Re: [PATCH v2 4/4] memcg: synchronously enforce memory.high for large overcharges

[Shakeel Butt <shakeelb@google.com>]

On Thu, Feb 10, 2022 at 10:49 PM Shakeel Butt <shakeelb@google.com> wrote:
>
> The high limit is used to throttle the workload without invoking the
> oom-killer. Recently we tried to use the high limit to right size our
> internal workloads. More specifically dynamically adjusting the limits
> of the workload without letting the workload get oom-killed. However due
> to the limitation of the implementation of high limit enforcement, we
> observed the mechanism fails for some real workloads.
>
> The high limit is enforced on return-to-userspace i.e. the kernel let
> the usage goes over the limit and when the execution returns to
> userspace, the high reclaim is triggered and the process can get
> throttled as well. However this mechanism fails for workloads which do
> large allocations in a single kernel entry e.g. applications that
> mlock() a large chunk of memory in a single syscall. Such applications
> bypass the high limit and can trigger the oom-killer.
>
> To make high limit enforcement more robust, this patch makes the limit
> enforcement synchronous only if the accumulated overcharge becomes
> larger than MEMCG_CHARGE_BATCH. So, most of the allocations would still
> be throttled on the return-to-userspace path but only the extreme
> allocations which accumulates large amount of overcharge without
> returning to the userspace will be throttled synchronously. The value
> MEMCG_CHARGE_BATCH is a bit arbitrary but most of other places in the
> memcg codebase uses this constant therefore for now uses the same one.
>
> Signed-off-by: Shakeel Butt <shakeelb@google.com>

Any comments or concerns on this patch? Otherwise I would ask Andrew
to add this series into the mm tree.

> ---
> Changes since v1:
> - Based on Roman's comment simply the sync enforcement and only target
>   the extreme cases.
>
>  mm/memcontrol.c | 5 +++++
>  1 file changed, 5 insertions(+)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 292b0b99a2c7..0da4be4798e7 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -2703,6 +2703,11 @@ static int try_charge_memcg(struct mem_cgroup *memcg, gfp_t gfp_mask,
>                 }
>         } while ((memcg = parent_mem_cgroup(memcg)));
>
> +       if (current->memcg_nr_pages_over_high > MEMCG_CHARGE_BATCH &&
> +           !(current->flags & PF_MEMALLOC) &&
> +           gfpflags_allow_blocking(gfp_mask)) {
> +               mem_cgroup_handle_over_high();
> +       }
>         return 0;
>  }
>
> --
> 2.35.1.265.g69c8d7142f-goog
>