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

[Yu Zhao]

On Tue, Apr 26, 2022 at 3:30 PM Andrew Morton <akpm at linux-foundation.org> wrote:
>
> On Tue, 26 Apr 2022 00:59:37 -0600 Yu Zhao <yuzhao at google.com> wrote:
>
> > On Fri, Apr 15, 2022 at 10:20 PM Andrew Morton
> > <akpm at linux-foundation.org> wrote:
> > >
> > > On Fri, 15 Apr 2022 18:03:16 -0600 Yu Zhao <yuzhao at google.com> 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
>
> So will these interfaces be moved into sysfs?

So the debugfs interface from this patch provides:
1. proactive reclaim,
2. working set estimation.

The sysfs interface for item 1 is being finalized by [4], and it's a
subset of this debugfs interface because we want it to be more
general. The sysfs interface for item 2 will be eventually proposed as
well, with the same approach. It will look like a histogram in which
the active/inactive LRU has two bins whereas MGLRU has more bins. Bins
contain pages and multiple bins represent different hotness/coldness.
Since [4] took about two years, I'd say this histogram-like interface
would take no less than a couple of years as well.

This debugfs interface stays even after that, and it will serve its
true purpose (debugging), not a substitute for the sysfs interfaces.

> > > 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@google.com/
>
> Sure, if one organization is involved in both the userspace code and
> the kernel interfaces then the alteration of kernel interfaces can be
> handled in a coordinated fashion.
>
> But releasing interfaces to the whole world is a different deal.  It's
> acceptable to say "this is in debugfs for now because it's a work
> in progress" but it sounds like mglru's interfaces are beyond that
> stage?

Correct. It's a WIP in the sense of "know what needs to be done but
can't get it done immediately", not "don't know what's next; try this
for now".