Re: [PATCH] mm: memcontrol: don't throttle dying tasks on memory.high

[Roman Gushchin <roman.gushchin@linux.dev>]

On Thu, Jan 11, 2024 at 08:29:02AM -0500, Johannes Weiner wrote:
> While investigating hosts with high cgroup memory pressures, Tejun
> found culprit zombie tasks that had were holding on to a lot of
> memory, had SIGKILL pending, but were stuck in memory.high reclaim.
> 
> In the past, we used to always force-charge allocations from tasks
> that were exiting in order to accelerate them dying and freeing up
> their rss. This changed for memory.max in a4ebf1b6ca1e ("memcg:
> prohibit unconditional exceeding the limit of dying tasks"); it noted
> that this can cause (userspace inducable) containment failures, so it
> added a mandatory reclaim and OOM kill cycle before forcing charges.
> At the time, memory.high enforcement was handled in the userspace
> return path, which isn't reached by dying tasks, and so memory.high
> was still never enforced by dying tasks.
> 
> When c9afe31ec443 ("memcg: synchronously enforce memory.high for large
> overcharges") added synchronous reclaim for memory.high, it added
> unconditional memory.high enforcement for dying tasks as well. The
> callstack shows that this path is where the zombie is stuck in.
> 
> We need to accelerate dying tasks getting past memory.high, but we
> cannot do it quite the same way as we do for memory.max: memory.max is
> enforced strictly, and tasks aren't allowed to move past it without
> FIRST reclaiming and OOM killing if necessary. This ensures very small
> levels of excess. With memory.high, though, enforcement happens lazily
> after the charge, and OOM killing is never triggered. A lot of
> concurrent threads could have pushed, or could actively be pushing,
> the cgroup into excess. The dying task will enter reclaim on every
> allocation attempt, with little hope of restoring balance.
> 
> To fix this, skip synchronous memory.high enforcement on dying tasks
> altogether again. Update memory.high path documentation while at it.

It makes total sense to me.
Acked-by: Roman Gushchin <roman.gushchin@linux.dev>

However if tasks can stuck for a long time in the "high reclaim" state,
shouldn't we also handle the case when tasks are being killed during the
reclaim? E. g. something like this (completely untested):


diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index c4c422c81f93..9f971fc6aae8 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2465,6 +2465,9 @@ static unsigned long reclaim_high(struct mem_cgroup *memcg,
                    READ_ONCE(memcg->memory.high))
                        continue;

+               if (task_is_dying())
+                       break;
+
                memcg_memory_event(memcg, MEMCG_HIGH);

                psi_memstall_enter(&pflags);
@@ -2645,6 +2648,9 @@ void mem_cgroup_handle_over_high(gfp_t gfp_mask)
        current->memcg_nr_pages_over_high = 0;

 retry_reclaim:
+       if (task_is_dying())
+               return;
+
        /*
         * The allocating task should reclaim at least the batch size, but for
         * subsequent retries we only want to do what's necessary to prevent oom