blk-throttle: generalize update_disptime optimization in blk_throtl_bio()
When blk_throtl_bio() wants to queue a bio to a tg (throtl_grp), it
avoids invoking tg_update_disptime() and
throtl_schedule_next_dispatch() if the tg already has bios queued in
that direction. As a new bio is appeneded after the existing ones, it
can't change the tg's next dispatch time or the parent's dispatch
schedule.
This optimization is currently open coded in blk_throtl_bio().
Whether the target biolist was occupied was recorded in a local
variable and later used to skip disptime update. This patch moves
generalizes it so that throtl_add_bio_tg() sets a new flag
THROTL_TG_WAS_EMPTY if the biolist was empty before the new bio was
added. tg_update_disptime() clears the flag automatically.
blk_throtl_bio() is updated to simply test the flag before updating
disptime.
This patch doesn't make any functional differences now but will enable
using the same optimization for recursive dispatch.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Vivek Goyal <vgoyal@redhat.com>
diff --git a/block/blk-throttle.c b/block/blk-throttle.c
index 154bd63..ec9397f 100644
--- a/block/blk-throttle.c
+++ b/block/blk-throttle.c
@@ -46,6 +46,7 @@
enum tg_state_flags {
THROTL_TG_PENDING = 1 << 0, /* on parent's pending tree */
+ THROTL_TG_WAS_EMPTY = 1 << 1, /* bio_lists[] became non-empty */
};
#define rb_entry_tg(node) rb_entry((node), struct throtl_grp, rb_node)
@@ -712,6 +713,15 @@
struct throtl_service_queue *sq = &tg->service_queue;
bool rw = bio_data_dir(bio);
+ /*
+ * If @tg doesn't currently have any bios queued in the same
+ * direction, queueing @bio can change when @tg should be
+ * dispatched. Mark that @tg was empty. This is automatically
+ * cleaered on the next tg_update_disptime().
+ */
+ if (!sq->nr_queued[rw])
+ tg->flags |= THROTL_TG_WAS_EMPTY;
+
bio_list_add(&sq->bio_lists[rw], bio);
/* Take a bio reference on tg */
blkg_get(tg_to_blkg(tg));
@@ -740,6 +750,9 @@
throtl_dequeue_tg(tg, parent_sq);
tg->disptime = disptime;
throtl_enqueue_tg(tg, parent_sq);
+
+ /* see throtl_add_bio_tg() */
+ tg->flags &= ~THROTL_TG_WAS_EMPTY;
}
static void tg_dispatch_one_bio(struct throtl_grp *tg, bool rw,
@@ -1061,7 +1074,7 @@
struct throtl_data *td = q->td;
struct throtl_grp *tg;
struct throtl_service_queue *sq;
- bool rw = bio_data_dir(bio), update_disptime = true;
+ bool rw = bio_data_dir(bio);
struct blkcg *blkcg;
bool throttled = false;
@@ -1097,16 +1110,10 @@
sq = &tg->service_queue;
- if (sq->nr_queued[rw]) {
- /*
- * There is already another bio queued in same dir. No
- * need to update dispatch time.
- */
- update_disptime = false;
+ /* throtl is FIFO - if other bios are already queued, should queue */
+ if (sq->nr_queued[rw])
goto queue_bio;
- }
-
/* Bio is with-in rate limit of group */
if (tg_may_dispatch(tg, bio, NULL)) {
throtl_charge_bio(tg, bio);
@@ -1138,7 +1145,8 @@
throtl_add_bio_tg(bio, tg, &q->td->service_queue);
throttled = true;
- if (update_disptime) {
+ /* update @tg's dispatch time if @tg was empty before @bio */
+ if (tg->flags & THROTL_TG_WAS_EMPTY) {
tg_update_disptime(tg, &td->service_queue);
throtl_schedule_next_dispatch(td);
}