[PATCH tip/core/rcu 03/16] documentation: Slow systems can stall RCU grace periods

From: Paul E. McKenney
Date: Wed Oct 04 2017 - 17:22:02 EST

If a fast system has a worst-case grace-period duration of (say) ten
seconds, then running the same workload on a system ten times as slow
will get you an RCU CPU stall warning given default stall-warning
timeout settings. This commit therefore adds this possibility to

Reported-by: Daniel Lezcano <daniel.lezcano@xxxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Documentation/RCU/stallwarn.txt | 6 ++++++
1 file changed, 6 insertions(+)

diff --git a/Documentation/RCU/stallwarn.txt b/Documentation/RCU/stallwarn.txt
index 21b8913acbdf..238acbd94917 100644
--- a/Documentation/RCU/stallwarn.txt
+++ b/Documentation/RCU/stallwarn.txt
@@ -70,6 +70,12 @@ o A periodic interrupt whose handler takes longer than the time
considerably longer than normal, which can in turn result in
RCU CPU stall warnings.

+o Testing a workload on a fast system, tuning the stall-warning
+ timeout down to just barely avoid RCU CPU stall warnings, and then
+ running the same workload with the same stall-warning timeout on a
+ slow system. Note that thermal throttling and on-demand governors
+ can cause a single system to be sometimes fast and sometimes slow!
o A hardware or software issue shuts off the scheduler-clock
interrupt on a CPU that is not in dyntick-idle mode. This
problem really has happened, and seems to be most likely to