[PATCH] sched/rt: don't try to balance rt_runtime when it is futile

[Paul Gortmaker]

As of the old commit ac086bc22997a2be24fc40fc8d46522fe7e03d11
("sched: rt-group: smp balancing") the concept of borrowing per
cpu rt_runtime from one core to another was introduced.

However, this prevents the RT throttling message from ever being
emitted when someone does a common (but mistaken) attempt at
using too much CPU in RT context.  Consider the following test:

  echo "main() {for(;;);}" > full_load.c
  gcc full_load.c -o full_load
  taskset -c 1 ./full_load &
  chrt -r -p 80 `pidof full_load`

When run on x86_64 defconfig, what happens is as follows:

-task runs on core1 for 95% of an rt_period as documented in
 the file Documentation/scheduler/sched-rt-group.txt

-at 95%, the code in balance_runtime sees this threshold and
 calls do_balance_runtime()

-do_balance_runtime sees that core 1 is in need, and does this:
	---------------
        if (rt_rq->rt_runtime + diff > rt_period)
                diff = rt_period - rt_rq->rt_runtime;
        iter->rt_runtime -= diff;
        rt_rq->rt_runtime += diff;
	---------------
 which extends core1's rt_runtime by 5%, making it 100% of rt_period
 by stealing 5% from core0 (or possibly some other core).

However, the next time core1's rt_rq enters sched_rt_runtime_exceeded(),
we hit this near the top of that function:
	---------------
        if (runtime >= sched_rt_period(rt_rq))
                return 0;
	---------------
and hence we'll _never_ look at/set any of the throttling checks and
messages in sched_rt_runtime_exceeded().  Instead, we will happily
plod along for CONFIG_RCU_CPU_STALL_TIMEOUT seconds, at which point
the RCU subsystem will get angry and trigger an NMI in response to
what it rightly sees as a WTF situation.

Granted, there are lots of ways you can do bad things to yourself with
RT, but in the current zeitgeist of multicore systems with people
dedicating individual cores to individual tasks, I'd say the above is
common enough that we should react to it sensibly, and an RCU stall
really doesn't translate well to an end user vs a simple message that
says "throttling activated".

One way to get the throttle message instead of the ambiguous and lengthy
NMI triggered all core backtrace of the RCU stall is to change the
SCHED_FEAT(RT_RUNTIME_SHARE, true) to false.  One could make a good
case for this being the default for the out-of-tree preempt-rt series,
since folks using that are more apt to be manually tuning the system
and won't want an invisible hand coming in and making changes.

However, in mainline, where it is more likely that there will be
n+x (x>0) RT tasks on an n core system, we can leave the sharing on,
and still avoid the RCU stalls by noting that there is no point in
trying to balance when there are no tasks to migrate, or only a
single RT task is present.  Inflating the rt_runtime does nothing
in this case other than defeat sched_rt_runtime_exceeded().

Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Paul Gortmaker <paul.gortmaker@xxxxxxxxxxxxx>
---

[I'd mentioned a similar use case here: https://lkml.org/lkml/2013/3/6/338
 and tglx asked why they wouldn't see the throttle message; it is only
 now that I had a chance to dig in and figure out why.  Oh, and the patch
 is against linux-next, in case that matters...]

 kernel/sched/rt.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index ea4d500..698aac9 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -774,6 +774,15 @@ static int balance_runtime(struct rt_rq *rt_rq)
 	if (!sched_feat(RT_RUNTIME_SHARE))
 		return more;
 
+	/*
+	 * Stealing from another core won't help us at all if
+	 * we have nothing to migrate over there, or only one
+	 * task that is running up all the rt_time.  In fact it
+	 * will just inhibit the throttling message in that case.
+	 */
+	if (!rt_rq->rt_nr_migratory || rt_rq->rt_nr_total == 1)
+		return more;
+
 	if (rt_rq->rt_time > rt_rq->rt_runtime) {
 		raw_spin_unlock(&rt_rq->rt_runtime_lock);
 		more = do_balance_runtime(rt_rq);
-- 
1.8.2.3

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/