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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>
This commit is contained in:
Tejun Heo 2013-05-14 13:52:35 -07:00
parent 651930bc1c
commit 0e9f4164ba
1 changed files with 18 additions and 10 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

28
block/blk-throttle.c
View File

@ -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);
}