Re: [PATCH] memcg: allow exiting tasks to write back data to swap

From: Balbir Singh
Date: Wed Dec 11 2024 - 18:16:00 EST


On 12/12/24 02:53, Rik van Riel wrote:
> A task already in exit can get stuck trying to allocate pages, if its
> cgroup is at the memory.max limit, the cgroup is using zswap, but
> zswap writeback is enabled, and the remaining memory in the cgroup is
> not compressible.
>
> This seems like an unlikely confluence of events, but it can happen
> quite easily if a cgroup is OOM killed due to exceeding its memory.max
> limit, and all the tasks in the cgroup are trying to exit simultaneously.
>
> When this happens, it can sometimes take hours for tasks to exit,
> as they are all trying to squeeze things into zswap to bring the group's
> memory consumption below memory.max.
>
> Allowing these exiting programs to push some memory from their own
> cgroup into swap allows them to quickly bring the cgroup's memory
> consumption below memory.max, and exit in seconds rather than hours.
>
> Loading this fix as a live patch on a system where a workload got stuck
> exiting allowed the workload to exit within a fraction of a second.
>
> Signed-off-by: Rik van Riel <riel@xxxxxxxxxxx>
> ---
> mm/memcontrol.c | 9 +++++++++
> 1 file changed, 9 insertions(+)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 7b3503d12aaf..03d77e93087e 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -5371,6 +5371,15 @@ bool mem_cgroup_zswap_writeback_enabled(struct mem_cgroup *memcg)
> if (!zswap_is_enabled())
> return true;
>
> + /*
> + * Always allow exiting tasks to push data to swap. A process in
> + * the middle of exit cannot get OOM killed, but may need to push
> + * uncompressible data to swap in order to get the cgroup memory
> + * use below the limit, and make progress with the exit.
> + */
> + if ((current->flags & PF_EXITING) && memcg == mem_cgroup_from_task(current))
> + return true;
> +
> for (; memcg; memcg = parent_mem_cgroup(memcg))
> if (!READ_ONCE(memcg->zswap_writeback))
> return false;

Rik,

I am unable to understand the motivation here, so we want
mem_cgroup_zswap_writeback_enabled() to return true, it only
returns false if a memcg in the hierarchy has zswap_writeback
set to 0 (false). In my git-grep I can't seem to find how/why
that may be the case. I can see memcg starts of with the value
set to true, if CONFIG_ZSWAP is enabled.

Your changelog above makes sense, but I am unable to map it to
the code changes.

Balbir