Re: [PATCH v10 12/14] mm: multi-gen LRU: debugfs interface

[Yu Zhao]

On Fri, Apr 15, 2022 at 10:20 PM Andrew Morton
<akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> On Fri, 15 Apr 2022 18:03:16 -0600 Yu Zhao <yuzhao@xxxxxxxxxx> wrote:
>
> > > Presumably sysfs is the place.  Fully documented and with usage
> > > examples in the changelog so we can carefully review the proposed
> > > extensions to Linux's ABI.  Extensions which must be maintained
> > > unchanged for all time.
> >
> > Eventually, yes. There still is a long way to go. Rest assured, this
> > is something Google will keep investing resources on.
>
> So.  The plan is to put these interfaces in debugfs for now, with a
> view to migrating stabilized interfaces into sysfs (or procfs or
> whatever) once end-user requirements and use cases are better
> understood?

The requirements are well understood and the use cases are proven,
e.g., Google [1], Meta [2] and Alibaba [3].

[1] https://dl.acm.org/doi/10.1145/3297858.3304053
[2] https://dl.acm.org/doi/10.1145/3503222.3507731
[3] https://gitee.com/anolis/cloud-kernel/blob/release-5.10/mm/kidled.c

> If so, that sounds totally great to me.  But it should have been in
> the darn changelog!  This is the sort of thing which we care about most
> keenly.
>
> It would be helpful for reviewers to understand the proposed timeline
> for this process, because the entire feature isn't really real until
> this is completed, is it?  I do think we should get this nailed down
> relatively rapidly, otherwise people will be reluctant to invest much
> into a moving target.
>
> And I must say, I see dissonance between the overall maturity of the
> feature as described in these emails versus the immaturity of these
> userspace control interfaces.  What's happening there?

Very observant. To answer both of the questions above: each iteration
of the entire stack is a multi-year effort.

Given its ROI, companies I know of constantly pour money into this
area. Given its scale, this debugfs is the least of their concerns. A
good example is the proactive reclaim sysfs interface [4]. It's been
used at Google for many years and at Meta for a few years. We only
started finalizing it recently.

[4] https://lore.kernel.org/r/20220425190040.2475377-1-yosryahmed@xxxxxxxxxx/