sbitmap: optimize wakeup check
Even if we have no waiters on any of the sbitmap_queue wait states, we
still have to loop every entry to check. We do this for every IO, so
the cost adds up.
Shift a bit of the cost to the slow path, when we actually have waiters.
Wrap prepare_to_wait_exclusive() and finish_wait(), so we can maintain
an internal count of how many are currently active. Then we can simply
check this count in sbq_wake_ptr() and not have to loop if we don't
have any sleepers.
Convert the two users of sbitmap with waiting, blk-mq-tag and iSCSI.
Reviewed-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/include/linux/sbitmap.h b/include/linux/sbitmap.h
index 81359d4..92806a2 100644
--- a/include/linux/sbitmap.h
+++ b/include/linux/sbitmap.h
@@ -135,6 +135,11 @@
*/
struct sbq_wait_state *ws;
+ /*
+ * @ws_active: count of currently active ws waitqueues
+ */
+ atomic_t ws_active;
+
/**
* @round_robin: Allocate bits in strict round-robin order.
*/
@@ -552,4 +557,33 @@
*/
void sbitmap_queue_show(struct sbitmap_queue *sbq, struct seq_file *m);
+struct sbq_wait {
+ int accounted;
+ struct wait_queue_entry wait;
+};
+
+#define DEFINE_SBQ_WAIT(name) \
+ struct sbq_wait name = { \
+ .accounted = 0, \
+ .wait = { \
+ .private = current, \
+ .func = autoremove_wake_function, \
+ .entry = LIST_HEAD_INIT((name).wait.entry), \
+ } \
+ }
+
+/*
+ * Wrapper around prepare_to_wait_exclusive(), which maintains some extra
+ * internal state.
+ */
+void sbitmap_prepare_to_wait(struct sbitmap_queue *sbq,
+ struct sbq_wait_state *ws,
+ struct sbq_wait *sbq_wait, int state);
+
+/*
+ * Must be paired with sbitmap_prepare_to_wait().
+ */
+void sbitmap_finish_wait(struct sbitmap_queue *sbq, struct sbq_wait_state *ws,
+ struct sbq_wait *sbq_wait);
+
#endif /* __LINUX_SCALE_BITMAP_H */