linux-kernel - Re: [PATCH RFC 06/12] mm/gup: Drop folio_fast_pin

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Date:   Thu, 30 Nov 2023 16:30:58 -0500
From:   Peter Xu <peterx@...hat.com>
To:     Ryan Roberts <ryan.roberts@....com>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Christoph Hellwig <hch@...radead.org>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Andrea Arcangeli <aarcange@...hat.com>,
        James Houghton <jthoughton@...gle.com>,
        Lorenzo Stoakes <lstoakes@...il.com>,
        David Hildenbrand <david@...hat.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        John Hubbard <jhubbard@...dia.com>,
        Yang Shi <shy828301@...il.com>,
        Rik van Riel <riel@...riel.com>,
        Hugh Dickins <hughd@...gle.com>,
        Jason Gunthorpe <jgg@...dia.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linuxppc-dev@...ts.ozlabs.org, Mike Rapoport <rppt@...nel.org>,
        Mike Kravetz <mike.kravetz@...cle.com>
Subject: Re: [PATCH RFC 06/12] mm/gup: Drop folio_fast_pin_allowed() in
 hugepd processing

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