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From:   Julius Hemanth Pitti <jpitti@cisco.com>
To:     hannes@cmpxchg.org, mhocko@kernel.org, vdavydov.dev@gmail.com,
        akpm@linux-foundation.org, guro@fb.com
Cc:     cgroups@vger.kernel.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, xe-linux-external@cisco.com,
        Julius Hemanth Pitti <jpitti@cisco.com>
Subject: [PATCH v2] mm: memcg: yield cpu when we fail to charge pages
Date:   Tue,  8 Sep 2020 13:14:26 -0700
Message-Id: <20200908201426.14837-1-jpitti@cisco.com>
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Sender: linux-kernel-owner@vger.kernel.org
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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@cisco.com>
Acked-by: Roman Gushchin <guro@fb.com>
---

Changes in v2:
 - Added comments.
 - Added "Acked-by: Roman Gushchin <guro@fb.com>".
---
 mm/memcontrol.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index cfa6cbad21d5..4f293bf8c7ed 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2745,6 +2745,15 @@ static int try_charge(struct mem_cgroup *memcg, gfp_t gfp_mask,
 	if (fatal_signal_pending(current))
 		goto force;
 
+	/*
+	 * We failed to charge even after retries, give oom_reaper or
+	 * other process a change to make some free pages.
+	 *
+	 * On non-preemptive, Non-SMP system, this is critical, else
+	 * we keep retrying with no success, forever.
+	 */
+	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
-- 
2.17.1