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Subject[PATCH 3/6] 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>
---
kernel/sched_rt.c | 42 ++++++++++++++++++++++++++++++++++++++++++
1 file changed, 42 insertions(+)

Index: linux-2.6/kernel/sched_rt.c
===================================================================
--- linux-2.6.orig/kernel/sched_rt.c
+++ linux-2.6/kernel/sched_rt.c
@@ -231,6 +231,9 @@ static inline struct rt_bandwidth *sched
#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_
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

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
}

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 *
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);

--



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