scftorture: Count single_rpc offline failures in statistics outputgrafted

scf_torture_stats_print() aggregates each invoker thread's counters
into a local scf_statistics structure before printing, but the
n_single_rpc_ofl field is missing from the aggregation loop.  As a
result, the "single_rpc_ofl" value printed in the statistics line is
always zero, even when smp_call_function_single() invocations for the
RPC test have failed due to offline CPUs and been counted by the
invoker threads.

Add the missing accumulation so that the printed value reflects the
actual counts.

Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

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 */