Re: [PATCH v3 0/7] mm/kvm: Improve parallelism for access bit harvesting

[Oliver Upton <oliver.upton@linux.dev>]

On Fri, Apr 19, 2024 at 01:57:03PM -0700, James Houghton wrote:
> On Fri, Apr 12, 2024 at 11:41 AM David Matlack <dmatlack@google.com> wrote:
> >
> > On 2024-04-01 11:29 PM, James Houghton wrote:
> > > This patchset adds a fast path in KVM to test and clear access bits on
> > > sptes without taking the mmu_lock. It also adds support for using a
> > > bitmap to (1) test the access bits for many sptes in a single call to
> > > mmu_notifier_test_young, and to (2) clear the access bits for many ptes
> > > in a single call to mmu_notifier_clear_young.
> >
> > How much improvement would we get if we _just_ made test/clear_young
> > lockless on x86 and hold the read-lock on arm64? And then how much
> > benefit does the bitmap look-around add on top of that?

Thanks David for providing the suggestion.

> I don't have these results right now. For the next version I will (1)
> separate the series into the locking change and the bitmap change, and
> I will (2) have performance data for each change separately. It is
> conceivable that the bitmap change should just be considered as a
> completely separate patchset.

That'd be great. Having the performance numbers will make it even more
compelling, but I'd be tempted to go for the lock improvement just
because it doesn't add any new complexity and leverages existing patterns
in the architectures that people seem to want improvements for.

The bitmap interface, OTOH, is rather complex. At least the current
implementation breaks some of the isolation we have between the MMU code
and the page table walker library on arm64, which I'm not ecstatic about.
It _could_ be justified by a massive performance uplift over locking, but
it'd have to be a sizable win.

-- 
Thanks,
Oliver