Re: [PATCH RFC] mm/memcontrol: reclaim severe usage over high limit in get_user_pages loop

[Yang Shi]

On Fri, Aug 2, 2019 at 2:35 AM Michal Hocko <mhocko@xxxxxxxxxx> wrote:
>
> On Thu 01-08-19 14:00:51, Yang Shi wrote:
> > On Mon, Jul 29, 2019 at 11:48 AM Michal Hocko <mhocko@xxxxxxxxxx> wrote:
> > >
> > > On Mon 29-07-19 10:28:43, Yang Shi wrote:
> > > [...]
> > > > I don't worry too much about scale since the scale issue is not unique
> > > > to background reclaim, direct reclaim may run into the same problem.
> > >
> > > Just to clarify. By scaling problem I mean 1:1 kswapd thread to memcg.
> > > You can have thousands of memcgs and I do not think we really do want
> > > to create one kswapd for each. Once we have a kswapd thread pool then we
> > > get into a tricky land where a determinism/fairness would be non trivial
> > > to achieve. Direct reclaim, on the other hand is bound by the workload
> > > itself.
> >
> > Yes, I agree thread pool would introduce more latency than dedicated
> > kswapd thread. But, it looks not that bad in our test. When memory
> > allocation is fast, even though dedicated kswapd thread can't catch
> > up. So, such background reclaim is best effort, not guaranteed.
> >
> > I don't quite get what you mean about fairness. Do you mean they may
> > spend excessive cpu time then cause other processes starvation? I
> > think this could be mitigated by properly organizing and setting
> > groups. But, I agree this is tricky.
>
> No, I meant that the cost of reclaiming a unit of charges (e.g.
> SWAP_CLUSTER_MAX) is not constant and depends on the state of the memory
> on LRUs. Therefore any thread pool mechanism would lead to unfair
> reclaim and non-deterministic behavior.

Yes, the cost depends on the state of pages, but I still don't quite
understand what does "unfair" refer to in this context. Do you mean
some cgroups may reclaim much more than others? Or the work may take
too long so it can't not serve other cgroups in time?

>
> I can imagine a middle ground where the background reclaim would have to
> be an opt-in feature and a dedicated kernel thread would be assigned to
> the particular memcg (hierarchy).

Yes, it is opt-in by defining a proper "water mark". As long as "water
mark" is defined (0, 100), the "kswapd" work would be queued once the
usage is greater than "water mark", then it would exit once the usage
is under "water mark". If "water mark" is 0, it will never queue any
background reclaim work.

We did use dedicated kernel thread for each cgroup, but it turns out
it is also tricky and error prone to manage the kernel thread,
workqueue sounds much more simple and less error prone.

> --
> Michal Hocko
> SUSE Labs