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sched: add some comments to the bandwidth code 

Hopefully clarify some of this code a little.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
This commit is contained in:
Peter Zijlstra 2008-09-23 15:33:43 +02:00 committed by Ingo Molnar
parent 940959e939
commit 78333cdd0e
1 changed files with 42 additions and 0 deletions
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42
kernel/sched_rt.c
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@ -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);