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.
>
> Fixes: c9afe31ec443 ("memcg: synchronously enforce memory.high for large overcharges")
> Reported-by: Tejun Heo <tj@xxxxxxxxxx>
> Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
Acked-by: Shakeel Butt <shakeelb@xxxxxxxxxx>
