Re: [PATCH resend] memcg: introduce per-memcg reclaim interface

Johannes Weiner
2022-04-01T21:07:39Z

On Fri, Apr 01, 2022 at 01:14:35PM -0700, Wei Xu wrote:
> On Fri, Apr 1, 2022 at 8:22 AM Johannes Weiner <hannes@cmpxchg.org> wrote:
> > On Thu, Mar 31, 2022 at 09:05:15PM -0700, Wei Xu wrote:
> > > It is better to return an error code (e.g. -EBUSY) when
> > > memory_reclaim() fails to reclaim nr_to_reclaim bytes of memory,
> > > except if the cgroup memory usage is already 0. We can also return
> > > -EINVAL if nr_to_reclaim is too large (e.g. > limit).
> >
> > For -EBUSY, are you thinking of a specific usecase where that would
> > come in handy? I'm not really opposed to it, but couldn't convince
> > myself of the practical benefits of it, either.
> >
> > Keep in mind that MAX_RECLAIM_RETRIES failed reclaim attempts usually
> > constitute an OOM situation: memory.max will issue kills and
> > memory.high will begin crippling throttling. In what scenario would
> > you want to keep reclaiming a workload that is considered OOM?
> >
> > Certainly, proactive reclaim that wants to purge only the cold tail of
> > the workload wouldn't retry. Meta's version of this patch actually
> > does return -EAGAIN on reclaim failure, but the userspace daemon
> > doesn't do anything with it, so I didn't bring it up.
>
> -EAGAIN sounds good, too. Given that the userspace requests to
> reclaim a specified number of bytes, I think it is generally better to
> tell the userspace whether the request has been successfully
> fulfilled. Ideally, it would be even better to return how many bytes
> that have been reclaimed, though that is not easy to do through the
> cgroup interface. The userspace can choose to ignore the return value
> or log a message/update some stats (which Google does) for the
> monitoring purpose.

Fair enough, thanks for your thoughts. No objection from me!