Re: [RFC PATCH] mm/rmap: use release ordering for small-folio mapcount removal

["Lorenzo Stoakes (ARM)" <ljs@kernel.org>]

On Wed, Nov 04, 2026 at 02:37:39PM +0100, David Hildenbrand (Arm) wrote:
> On 11/4/26 10:46, Pranith Kumar wrote:
> > Hi David,
> >
> > Thank you for looking at the patch.
> >
> > On Wed, Nov 4, 2026 at 1:16 AM David Hildenbrand (Arm) <david@kernel.org> wrote:
> >>
> >>
> >>> In an arm64 page map/unmap microbenchmark with 20,480 minor
> >>> faults, simulated cycles fell by 1.54%.
> >>
> >> Thanks for the patch.
> >>
> >> While I see value, I am not convinced this is the right time for this
> >> micro-optimization while we are busy with other things.
> >>
> >> This needs a better evaluation: 1.54% in a micro-benchmark without knowing the
> >> stddev isn't particularly helpful.
> >>
> >
> > This result is from a deterministic simulation, hence no stddev.
>
> Ah, I'd assume even a deterministic simulator would have some variance due to
> preemption, TLB flushes etc.
>
> But the bigger question is: which arm64 variant is simulated? I'd assume it
> might vary heavily no?

I mean 1.54%, theoretically is even less convincing for me :)

In any case for something touching a sensitive memory barrier operation to have
a stronger argument put forward for release ordering being suitable than simply
stating it's fine in the commit message and updating a comment.

--
Cheers, Lorenzo