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

[Shakeel Butt <shakeelb@google.com>]

On Fri, Feb 11, 2022 at 4:13 AM Chris Down <chris@chrisdown.name> wrote:
>
[...]
> >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.
>
> Note that mem_cgroup_handle_over_high() has its own allocator throttling grace
> period, where it bails out if the penalty to apply is less than 10ms. The
> reclaim will still happen, though. So throttling might not happen even for
> roughly MEMCG_CHARGE_BATCH-sized allocations, depending on the overall size of
> the cgroup and its protection.
>

Here by throttling, I meant both reclaim and
schedule_timeout_killable(). I don't want to say low level details
which might change in future.

[...]
>
> Thanks, I was going to comment on v1 that I prefer to keep the implementation
> of mem_cgroup_handle_over_high if possible since we know that the mechanism has
> been safe in production over the past few years.
>
> One question I have is about throttling. It looks like this new
> mem_cgroup_handle_over_high callsite may mean that throttling is invoked more
> than once on a misbehaving workload that's failing to reclaim since the
> throttling could be invoked both here and in return to userspace, right? That
> might not be a problem, but we should think about the implications of that,
> especially in relation to MEMCG_MAX_HIGH_DELAY_JIFFIES.
>

Please note that mem_cgroup_handle_over_high() clears
memcg_nr_pages_over_high and if on the return-to-userspace path
mem_cgroup_handle_over_high() finds that memcg_nr_pages_over_high is
non-zero, then it means the task has further accumulated the charges
over high limit after a possibly synchronous
memcg_nr_pages_over_high() call.

> Maybe we should record if throttling happened previously and avoid doing it
> again for this entry into kernelspace? Not certain that's the right answer, but
> we should think about what the new semantics should be.

For now, I will keep this as is and will add a comment in the code and
a mention in the commit message about it. I will wait for others to
comment before sending the next version and thanks for taking a look.