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blk-throttle: implement dispatch looping

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throtl_select_dispatch() only dispatches throtl_quantum bios on each
invocation.  blk_throtl_dispatch_work_fn() in turn depends on
throtl_schedule_next_dispatch() scheduling the next dispatch window
immediately so that undue delays aren't incurred.  This effectively
chains multiple dispatch work item executions back-to-back when there
are more than throtl_quantum bios to dispatch on a given tick.

There is no reason to finish the current work item just to repeat it
immediately.  This patch makes throtl_schedule_next_dispatch() return
%false without doing anything if the current dispatch window is still
open and updates blk_throtl_dispatch_work_fn() repeat dispatching
after cpu_relax() on %false return.

This change will help implementing hierarchy support as dispatching
will be done from pending_timer and immediate reschedule of timer
function isn't supported and doesn't make much sense.

While this patch changes how dispatch behaves when there are more than
throtl_quantum bios to dispatch on a single tick, the behavior change
is immaterial.

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 13:52:37 -07:00
parent 69df0ab030
commit 7f52f98c2a
1 changed files with 56 additions and 26 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

82
block/blk-throttle.c
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@ -467,24 +467,41 @@ static void throtl_schedule_pending_timer(struct throtl_service_queue *sq,
expires - jiffies, jiffies);
}
static void throtl_schedule_next_dispatch(struct throtl_service_queue *sq)
/**
* throtl_schedule_next_dispatch - schedule the next dispatch cycle
* @sq: the service_queue to schedule dispatch for
* @force: force scheduling
*
* Arm @sq->pending_timer so that the next dispatch cycle starts on the
* dispatch time of the first pending child. Returns %true if either timer
* is armed or there's no pending child left. %false if the current
* dispatch window is still open and the caller should continue
* dispatching.
*
* If @force is %true, the dispatch timer is always scheduled and this
* function is guaranteed to return %true. This is to be used when the
* caller can't dispatch itself and needs to invoke pending_timer
* unconditionally. Note that forced scheduling is likely to induce short
* delay before dispatch starts even if @sq->first_pending_disptime is not
* in the future and thus shouldn't be used in hot paths.
*/
static bool throtl_schedule_next_dispatch(struct throtl_service_queue *sq,
bool force)
{
struct throtl_data *td = sq_to_td(sq);
/* any pending children left? */
if (!sq->nr_pending)
return;
return true;
update_min_dispatch_time(sq);
/* is the next dispatch time in the future? */
if (time_after(sq->first_pending_disptime, jiffies)) {
if (force || time_after(sq->first_pending_disptime, jiffies)) {
throtl_schedule_pending_timer(sq, sq->first_pending_disptime);
return;
return true;
}
/* kick immediate execution */
queue_work(kthrotld_workqueue, &td->dispatch_work);
/* tell the caller to continue dispatching */
return false;
}
static inline void throtl_start_new_slice(struct throtl_grp *tg, bool rw)
@ -930,31 +947,39 @@ void blk_throtl_dispatch_work_fn(struct work_struct *work)
dispatch_work);
struct throtl_service_queue *sq = &td->service_queue;
struct request_queue *q = td->queue;
unsigned int nr_disp = 0;
struct bio_list bio_list_on_stack;
struct bio *bio;
struct blk_plug plug;
int rw;
bool dispatched = false;
int rw, ret;
spin_lock_irq(q->queue_lock);
bio_list_init(&bio_list_on_stack);
throtl_log(sq, "dispatch nr_queued=%u read=%u write=%u",
td->nr_queued[READ] + td->nr_queued[WRITE],
td->nr_queued[READ], td->nr_queued[WRITE]);
while (true) {
throtl_log(sq, "dispatch nr_queued=%u read=%u write=%u",
td->nr_queued[READ] + td->nr_queued[WRITE],
td->nr_queued[READ], td->nr_queued[WRITE]);
nr_disp = throtl_select_dispatch(sq);
if (nr_disp) {
for (rw = READ; rw <= WRITE; rw++) {
bio_list_merge(&bio_list_on_stack, &sq->bio_lists[rw]);
bio_list_init(&sq->bio_lists[rw]);
ret = throtl_select_dispatch(sq);
if (ret) {
for (rw = READ; rw <= WRITE; rw++) {
bio_list_merge(&bio_list_on_stack, &sq->bio_lists[rw]);
bio_list_init(&sq->bio_lists[rw]);
}
throtl_log(sq, "bios disp=%u", ret);
dispatched = true;
}
throtl_log(sq, "bios disp=%u", nr_disp);
}
throtl_schedule_next_dispatch(sq);
if (throtl_schedule_next_dispatch(sq, false))
break;
/* this dispatch windows is still open, relax and repeat */
spin_unlock_irq(q->queue_lock);
cpu_relax();
spin_lock_irq(q->queue_lock);
}
spin_unlock_irq(q->queue_lock);
@ -962,7 +987,7 @@ void blk_throtl_dispatch_work_fn(struct work_struct *work)
* If we dispatched some requests, unplug the queue to make sure
* immediate dispatch
*/
if (nr_disp) {
if (dispatched) {
blk_start_plug(&plug);
while((bio = bio_list_pop(&bio_list_on_stack)))
generic_make_request(bio);
@ -1078,7 +1103,7 @@ static int tg_set_conf(struct cgroup *cgrp, struct cftype *cft, const char *buf,
if (tg->flags & THROTL_TG_PENDING) {
tg_update_disptime(tg);
throtl_schedule_next_dispatch(sq->parent_sq);
throtl_schedule_next_dispatch(sq->parent_sq, true);
}
blkg_conf_finish(&ctx);
@ -1229,10 +1254,15 @@ queue_bio:
throtl_add_bio_tg(bio, tg);
throttled = true;
/* update @tg's dispatch time if @tg was empty before @bio */
/*
* Update @tg's dispatch time and force schedule dispatch if @tg
* was empty before @bio. The forced scheduling isn't likely to
* cause undue delay as @bio is likely to be dispatched directly if
* its @tg's disptime is not in the future.
*/
if (tg->flags & THROTL_TG_WAS_EMPTY) {
tg_update_disptime(tg);
throtl_schedule_next_dispatch(tg->service_queue.parent_sq);
throtl_schedule_next_dispatch(tg->service_queue.parent_sq, true);
}
out_unlock: