[PATCH v4 2/7] mm: multi-gen LRU: Have secondary MMUs participate in aging

[Yu Zhao]

On Wed, May 29, 2024 at 3:59 PM Sean Christopherson <seanjc at google.com> wrote:
>
> On Wed, May 29, 2024, Yu Zhao wrote:
> > On Wed, May 29, 2024 at 12:05 PM James Houghton <jthoughton at google.com> wrote:
> > >
> > > Secondary MMUs are currently consulted for access/age information at
> > > eviction time, but before then, we don't get accurate age information.
> > > That is, pages that are mostly accessed through a secondary MMU (like
> > > guest memory, used by KVM) will always just proceed down to the oldest
> > > generation, and then at eviction time, if KVM reports the page to be
> > > young, the page will be activated/promoted back to the youngest
> > > generation.
> >
> > Correct, and as I explained offline, this is the only reasonable
> > behavior if we can't locklessly walk secondary MMUs.
> >
> > Just for the record, the (crude) analogy I used was:
> > Imagine a large room with many bills ($1, $5, $10, ...) on the floor,
> > but you are only allowed to pick up 10 of them (and put them in your
> > pocket). A smart move would be to survey the room *first and then*
> > pick up the largest ones. But if you are carrying a 500 lbs backpack,
> > you would just want to pick up whichever that's in front of you rather
> > than walk the entire room.
> >
> > MGLRU should only scan (or lookaround) secondary MMUs if it can be
> > done lockless. Otherwise, it should just fall back to the existing
> > approach, which existed in previous versions but is removed in this
> > version.
>
> IIUC, by "existing approach" you mean completely ignore secondary MMUs that don't
> implement a lockless walk?

No, the existing approach only checks secondary MMUs for LRU folios,
i.e., those at the end of the LRU list. It might not find the best
candidates (the coldest ones) on the entire list, but it doesn't pay
as much for the locking. MGLRU can *optionally* scan MMUs (secondary
included) to find the best candidates, but it can only be a win if the
scanning incurs a relatively low overhead, e.g., done locklessly for
the secondary MMU. IOW, this is a balance between the cost of
reclaiming not-so-cold (warm) folios and that of finding the coldest
folios.

Scanning host MMUs is likely to be a win because 1) there is usually
access locality 2) there is no coarsed locking. If neither holds,
scanning secondary MMUs would likely be a loss. And 1) is generally
weaker for secondary MMUs, since it's about (guest) physical address
space.