Re: [PATCH v3 00/15] mm/memory: optimize fork() with PTE-mapped THP

Ryan Roberts
2024-01-31T10:43:32Z

On 29/01/2024 12:46, David Hildenbrand wrote:
> Now that the rmap overhaul[1] is upstream that provides a clean interface
> for rmap batching, let's implement PTE batching during fork when processing
> PTE-mapped THPs.
>
> This series is partially based on Ryan's previous work[2] to implement
> cont-pte support on arm64, but its a complete rewrite based on [1] to
> optimize all architectures independent of any such PTE bits, and to
> use the new rmap batching functions that simplify the code and prepare
> for further rmap accounting changes.
>
> We collect consecutive PTEs that map consecutive pages of the same large
> folio, making sure that the other PTE bits are compatible, and (a) adjust
> the refcount only once per batch, (b) call rmap handling functions only
> once per batch and (c) perform batch PTE setting/updates.
>
> While this series should be beneficial for adding cont-pte support on
> ARM64[2], it's one of the requirements for maintaining a total mapcount[3]
> for large folios with minimal added overhead and further changes[4] that
> build up on top of the total mapcount.
>
> Independent of all that, this series results in a speedup during fork with
> PTE-mapped THP, which is the default with THPs that are smaller than a PMD
> (for example, 16KiB to 1024KiB mTHPs for anonymous memory[5]).
>
> On an Intel Xeon Silver 4210R CPU, fork'ing with 1GiB of PTE-mapped folios
> of the same size (stddev < 1%) results in the following runtimes
> for fork() (shorter is better):
>
> Folio Size | v6.8-rc1 | New | Change
> ------------------------------------------
> 4KiB | 0.014328 | 0.014035 | - 2%
> 16KiB | 0.014263 | 0.01196 | -16%
> 32KiB | 0.014334 | 0.01094 | -24%
> 64KiB | 0.014046 | 0.010444 | -26%
> 128KiB | 0.014011 | 0.010063 | -28%
> 256KiB | 0.013993 | 0.009938 | -29%
> 512KiB | 0.013983 | 0.00985 | -30%
> 1024KiB | 0.013986 | 0.00982 | -30%
> 2048KiB | 0.014305 | 0.010076 | -30%

Just a heads up that I'm seeing some strange results on Apple M2. Fork for
order-0 is seemingly costing ~17% more. I'm using GCC 13.2 and was pretty sure I
didn't see this problem with version 1; although that was on a different
baseline and I've thrown the numbers away so will rerun and try to debug this.

| kernel | mean_rel | std_rel |
|:------------|-----------:|----------:|
| mm-unstable | 0.0% | 1.1% |
| patch 1 | -2.3% | 1.3% |
| patch 10 | -2.9% | 2.7% |
| patch 11 | 13.5% | 0.5% |
| patch 12 | 15.2% | 1.2% |
| patch 13 | 18.2% | 0.7% |
| patch 14 | 20.5% | 1.0% |
| patch 15 | 17.1% | 1.6% |
| patch 15 | 16.7% | 0.8% |

fork for order-9 is looking good (-20%), and for the zap series, munmap is
looking good, but dontneed is looking poor for both order-0 and 9. But one thing
at a time... let's concentrate on fork order-0 first.

Note that I'm still using the "old" benchmark code. Could you resend me the link
to the new code? Although I don't think there should be any effect for order-0
anyway, if I understood your changes correctly?

