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Message-ID: <a81dc390-8b10-4ce9-b72f-57f253e77af3@arm.com>
Date: Tue, 5 Dec 2023 11:30:56 +0000
From: Ryan Roberts <ryan.roberts@....com>
To: David Hildenbrand <david@...hat.com>,
Catalin Marinas <catalin.marinas@....com>,
Will Deacon <will@...nel.org>,
Ard Biesheuvel <ardb@...nel.org>,
Marc Zyngier <maz@...nel.org>,
Oliver Upton <oliver.upton@...ux.dev>,
James Morse <james.morse@....com>,
Suzuki K Poulose <suzuki.poulose@....com>,
Zenghui Yu <yuzenghui@...wei.com>,
Andrey Ryabinin <ryabinin.a.a@...il.com>,
Alexander Potapenko <glider@...gle.com>,
Andrey Konovalov <andreyknvl@...il.com>,
Dmitry Vyukov <dvyukov@...gle.com>,
Vincenzo Frascino <vincenzo.frascino@....com>,
Andrew Morton <akpm@...ux-foundation.org>,
Anshuman Khandual <anshuman.khandual@....com>,
Matthew Wilcox <willy@...radead.org>,
Yu Zhao <yuzhao@...gle.com>,
Mark Rutland <mark.rutland@....com>,
Kefeng Wang <wangkefeng.wang@...wei.com>,
John Hubbard <jhubbard@...dia.com>, Zi Yan <ziy@...dia.com>,
Barry Song <21cnbao@...il.com>,
Alistair Popple <apopple@...dia.com>,
Yang Shi <shy828301@...il.com>
Cc: linux-arm-kernel@...ts.infradead.org, linux-mm@...ck.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 01/15] mm: Batch-copy PTE ranges during fork()
On 04/12/2023 17:27, David Hildenbrand wrote:
>>
>> With rmap batching from [1] -- rebased+changed on top of that -- we could turn
>> that into an effective (untested):
>>
>> if (page && folio_test_anon(folio)) {
>> + nr = folio_nr_pages_cont_mapped(folio, page, src_pte, addr, end,
>> + pte, enforce_uffd_wp, &nr_dirty,
>> + &nr_writable);
>> /*
>> * If this page may have been pinned by the parent process,
>> * copy the page immediately for the child so that we'll always
>> * guarantee the pinned page won't be randomly replaced in the
>> * future.
>> */
>> - folio_get(folio);
>> - if (unlikely(folio_try_dup_anon_rmap_pte(folio, page,
>> src_vma))) {
>> + folio_ref_add(folio, nr);
>> + if (unlikely(folio_try_dup_anon_rmap_ptes(folio, page, nr,
>> src_vma))) {
>> /* Page may be pinned, we have to copy. */
>> - folio_put(folio);
>> - return copy_present_page(dst_vma, src_vma, dst_pte,
>> src_pte,
>> - addr, rss, prealloc, page);
>> + folio_ref_sub(folio, nr);
>> + ret = copy_present_page(dst_vma, src_vma, dst_pte,
>> + src_pte, addr, rss, prealloc,
>> + page);
>> + return ret == 0 ? 1 : ret;
>> }
>> - rss[MM_ANONPAGES]++;
>> + rss[MM_ANONPAGES] += nr;
>> } else if (page) {
>> - folio_get(folio);
>> - folio_dup_file_rmap_pte(folio, page);
>> - rss[mm_counter_file(page)]++;
>> + nr = folio_nr_pages_cont_mapped(folio, page, src_pte, addr, end,
>> + pte, enforce_uffd_wp, &nr_dirty,
>> + &nr_writable);
>> + folio_ref_add(folio, nr);
>> + folio_dup_file_rmap_ptes(folio, page, nr);
>> + rss[mm_counter_file(page)] += nr;
>> }
>>
>>
>> We'll have to test performance, but it could be that we want to specialize
>> more on !folio_test_large(). That code is very performance-sensitive.
>>
>>
>> [1] https://lkml.kernel.org/r/20231204142146.91437-1-david@redhat.com
>
> So, on top of [1] without rmap batching but with a slightly modified version of
Can you clarify what you mean by "without rmap batching"? I thought [1]
implicitly adds rmap batching? (e.g. folio_dup_file_rmap_ptes(), which you've
added in the code snippet above).
> yours (that keeps the existing code structure as pointed out and e.g., updates
> counter updates), running my fork() microbenchmark with a 1 GiB of memory:
>
> Compared to [1], with all order-0 pages it gets 13--14% _slower_ and with all
> PTE-mapped THP (order-9) it gets ~29--30% _faster_.
What test are you running - I'd like to reproduce if possible, since it sounds
like I've got some work to do to remove the order-0 regression.
>
> So looks like we really want to have a completely seprate code path for
> "!folio_test_large()" to keep that case as fast as possible. And "Likely" we
> want to use "likely(!folio_test_large()". ;)
Yuk, but fair enough. If I can repro the perf numbers, I'll have a go a
reworking this.
I think you're also implicitly suggesting that this change needs to depend on
[1]? Which is a shame...
I guess I should also go through a similar exercise for patch 2 in this series.
>
> Performing rmap batching on top of that code only slightly (another 1% or so)
> improves performance in the PTE-mapped THP (order-9) case right now, in contrast
> to other rmap batching. Reason is as all rmap code gets inlined here and we're
> only doing subpage mapcount updates + PAE handling.
>
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