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>

1 files changed, 42 insertions, 0 deletions

diff --git a/kernel/sched_rt.c b/kernel/sched_rt.c
index 2e228bd5395..d570a8cc4fc 100644
--- a/kernel/sched_rt.c
+++ b/kernel/sched_rt.c
@@ -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);