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

From: Wanpeng Li
Date: Thu Nov 27 2014 - 06:36:55 EST

Hi Paul,
On 5/15/14, 3:11 AM, Paul Gortmaker wrote:
[Added Frederic to Cc: since we are now talking nohz stuff]

[Re: [PATCH] sched/rt: don't try to balance rt_runtime when it is futile] On 14/05/2014 (Wed 08:44) Paul E. McKenney wrote:

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.
Agreed; wanting to test that locally for myself meant moving to a more
modern machine, as the older PentiumD doesn't support NO_HZ_FULL. But

Could you point out which hw feature support NO_HZ_FULL? How to check it through cpuid?

Regards,
Wanpeng Li

on the newer box (dual socket six cores in each) I found the stall
harder to trigger w/o going back to using the threadirqs boot arg as
used in the earlier lkml post referenced below. (Why? Not sure...)

Once I did that though (boot vanilla linux-next with threadirqs) I
confirmed what you said; i.e. that we would reliably get a stall with
the defconfig of NOHZ_IDLE=y but not with NOHZ_FULL=y (and hence also
RCU_USER_QS=y).

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...

[...]

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))
Yes, I think special casing nohz_full can make sense, but maybe not
exactly here in balance_runtime? Since the underlying reasoning doesn't
change on nohz_full ; if only one task is present, or nothing can
migrate, then the call to do_balance_runtime is largely useless - we'll
walk possibly all cpus in search of an rt_rq to steal from, and what we
steal, we can't use - so we've artificially crippled the other rt_rq for
nothing other than to artifically inflate our rt_runtime and thus allow
100% usage.

Given that, perhaps a separate change to sched_rt_runtime_exceeded()
that works out the CPU from the rt_rq, and returns zero if it is a
nohz_full cpu? Does that make sense? Then the nohz_full people won't
get the throttling message even if they go 100%.

Paul.
--

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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