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

[Roman Gushchin <guro@fb.com>]

On Thu, Feb 10, 2022 at 10:49:17PM -0800, Shakeel Butt 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>
> ---
> Changes since v1:
> - Based on Roman's comment simply the sync enforcement and only target
>   the extreme cases.

Reviewed-by: Roman Gushchin <guro@fb.com>

This version indeed looks more safe to me.

Thanks!