[PATCH] sched/rt: Document why has_pushable_tasks() isn't called with a runqueue lock
From: Steven Rostedt
Date: Tue Feb 28 2017 - 18:32:41 EST
From: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
While reviewing the RT scheduling IPI logic, I was thinking that it was
a bug that has_pushable_tasks(rq) was not called under the runqueue
lock. But then I realized that there isn't a case where a race would
cause a problem, as to update has_pushable_tasks() would trigger a
push_rt_task() call from the CPU doing the update.
This subtle logic deserves a comment.
Signed-off-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
---
diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index 4101f9d..f39449b 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -1976,6 +1976,16 @@ static void try_to_push_tasks(void *arg)
src_rq = rq_of_rt_rq(rt_rq);
again:
+ /*
+ * Normally, has_pushable_tasks() would be performed within the
+ * runqueue lock being held. But if it was not set when entering
+ * this hard interrupt handler function, then to have it set would
+ * require a wake up. A wake up of an RT task will either cause a
+ * schedule if the woken task is higher priority than the running
+ * task, or it would try to do a push from the CPU doing the wake
+ * up. Grabbing the runqueue lock in such a case would more likely
+ * just cause unnecessary contention.
+ */
if (has_pushable_tasks(rq)) {
raw_spin_lock(&rq->lock);
push_rt_task(rq);