On Fri, Feb 5, 2021 at 4:24 PM Michal Hocko [off-list ref] wrote:

On Fri 05-02-21 14:23:10, Muchun Song wrote:

quoted

We call memcg_oom_recover() in the uncharge_batch() to wakeup OOM task
when page uncharged, but for the slab pages, we do not do this when page
uncharged.

How does the patch deal with this?

When we uncharge a slab page via __memcg_kmem_uncharge,
actually, this path forgets to do this for us compared to
uncharge_batch(). Right?

quoted

When we drain per cpu stock, we also should do this.

Can we have anything the per-cpu stock while entering the OOM path. IIRC
we do drain all cpus before entering oom path.

You are right. I did not notice this. Thank you.

quoted

The memcg_oom_recover() is small, so make it inline.

Does this lead to any code generation improvements? I would expect
compiler to be clever enough to inline static functions if that pays
off. If yes make this a patch on its own.

I have disassembled the code, I see memcg_oom_recover is not
inline. Maybe because memcg_oom_recover has a lot of callers.
Just guess.

(gdb) disassemble uncharge_batch
 [...]
 0xffffffff81341c73 <+227>: callq  0xffffffff8133c420 <page_counter_uncharge>
 0xffffffff81341c78 <+232>: jmpq   0xffffffff81341bc0 <uncharge_batch+48>
 0xffffffff81341c7d <+237>: callq  0xffffffff8133e2c0 <memcg_oom_recover>

quoted

And the parameter
of memcg cannot be NULL, so remove the check.

2bd9bb206b338 has introduced the check without any explanation
whatsoever. I indeed do not see any potential path to provide a NULL
memcg here. This is an internal function so the check is unnecessary
indeed. Please make it a patch on its own.

OK. Will do this. Thanks.

quoted

Signed-off-by: Muchun Song <redacted>
---
 mm/memcontrol.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 8c035846c7a4..8569f4dbea2a 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c

@@ -1925,7 +1925,7 @@ static int memcg_oom_wake_function(wait_queue_entry_t *wait,
      return autoremove_wake_function(wait, mode, sync, arg);
 }

-static void memcg_oom_recover(struct mem_cgroup *memcg)
+static inline void memcg_oom_recover(struct mem_cgroup *memcg)
 {
      /*
       * For the following lockless ->under_oom test, the only required

@@ -1935,7 +1935,7 @@ static void memcg_oom_recover(struct mem_cgroup *memcg)
       * achieved by invoking mem_cgroup_mark_under_oom() before
       * triggering notification.
       */
-     if (memcg && memcg->under_oom)
+     if (memcg->under_oom)
              __wake_up(&memcg_oom_waitq, TASK_NORMAL, 0, memcg);
 }

@@ -2313,6 +2313,7 @@ static void drain_stock(struct memcg_stock_pcp *stock)
              page_counter_uncharge(&old->memory, stock->nr_pages);
              if (do_memsw_account())
                      page_counter_uncharge(&old->memsw, stock->nr_pages);
+             memcg_oom_recover(old);
              stock->nr_pages = 0;
      }

--

2.11.0

--
Michal Hocko
SUSE Labs