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Subject[PATCH 10/33] blk-throttle: remove pointless throtl_nr_queued() optimizations
Date
throtl_nr_queued() is used in several places to avoid performing
certain operations when the throtl_data is empty. This usually is
useless as those paths usually aren't traveled if there's no bio
queued.

* throtl_schedule_delayed_work() skips scheduling dispatch work item
if @td doesn't have any bios queued; however, the only case it can
be called when @td is empty is from tg_set_conf() which isn't
something we should be optimizing for.

* throtl_schedule_next_dispatch() takes a quick exit if @td is empty;
however, right after that it triggers BUG if the service tree is
empty. The two conditions are equivalent and it can just test
@st->count for the quick exit.

* blk_throtl_dispatch_work_fn() skips dispatch if @td is empty. This
work function isn't usually invoked when @td is empty. The only
possibility is from tg_set_conf() and when it happens the normal
dispatching path can handle empty @td fine. No need to add special
skip path.

This patch removes the above three unnecessary optimizations, which
leave throtl_log() call in blk_throtl_dispatch_work_fn() the only user
of throtl_nr_queued(). Remove throtl_nr_queued() and open code it in
throtl_log(). I don't think we need td->nr_queued[] at all. Maybe we
can remove it later.

Signed-off-by: Tejun Heo <tj@kernel.org>
---
block/blk-throttle.c | 29 +++++++----------------------
1 file changed, 7 insertions(+), 22 deletions(-)

diff --git a/block/blk-throttle.c b/block/blk-throttle.c
index 507b1c6..dbeef30 100644
--- a/block/blk-throttle.c
+++ b/block/blk-throttle.c
@@ -166,11 +166,6 @@ THROTL_TG_FNS(on_rr);
#define throtl_log(td, fmt, args...) \
blk_add_trace_msg((td)->queue, "throtl " fmt, ##args)

-static inline unsigned int total_nr_queued(struct throtl_data *td)
-{
- return td->nr_queued[0] + td->nr_queued[1];
-}
-
/*
* Worker for allocating per cpu stat for tgs. This is scheduled on the
* system_wq once there are some groups on the alloc_list waiting for
@@ -402,25 +397,18 @@ static void throtl_schedule_delayed_work(struct throtl_data *td,
{
struct delayed_work *dwork = &td->dispatch_work;

- if (total_nr_queued(td)) {
- mod_delayed_work(kthrotld_workqueue, dwork, delay);
- throtl_log(td, "schedule work. delay=%lu jiffies=%lu",
- delay, jiffies);
- }
+ mod_delayed_work(kthrotld_workqueue, dwork, delay);
+ throtl_log(td, "schedule work. delay=%lu jiffies=%lu", delay, jiffies);
}

static void throtl_schedule_next_dispatch(struct throtl_data *td)
{
struct throtl_rb_root *st = &td->tg_service_tree;

- /*
- * If there are more bios pending, schedule more work.
- */
- if (!total_nr_queued(td))
+ /* any pending children left? */
+ if (!st->count)
return;

- BUG_ON(!st->count);
-
update_min_dispatch_time(st);

if (time_before_eq(st->min_disptime, jiffies))
@@ -844,14 +832,11 @@ void blk_throtl_dispatch_work_fn(struct work_struct *work)

spin_lock_irq(q->queue_lock);

- if (!total_nr_queued(td))
- goto out;
-
bio_list_init(&bio_list_on_stack);

throtl_log(td, "dispatch nr_queued=%u read=%u write=%u",
- total_nr_queued(td), td->nr_queued[READ],
- td->nr_queued[WRITE]);
+ td->nr_queued[READ] + td->nr_queued[WRITE],
+ td->nr_queued[READ], td->nr_queued[WRITE]);

nr_disp = throtl_select_dispatch(td, &bio_list_on_stack);

@@ -859,7 +844,7 @@ void blk_throtl_dispatch_work_fn(struct work_struct *work)
throtl_log(td, "bios disp=%u", nr_disp);

throtl_schedule_next_dispatch(td);
-out:
+
spin_unlock_irq(q->queue_lock);

/*
--
1.8.1.4


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