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

[Balbir Singh <balbirs@nvidia.com>]

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@surriel.com>
> ---
>  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