[LTP] [PATCH v1] mmapstress06: Rate limit page-dirtying loop to prevent MemCG OOM-kills

[Cyril Hrubis]

Hi!
> yes, basically we re-implement some sort of OOM-killer/memory.reclaim
> inside the code, instead of using them directly. That means we need to
> disable OOM killer, loop over the current swap status and eventually
> raise an error if after N iterations pages are not swapped.
> 
> But I feel it's not deterministic because on a really slow device we
> might run out of iterations. I still think memory.reclaim is the best
> choice for v2 in this case, because it will wait until a certain amount
> of memory has been reclaimed and eventually raise an error.
> 
> @Cyril do you have any idea on how to proceed here?	

If the goal is to make sure that kernel swaps when memory.max is reached
we may as well slow down the dirtying loop so that it dirties memory
with speed 20MB/s when it gets close to the limit.

Something as:

diff --git a/testcases/kernel/mem/mmapstress/mmapstress06.c b/testcases/kernel/mem/mmapstress/mmapstress06.c
index 4e6023399..2c0c6fb02 100644
--- a/testcases/kernel/mem/mmapstress/mmapstress06.c
+++ b/testcases/kernel/mem/mmapstress/mmapstress06.c
@@ -59,8 +59,9 @@ static void run_test(void)
 
                for (size_t i = 0; i < map_size; i += page_size) {
                        mmapaddr[i] = 'a';
-                       if ((i % (2 * 1024 * 1024)) == 0)
-                               usleep(1000);
+
+                       if (i >= mem_limit && !(i % (2 * 1024 * 1024)))
+                               usleep(100000);
                }

The testrun increses to 6s from 1s but it makes the OOM very unlikely to
happen.

-- 
Cyril Hrubis
chrubis@suse.cz