On Wed, May 14, 2014 at 11:08:35AM -0400, Paul Gortmaker wrote:
> 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.
In theory, one way of making RCU OK with an RT usermode CPU hog is to
build with Frederic's CONFIG_NO_HZ_FULL=y. This will cause RCU to see
CPUs having a single runnable usermode task as idle, preventing the RCU
CPU stall warning. This does work well for mainline kernel in the lab.
In practice, not sure how much testing CONFIG_NO_HZ_FULL=y has received
for -rt kernels in production environments.
But leaving practice aside for the moment...
> 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)
How about something like the following to take NO_HZ_FULL into account?
+ if ((!rt_rq->rt_nr_migratory || rt_rq->rt_nr_total == 1) &&
+ !tick_nohz_full_cpu(cpu))
Thanx, Paul
> + 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
>
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