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

[David Matlack]

On Fri, May 31, 2024 at 2:06 PM David Matlack <dmatlack at google.com> wrote:
>
> On Fri, May 31, 2024 at 1:31 PM Yu Zhao <yuzhao at google.com> wrote:
> >
> > On Fri, May 31, 2024 at 1:24 AM Oliver Upton <oliver.upton at linux.dev> wrote:
> > >
> > > On Wed, May 29, 2024 at 03:03:21PM -0600, 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.
> > >
> > > Grabbing the MMU lock for write to scan sucks, no argument there. But
> > > can you please be specific about the impact of read lock v. RCU in the
> > > case of arm64? I had asked about this before and you never replied.
> > >
> > > My concern remains that adding support for software table walkers
> > > outside of the MMU lock entirely requires more work than just deferring
> > > the deallocation to an RCU callback. Walkers that previously assumed
> > > 'exclusive' access while holding the MMU lock for write must now cope
> > > with volatile PTEs.
> > >
> > > Yes, this problem already exists when hardware sets the AF, but the
> > > lock-free walker implementation needs to be generic so it can be applied
> > > for other PTE bits.
> >
> > Direct reclaim is multi-threaded and each reclaimer can take the mmu
> > lock for read (testing the A-bit) or write (unmapping before paging
> > out) on arm64. The fundamental problem of using the readers-writer
> > lock in this case is priority inversion: the readers have lower
> > priority than the writers, so ideally, we don't want the readers to
> > block the writers at all.
> >
> > Using my previous (crude) analogy: puting the bill right in front of
> > you (the writers) profits immediately whereas searching for the
> > largest bill (the readers) can be futile.
> >
> > As I said earlier, I prefer we drop the arm64 support for now, but I
> > will not object to taking the mmu lock for read when clearing the
> > A-bit, as long as we fully understand the problem here and document it
> > clearly.
>
> FWIW, Google Cloud has been doing proactive reclaim and kstaled-based
> aging (a Google-internal page aging daemon, for those outside of
> Google) for many years on x86 VMs with the A-bit harvesting
> under the write-lock. So I'm skeptical that making ARM64 lockless is
> necessary to allow Secondary MMUs to participate in MGLRU aging with
> acceptable performance for Cloud usecases. I don't even think it's
> necessary on x86 but it's a simple enough change that we might as well
> just do it.

The obvious caveat here: If MGLRU aging and kstaled aging are
substantially different in how frequently they trigger mmu_notifiers,
then my analysis may not be correct. I'm hoping Yu you can shed some
light on that. I'm also operating under the assumption that Secondary
MMUs are only participating in aging, and not look-around (i.e. what
is implemented in v4).

>
> I suspect under pathological conditions (host under intense memory
> pressure and high rate of reclaim occurring) making A-bit harvesting
> lockless will perform better. But under such conditions VM performance
> is likely going to suffer regardless. In a Cloud environment we deal
> with that through other mechanisms to reduce the rate of reclaim and
> make the host healthy.
>
> For these reasons, I think there's value in giving users the option to
> enable Secondary MMUs participation MGLRU aging even when A-bit
> test/clearing is not done locklessly. I believe this was James' intent
> with the Kconfig. Perhaps a default-off writable module parameter
> would be better to avoid distros accidentally turning it on?
>
> If and when there is a usecase for optimizing VM performance under
> pathological reclaim conditions on ARM, we can make it lockless then.