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Commit 6a525600 authored by Tejun Heo's avatar Tejun Heo
Browse files

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: default avatarTejun Heo <tj@kernel.org>
Acked-by: default avatarVivek Goyal <vgoyal@redhat.com>
parent a9131a27

block/blk-throttle.c

+7 −22
Original line number Diff line number Diff line
@@ -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);

	}

	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);



	/*