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

[Yosry Ahmed]

On Thu, Jan 11, 2024 at 5:29 AM Johannes Weiner <hannes@xxxxxxxxxxx> 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>

LGTM with a couple of nits below:

Reviewed-by: Yosry Ahmed <yosryahmed@xxxxxxxxxx>

> ---
>  mm/memcontrol.c | 24 +++++++++++++++++++++---
>  1 file changed, 21 insertions(+), 3 deletions(-)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 73692cd8c142..aca879995022 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -2603,8 +2603,9 @@ static unsigned long calculate_high_delay(struct mem_cgroup *memcg,
>  }
>
>  /*
> - * Scheduled by try_charge() to be executed from the userland return path
> - * and reclaims memory over the high limit.
> + * Reclaims memory over the high limit. Called directly from
> + * try_charge() when possible, but also scheduled to be called from
> + * the userland return path where reclaim is always able to block.
>   */

nit: The term "scheduled" here is deceptive imo, it makes me think of
queue_work() and friends, when it is directly called from
resume_user_mode_work(). Can we change the terminology to "called from
the userland return path" or directly reference
resume_user_mode_work() instead? Same applies to the added comment
below in try_charge_memcg().

nit: "when possible" is not entirely accurate, it makes it seem like
we call mem_cgroup_handle_over_high() whenever we can (which means
gfpflags_allow_blocking() imo). We actually choose not to call it in
some situations, and this patch is adding one such situation. So
perhaps "when possible and desirable" or just "when appropriate".

>  void mem_cgroup_handle_over_high(gfp_t gfp_mask)
>  {
> @@ -2673,6 +2674,9 @@ void mem_cgroup_handle_over_high(gfp_t gfp_mask)
>         }
>
>         /*
> +        * Reclaim didn't manage to push usage below the limit, slow
> +        * this allocating task down.
> +        *
>          * If we exit early, we're guaranteed to die (since
>          * schedule_timeout_killable sets TASK_KILLABLE). This means we don't
>          * need to account for any ill-begotten jiffies to pay them off later.
> @@ -2867,8 +2871,22 @@ static int try_charge_memcg(struct mem_cgroup *memcg, gfp_t gfp_mask,
>                 }
>         } while ((memcg = parent_mem_cgroup(memcg)));
>
> +       /*
> +        * Reclaim is scheduled for the userland return path already,
> +        * but also attempt synchronous reclaim to avoid excessive
> +        * overrun while the task is still inside the kernel. If this
> +        * is successful, the return path will see it when it rechecks
> +        * the overage, and simply bail out.
> +        *
> +        * Skip if the task is already dying, though. Unlike
> +        * memory.max, memory.high enforcement isn't as strict, and
> +        * there is no OOM killer involved, which means the excess
> +        * could already be much bigger (and still growing) than it
> +        * could for memory.max; the dying task could get stuck in
> +        * fruitless reclaim for a long time, which isn't desirable.
> +        */
>         if (current->memcg_nr_pages_over_high > MEMCG_CHARGE_BATCH &&
> -           !(current->flags & PF_MEMALLOC) &&
> +           !(current->flags & PF_MEMALLOC) && !task_is_dying() &&
>             gfpflags_allow_blocking(gfp_mask)) {
>                 mem_cgroup_handle_over_high(gfp_mask);
>         }
> --
> 2.43.0
>
>