[patch] Re: rt scheduler may calculate wrong rt_time

[Mike Galbraith <efault@gmx.de>]

On Wed, 2011-04-27 at 19:51 +0200, Thomas Giesel wrote:
> > Hm.  Does forcing a clock update if we're idle when we release the
> > throttle do the trick?
> 
> It does. I tested it today and it works as expected. Even with ftrace I
> couldn't see any suspicious behaviour anymore.
> 
> Mike: Can you send the patch to the right people to get it into the
> kernel or should I do it? Or is Peter the right one already?

Peter is the right one.  Below is an ever so slightly different version.

sched, rt: update rq clock when unthrottling of an otherwise idle CPU

If an RT task is awakened while it's rt_rq is throttled, the time between
wakeup/enqueue and unthrottle/selection may be accounted as rt_time
if the CPU is idle.  Set rq->skip_clock_update negative upon throttle
release to tell put_prev_task() that we need a clock update.

Signed-off-by: Mike Galbraith <efault@gmx.de>
Reported-by: Thomas Giesel <skoe@directbox.com>

---
 kernel/sched.c    |    6 +++---
 kernel/sched_rt.c |    7 +++++++
 2 files changed, 10 insertions(+), 3 deletions(-)

Index: linux-2.6/kernel/sched.c
===================================================================
--- linux-2.6.orig/kernel/sched.c
+++ linux-2.6/kernel/sched.c
@@ -464,7 +464,7 @@ struct rq {
 	u64 nohz_stamp;
 	unsigned char nohz_balance_kick;
 #endif
-	unsigned int skip_clock_update;
+	int skip_clock_update;
 
 	/* capture load from *all* tasks on this cpu: */
 	struct load_weight load;
@@ -650,7 +650,7 @@ static void update_rq_clock(struct rq *r
 {
 	s64 delta;
 
-	if (rq->skip_clock_update)
+	if (rq->skip_clock_update > 0)
 		return;
 
 	delta = sched_clock_cpu(cpu_of(rq)) - rq->clock;
@@ -4125,7 +4125,7 @@ static inline void schedule_debug(struct
 
 static void put_prev_task(struct rq *rq, struct task_struct *prev)
 {
-	if (prev->on_rq)
+	if (prev->on_rq || rq->skip_clock_update < 0)
 		update_rq_clock(rq);
 	prev->sched_class->put_prev_task(rq, prev);
 }
Index: linux-2.6/kernel/sched_rt.c
===================================================================
--- linux-2.6.orig/kernel/sched_rt.c
+++ linux-2.6/kernel/sched_rt.c
@@ -562,6 +562,13 @@ static int do_sched_rt_period_timer(stru
 			if (rt_rq->rt_throttled && rt_rq->rt_time < runtime) {
 				rt_rq->rt_throttled = 0;
 				enqueue = 1;
+
+				/*
+				 * Force a clock update if the CPU was idle,
+				 * lest wakeup -> unthrottle time accumulate.
+				 */
+				if (rt_rq->rt_nr_running && rq->curr == rq->idle)
+					rq->skip_clock_update = -1;
 			}
 			if (rt_rq->rt_time || rt_rq->rt_nr_running)
 				idle = 0;