NULL, while mpscq_empty()
+ * reports false. The consumer must not treat such a NULL as "queue empty" - it
+ * should retry later. The window is two instructions wide, but a producer can
+ * be preempted inside it, so the consumer must not spin on it while holding
+ * resources the producer might need to make progress.
+ *
+ * The consumer side only supports a single consumer at a time, callers must
+ * provide their own serialization for it. The stub node is what allows the
+ * consumer to detach the final node without racing with the link stores of
+ * producers. This scheme also guarantees that the previous tail observed by
+ * mpscq_push() cannot be freed by the consumer until the push has linked it,
+ * which is what makes the deferred link store safe.
+ *
+ * The queue struct only holds the producer side. The consumer keeps its cursor
+ * (the oldest not yet handed out node) externally and passes it to mpscq_pop(),
+ * so that it can be placed on a different cacheline: the cursor is written for
+ * every pop, and having it share a line with ->tail would have the consumer
+ * invalidating the line that producers need for every push.
+ */
+static inline void mpscq_init(struct mpscq *q, struct llist_node **headp)
+{
+	q->tail = *headp = &q->stub;
+	q->stub.next = NULL;
+}
+
+/*
+ * Returns true if the queue holds no entries that mpscq_pop() hasn't handed out
+ * yet. May be called from any context. Note that !empty doesn't guarantee that
+ * mpscq_pop() will return an entry yet, see the in-flight producer window
+ * above.
+ */
+static inline bool mpscq_empty(struct mpscq *q)
+{
+	return READ_ONCE(q->tail) == &q->stub;
+}
+
+/*
+ * Push a node onto the queue. Safe against concurrent pushes from any context,
+ * and against the (single) consumer. Returns true if the queue was empty
+ * before this push.
+ */
+static inline bool mpscq_push(struct mpscq *q, struct llist_node *node)
+{
+	struct llist_node *prev;
+
+	node->next = NULL;
+	/*
+	 * xchg() implies a full barrier, so the initialization of the
+	 * entry (including ->next above) is visible before the node can
+	 * be reached, either via ->tail or via ->next chasing from the
+	 * head once the store below has linked it.
+	 */
+	prev = xchg(&q->tail, node);
+	WRITE_ONCE(prev->next, node);
+	return prev == &q->stub;
+}
+
+/*
+ * Pop the oldest node off the queue, or return NULL if no node is available.
+ * NULL is returned both when the queue is empty and when a producer has
+ * published a node via ->tail but hasn't linked it yet; use mpscq_empty() to
+ * tell the two apart. Single consumer only, with headp being the consumer
+ * cursor that mpscq_init() set up.
+ */
+static inline struct llist_node *mpscq_pop(struct mpscq *q,
+					   struct llist_node **headp)
+{
+	struct llist_node *head = *headp, *next;
+
+	if (head == &q->stub) {
+		head = READ_ONCE(head->next);
+		if (!head)
+			return NULL;
+		/*
+		 * The stub is now detached and stays quiescent until the
+		 * consumer reinserts it as the tail, so reset its ->next here,
+		 * ready for that.
+		 */
+		q->stub.next = NULL;
+		*headp = head;
+	}
+	next = READ_ONCE(head->next);
+	if (next) {
+		*headp = next;
+		return head;
+	}
+	/*
+	 * 'head' is the last linked node, it can only be handed out once the
+	 * stub has taken its place as the tail. If the cmpxchg fails, a
+	 * producer has made a new node the tail but hasn't linked 'head' to
+	 * it yet - bail and let the caller retry.
+	 */
+	if (try_cmpxchg(&q->tail, &head, &q->stub)) {
+		*headp = &q->stub;
+		return head;
+	}
+	return NULL;
+}
+
+/*
+ * Returns true if the most recent mpscq_pop() that returned a node also
+ * emptied the queue. Consumer must be serialized.
+ */
+static inline bool mpscq_pop_emptied(struct mpscq *q, struct llist_node *head)
+{
+	return head == &q->stub;
+}
+
+#endif /* IOU_MPSCQ_H */