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Subjectsched tree bisectability
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While trying to bisect a regression added by the sched tree merges
today, I found the following gem that broke compilation mid-bisect:

commit d0b27fa77854b149ad4af08b0fe47fe712a47ade
Author: Peter Zijlstra <a.p.zijlstra@chello.nl>
Date: Sat Apr 19 19:44:57 2008 +0200

sched: rt-group: synchonised bandwidth period

Various SMP balancing algorithms require that the bandwidth period
run in sync.

Possible improvements are moving the rt_bandwidth thing into root_domain
and keeping a span per rt_bandwidth which marks throttled cpus.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>


That change adds:

@@ -156,6 +161,80 @@ struct rt_prio_array {
struct list_head queue[MAX_RT_PRIO];
};

+struct rt_bandwidth {
+ ktime_t rt_period;
+ u64 rt_runtime;
+ struct hrtimer rt_period_timer;
+};
+

and then things like:

+static void start_rt_bandwidth(struct rt_bandwidth *rt_b)
+{
+ ktime_t now;
+
+ if (rt_b->rt_runtime == RUNTIME_INF)
+ return;
+
+ if (hrtimer_active(&rt_b->rt_period_timer))
+ return;
+
+ spin_lock(&rt_b->rt_runtime_lock);

rt_bandwidth does not have a rt_runtime_lock member, so the compile
fails. The very next changeset adds it:
commit ac086bc22997a2be24fc40fc8d46522fe7e03d11
Author: Peter Zijlstra <a.p.zijlstra@chello.nl>
Date: Sat Apr 19 19:44:58 2008 +0200

sched: rt-group: smp balancing

Currently the rt group scheduling does a per cpu runtime limit, however
the rt load balancer makes no guarantees about an equal spread of real-
time tasks, just that at any one time, the highest priority tasks run.

Solve this by making the runtime limit a global property by borrowing
excessive runtime from the other cpus once the local limit runs out.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

diff --git a/kernel/sched.c b/kernel/sched.c
index bb20323..313cd4f 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -164,6 +164,7 @@ struct rt_prio_array {
struct rt_bandwidth {
ktime_t rt_period;
u64 rt_runtime;
+ spinlock_t rt_runtime_lock;
struct hrtimer rt_period_timer;
};


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