[PATCH] mm: memcg: yield cpu when we fail to charge pages

From: Julius Hemanth Pitti
Date: Tue Sep 08 2020 - 14:59:06 EST

For non root CG, in try_charge(), we keep trying
to charge until we succeed. On non-preemptive
kernel, when we are OOM, this results in holding
CPU forever.

On SMP systems, this doesn't create a big problem
because oom_reaper get a change to kill victim
and make some free pages. However on a single-core
CPU (or cases where oom_reaper pinned to same CPU
where try_charge is executing), oom_reaper shall
never get scheduled and we stay in try_charge forever.

Steps to repo this on non-smp:
1. mount -t tmpfs none /sys/fs/cgroup
2. mkdir /sys/fs/cgroup/memory
3. mount -t cgroup none /sys/fs/cgroup/memory -o memory
4. mkdir /sys/fs/cgroup/memory/0
5. echo 40M > /sys/fs/cgroup/memory/0/memory.limit_in_bytes
6. echo $$ > /sys/fs/cgroup/memory/0/tasks
7. stress -m 5 --vm-bytes 10M --vm-hang 0

Signed-off-by: Julius Hemanth Pitti <jpitti@xxxxxxxxx>
mm/memcontrol.c | 2 ++
1 file changed, 2 insertions(+)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 0d6f3ea86738..4620d70267cb 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2652,6 +2652,8 @@ static int try_charge(struct mem_cgroup *memcg, gfp_t gfp_mask,
if (fatal_signal_pending(current))
goto force;

+ cond_resched();
* keep retrying as long as the memcg oom killer is able to make
* a forward progress or bypass the charge if the oom killer