Re: [PATCH RFC 06/12] mm/gup: Drop folio_fast_pin_allowed() in hugepd processing

Peter Xu
2023-11-30T21:31:09Z

On Fri, Nov 24, 2023 at 11:07:51AM -0500, Peter Xu wrote:
> On Fri, Nov 24, 2023 at 09:06:01AM +0000, Ryan Roberts wrote:
> > I don't have any micro-benchmarks for GUP though, if that's your question. Is
> > there an easy-to-use test I can run to get some numbers? I'd be happy to try it out.
>
> Thanks Ryan. Then nothing is needed to be tested if gup is not yet touched
> from your side, afaict. I'll see whether I can provide some rough numbers
> instead in the next post (I'll probably only be able to test it in a VM,
> though, but hopefully that should still reflect mostly the truth).

An update: I finished a round of 64K cont_pte test, in the slow gup micro
benchmark I see ~15% perf degrade with this patchset applied on a VM on top
of Apple M1.

Frankly that's even less than I expected, considering not only how slow gup
THP used to be, but also on the fact that that's a tight loop over slow
gup, which in normal cases shouldn't happen: "present" ptes normally goes
to fast-gup, while !present goes into a fault following it. I assume
that's why nobody cared slow gup for THP before. I think adding cont_pte
support shouldn't be very hard, but that will include making cont_pte idea
global just for arm64 and riscv Svnapot.

The current plan is I'll add that performance number into my commit message
only, as I don't ever expect any real workload will regress with it. Maybe
a global cont_pte api will be needed at some point, but perhaps not yet
feel strongly for this use case.

Please feel free to raise any concerns otherwise.

Thanks,

--
Peter Xu