On my tulsa x86-64 machine, kernel 2.6.25-rc5 couldn't boot randomly.
Basically, function __enable_runtime forgets to reset rt_rq->rt_throttled to 0.
When every cpu is up, per-cpu migration_thread is created and it runs very fast,
sometimes to mark the corresponding rt_rq->rt_throttled to 1 very quickly. After
all cpus are up, with below calling chain,
sched_init_smp => arch_init_sched_domains => build_sched_domains => ...
=> cpu_attach_domain => rq_attach_root => set_rq_online => ... => __enable_runtime,
__enable_runtime is called against every rt_rq again, so rt_rq->rt_time is reset to
0, but rt_rq->rt_throttled might be still 1. Later on function do_sched_rt_period_timer
couldn't reset it, and all RT tasks couldn't be scheduled to run on that cpu.
here is RT task migration_thread which is woken up when a task is migrated to another cpu.
Below patch fixes it against 2.6.27-rc5.
Signed-off-by: Zhang Yanmin <firstname.lastname@example.org>
diff -Nraup linux-2.6.27-rc5/kernel/sched_rt.c linux-2.6.27-rc5_fix/kernel/sched_rt.c
--- linux-2.6.27-rc5/kernel/sched_rt.c 2008-09-09 11:06:43.000000000 +0800
+++ linux-2.6.27-rc5_fix/kernel/sched_rt.c 2008-09-09 11:13:04.000000000 +0800
@@ -350,6 +350,7 @@ static void __enable_runtime(struct rq *
rt_rq->rt_runtime = rt_b->rt_runtime;
rt_rq->rt_time = 0;
+ rt_rq->rt_throttled = 0;