Commit 78333cdd authored by Peter Zijlstra's avatar Peter Zijlstra Committed by Ingo Molnar
Browse files

sched: add some comments to the bandwidth code 



Hopefully clarify some of this code a little.

Signed-off-by: default avatarPeter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
parent 940959e9

kernel/sched_rt.c

+42 −0
Original line number Diff line number Diff line
@@ -231,6 +231,9 @@ static inline struct rt_bandwidth *sched_rt_bandwidth(struct rt_rq *rt_rq) 
#endif /* CONFIG_RT_GROUP_SCHED */



#ifdef CONFIG_SMP

/*

 * We ran out of runtime, see if we can borrow some from our neighbours.

 */

static int do_balance_runtime(struct rt_rq *rt_rq)

{

	struct rt_bandwidth *rt_b = sched_rt_bandwidth(rt_rq);
@@ -250,9 +253,18 @@ static int do_balance_runtime(struct rt_rq *rt_rq) 
			continue;



		spin_lock(&iter->rt_runtime_lock);

		/*

		 * Either all rqs have inf runtime and there's nothing to steal

		 * or __disable_runtime() below sets a specific rq to inf to

		 * indicate its been disabled and disalow stealing.

		 */

		if (iter->rt_runtime == RUNTIME_INF)

			goto next;



		/*

		 * From runqueues with spare time, take 1/n part of their

		 * spare time, but no more than our period.

		 */

		diff = iter->rt_runtime - iter->rt_time;

		if (diff > 0) {

			diff = div_u64((u64)diff, weight);
@@ -274,6 +286,9 @@ next: 
	return more;

}



/*

 * Ensure this RQ takes back all the runtime it lend to its neighbours.

 */

static void __disable_runtime(struct rq *rq)

{

	struct root_domain *rd = rq->rd;
@@ -289,17 +304,33 @@ static void __disable_runtime(struct rq *rq) 


		spin_lock(&rt_b->rt_runtime_lock);

		spin_lock(&rt_rq->rt_runtime_lock);

		/*

		 * Either we're all inf and nobody needs to borrow, or we're

		 * already disabled and thus have nothing to do, or we have

		 * exactly the right amount of runtime to take out.

		 */

		if (rt_rq->rt_runtime == RUNTIME_INF ||

				rt_rq->rt_runtime == rt_b->rt_runtime)

			goto balanced;

		spin_unlock(&rt_rq->rt_runtime_lock);



		/*

		 * Calculate the difference between what we started out with

		 * and what we current have, that's the amount of runtime

		 * we lend and now have to reclaim.

		 */

		want = rt_b->rt_runtime - rt_rq->rt_runtime;



		/*

		 * Greedy reclaim, take back as much as we can.

		 */

		for_each_cpu_mask(i, rd->span) {

			struct rt_rq *iter = sched_rt_period_rt_rq(rt_b, i);

			s64 diff;



			/*

			 * Can't reclaim from ourselves or disabled runqueues.

			 */

			if (iter == rt_rq || iter->rt_runtime == RUNTIME_INF)

				continue;
@@ -319,8 +350,16 @@ static void __disable_runtime(struct rq *rq) 
		}



		spin_lock(&rt_rq->rt_runtime_lock);

		/*

		 * We cannot be left wanting - that would mean some runtime

		 * leaked out of the system.

		 */

		BUG_ON(want);

balanced:

		/*

		 * Disable all the borrow logic by pretending we have inf

		 * runtime - in which case borrowing doesn't make sense.

		 */

		rt_rq->rt_runtime = RUNTIME_INF;

		spin_unlock(&rt_rq->rt_runtime_lock);

		spin_unlock(&rt_b->rt_runtime_lock);
@@ -343,6 +382,9 @@ static void __enable_runtime(struct rq *rq) 
	if (unlikely(!scheduler_running))

		return;



	/*

	 * Reset each runqueue's bandwidth settings

	 */

	for_each_leaf_rt_rq(rt_rq, rq) {

		struct rt_bandwidth *rt_b = sched_rt_bandwidth(rt_rq);