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>

1 files changed, 18 insertions, 10 deletions

diff --git a/block/blk-throttle.c b/block/blk-throttle.c
index 154bd63719c5..ec9397f3eb0a 100644
--- a/block/blk-throttle.c
+++ b/block/blk-throttle.c
@@ -46,6 +46,7 @@ struct throtl_service_queue {
 
 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 @@ static void throtl_add_bio_tg(struct bio *bio, struct throtl_grp *tg,
         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 @@ static void tg_update_disptime(struct throtl_grp *tg,
         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 @@ bool blk_throtl_bio(struct request_queue *q, struct bio *bio)
         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 @@ bool blk_throtl_bio(struct request_queue *q, struct bio *bio)
 
         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 @@ queue_bio:
         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);
         }