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blk-throttle: remove pointless throtl_nr_queued() optimizations 

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>
Acked-by: Vivek Goyal <vgoyal@redhat.com>
This commit is contained in:
Tejun Heo 2013-05-14 13:52:32 -07:00
parent a9131a27e2
commit 6a525600ff
1 changed files with 7 additions and 22 deletions
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29
block/blk-throttle.c
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@ -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);
/*