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

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 26 Apr 2022 00:59:37 -0600 Yu Zhao <yuzhao@google.com> wrote:

> On Fri, Apr 15, 2022 at 10:20 PM Andrew Morton
> <akpm@linux-foundation.org> wrote:
> >
> > On Fri, 15 Apr 2022 18:03:16 -0600 Yu Zhao <yuzhao@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?

> > 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?