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

[Shakeel Butt]

On Tue, Sep 22, 2020 at 12:09 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
>
> On Tue 22-09-20 11:10:17, Shakeel Butt wrote:
> > On Tue, Sep 22, 2020 at 9:55 AM Michal Hocko <mhocko@xxxxxxxx> wrote:
> [...]
> > > Last but not least the memcg
> > > background reclaim is something that should be possible without a new
> > > interface.
> >
> > So, it comes down to adding more functionality/semantics to
> > memory.high or introducing a new simple interface. I am fine with
> > either of one but IMO convoluted memory.high might have a higher
> > maintenance cost.
>
> One idea would be to schedule a background worker (which work on behalf
> on the memcg) to do the high limit reclaim with high limit target as
> soon as the high limit is reached. There would be one work item for each
> memcg. Userspace would recheck the high limit on return to the userspace
> and do the reclaim if the excess is larger than a threshold, and sleep
> as the fallback.
>
> Excessive consumers would get throttled if the background work cannot
> keep up with the charge pace and most of them would return without doing
> any reclaim because there is somebody working on their behalf - and is
> accounted for that.
>
> The semantic of high limit would be preserved IMHO because high limit is
> actively throttled. Where that work is done shouldn't matter as long as
> it is accounted properly and memcg cannot outsource all the work to the
> rest of the system.
>
> Would something like that (with many details to be sorted out of course)
> be feasible?
>

Well what about the proactive reclaim use-case? You are targeting only
uswapd/background-reclaim use-case.

> If we do not want to change the existing semantic of high and want a new
> api then I think having another limit for the background reclaim then
> that would make more sense to me. It would resemble the global reclaim
> and kswapd model and something that would be easier to reason about.
> Comparing to echo $N > reclaim which might mean to reclaim any number
> pages around N.
> --

I am not really against the approach you are proposing but "echo $N >
reclaim" allows more flexibility and enables more use-cases.