>
> Note that these numbers are even better than the ones from v1 (verified
> over multiple reboots), even though there were only minimal code changes.
> Well, I removed a pte_mkclean() call for anon folios, maybe that also
> plays a role.
>
> But my experience is that fork() is extremely sensitive to code size,
> inlining, ... so I suspect we'll see on other architectures rather a change
> of -20% instead of -30%, and it will be easy to "lose" some of that speedup
> in the future by subtle code changes.
>
> Next up is PTE batching when unmapping. Only tested on x86-64.
> Compile-tested on most other architectures.
>
> v2 -> v3:
> * Rebased on mm-unstable
> * Picked up RB's
> * Updated documentation of wrprotect_ptes().
>
> v1 -> v2:
> * "arm64/mm: Make set_ptes() robust when OAs cross 48-bit boundary"
> -> Added patch from Ryan
> * "arm/pgtable: define PFN_PTE_SHIFT"
> -> Removed the arm64 bits
> * "mm/pgtable: make pte_next_pfn() independent of set_ptes()"
> * "arm/mm: use pte_next_pfn() in set_ptes()"
> * "powerpc/mm: use pte_next_pfn() in set_ptes()"
> -> Added to use pte_next_pfn() in some arch set_ptes() implementations
> I tried to make use of pte_next_pfn() also in the others, but it's
> not trivial because the other archs implement set_ptes() in their
> asm/pgtable.h. Future work.
> * "mm/memory: factor out copying the actual PTE in copy_present_pte()"
> -> Move common folio_get() out of if/else
> * "mm/memory: optimize fork() with PTE-mapped THP"
> -> Add doc for wrprotect_ptes
> -> Extend description to mention handling of pinned folios
> -> Move common folio_ref_add() out of if/else
> * "mm/memory: ignore dirty/accessed/soft-dirty bits in folio_pte_batch()"
> -> Be more conservative with dirt/soft-dirty, let the caller specify
> using flags
>
> [1] https://lkml.kernel.org/r/20231220224504.646757-1-david@redhat.com
> [2] https://lkml.kernel.org/r/20231218105100.172635-1-ryan.roberts@arm.com
> [3] https://lkml.kernel.org/r/20230809083256.699513-1-david@redhat.com
> [4] https://lkml.kernel.org/r/20231124132626.235350-1-david@redhat.com
> [5] https://lkml.kernel.org/r/20231207161211.2374093-1-ryan.roberts@arm.com
>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
> Cc: Ryan Roberts <ryan.roberts@arm.com>
> Cc: Russell King <linux@armlinux.org.uk>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Will Deacon <will@kernel.org>
> Cc: Dinh Nguyen <dinguyen@kernel.org>
> Cc: Michael Ellerman <mpe@ellerman.id.au>
> Cc: Nicholas Piggin <npiggin@gmail.com>
> Cc: Christophe Leroy <christophe.leroy@csgroup.eu>
> Cc: "Aneesh Kumar K.V" <aneesh.kumar@kernel.org>
> Cc: "Naveen N. Rao" <naveen.n.rao@linux.ibm.com>
> Cc: Paul Walmsley <paul.walmsley@sifive.com>
> Cc: Palmer Dabbelt <palmer@dabbelt.com>
> Cc: Albert Ou <aou@eecs.berkeley.edu>
> Cc: Alexander Gordeev <agordeev@linux.ibm.com>
> Cc: Gerald Schaefer <gerald.schaefer@linux.ibm.com>
> Cc: Heiko Carstens <hca@linux.ibm.com>
> Cc: Vasily Gorbik <gor@linux.ibm.com>
> Cc: Christian Borntraeger <borntraeger@linux.ibm.com>
> Cc: Sven Schnelle <svens@linux.ibm.com>
> Cc: "David S. Miller" <davem@davemloft.net>
> Cc: linux-arm-kernel@lists.infradead.org
> Cc: linuxppc-dev@lists.ozlabs.org
> Cc: linux-riscv@lists.infradead.org
> Cc: linux-s390@vger.kernel.org
> Cc: sparclinux@vger.kernel.org
>
> ---
>
> Andrew asked for a resend based on latest mm-unstable. I am sending this
> out earlier than I would usually have sent out the next version, so we can
> pull it into mm-unstable again now that v1 was dropped.
>
> David Hildenbrand (14):
> arm/pgtable: define PFN_PTE_SHIFT
> nios2/pgtable: define PFN_PTE_SHIFT
> powerpc/pgtable: define PFN_PTE_SHIFT
> riscv/pgtable: define PFN_PTE_SHIFT
> s390/pgtable: define PFN_PTE_SHIFT
> sparc/pgtable: define PFN_PTE_SHIFT
> mm/pgtable: make pte_next_pfn() independent of set_ptes()
> arm/mm: use pte_next_pfn() in set_ptes()
> powerpc/mm: use pte_next_pfn() in set_ptes()
> mm/memory: factor out copying the actual PTE in copy_present_pte()
> mm/memory: pass PTE to copy_present_pte()
> mm/memory: optimize fork() with PTE-mapped THP
> mm/memory: ignore dirty/accessed/soft-dirty bits in folio_pte_batch()
> mm/memory: ignore writable bit in folio_pte_batch()
>
> Ryan Roberts (1):
> arm64/mm: Make set_ptes() robust when OAs cross 48-bit boundary
>
> arch/arm/include/asm/pgtable.h | 2 +
> arch/arm/mm/mmu.c | 2 +-
> arch/arm64/include/asm/pgtable.h | 28 ++--
> arch/nios2/include/asm/pgtable.h | 2 +
> arch/powerpc/include/asm/pgtable.h | 2 +
> arch/powerpc/mm/pgtable.c | 5 +-
> arch/riscv/include/asm/pgtable.h | 2 +
> arch/s390/include/asm/pgtable.h | 2 +
> arch/sparc/include/asm/pgtable_64.h | 2 +
> include/linux/pgtable.h | 33 ++++-
> mm/memory.c | 212 ++++++++++++++++++++++------
> 11 files changed, 229 insertions(+), 63 deletions(-)
>
>
> base-commit: d162e170f1181b4305494843e1976584ddf2b72e