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

Roman Gushchin
2024-01-12T21:00:12Z

On Fri, Jan 12, 2024 at 11:04:06AM -0800, Shakeel Butt wrote:
> On Thu, Jan 11, 2024 at 11:28 AM Johannes Weiner <hannes@cmpxchg.org> wrote:
> >
> [...]
> >
> > From 6124a13cb073f5ff06b9c1309505bc937d65d6e5 Mon Sep 17 00:00:00 2001
> > From: Johannes Weiner <hannes@cmpxchg.org>
> > Date: Thu, 11 Jan 2024 07:18:47 -0500
> > Subject: [PATCH] mm: memcontrol: don't throttle dying tasks on memory.high
> >
> > 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@kernel.org>
> > Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
>
> Acked-by: Shakeel Butt <shakeelb@google.com>
>
> I am wondering if you have seen or suspected a similar issue but for
> remote memcg charging. For example pageout on a global reclaim which
> has to allocate buffers for some other memcg.

You mean dying tasks entering a direct reclaim mode?
Or kswapd being stuck in the reclaim path?