[PATCH v4 02/16] mm: Batch-copy PTE ranges during fork()

[David Hildenbrand]

On 20.12.23 10:57, Ryan Roberts wrote:
> On 20/12/2023 09:51, Ryan Roberts wrote:
>> On 20/12/2023 09:17, David Hildenbrand wrote:
>>> On 19.12.23 18:42, Ryan Roberts wrote:
>>>> On 19/12/2023 17:22, David Hildenbrand wrote:
>>>>> On 19.12.23 09:30, Ryan Roberts wrote:
>>>>>> On 18/12/2023 17:47, David Hildenbrand wrote:
>>>>>>> On 18.12.23 11:50, Ryan Roberts wrote:
>>>>>>>> Convert copy_pte_range() to copy a batch of ptes in one go. A given
>>>>>>>> batch is determined by the architecture with the new helper,
>>>>>>>> pte_batch_remaining(), and maps a physically contiguous block of memory,
>>>>>>>> all belonging to the same folio. A pte batch is then write-protected in
>>>>>>>> one go in the parent using the new helper, ptep_set_wrprotects() and is
>>>>>>>> set in one go in the child using the new helper, set_ptes_full().
>>>>>>>>
>>>>>>>> The primary motivation for this change is to reduce the number of tlb
>>>>>>>> maintenance operations that the arm64 backend has to perform during
>>>>>>>> fork, as it is about to add transparent support for the "contiguous bit"
>>>>>>>> in its ptes. By write-protecting the parent using the new
>>>>>>>> ptep_set_wrprotects() (note the 's' at the end) function, the backend
>>>>>>>> can avoid having to unfold contig ranges of PTEs, which is expensive,
>>>>>>>> when all ptes in the range are being write-protected. Similarly, by
>>>>>>>> using set_ptes_full() rather than set_pte_at() to set up ptes in the
>>>>>>>> child, the backend does not need to fold a contiguous range once they
>>>>>>>> are all populated - they can be initially populated as a contiguous
>>>>>>>> range in the first place.
>>>>>>>>
>>>>>>>> This code is very performance sensitive, and a significant amount of
>>>>>>>> effort has been put into not regressing performance for the order-0
>>>>>>>> folio case. By default, pte_batch_remaining() is compile constant 1,
>>>>>>>> which enables the compiler to simplify the extra loops that are added
>>>>>>>> for batching and produce code that is equivalent (and equally
>>>>>>>> performant) as the previous implementation.
>>>>>>>>
>>>>>>>> This change addresses the core-mm refactoring only and a separate change
>>>>>>>> will implement pte_batch_remaining(), ptep_set_wrprotects() and
>>>>>>>> set_ptes_full() in the arm64 backend to realize the performance
>>>>>>>> improvement as part of the work to enable contpte mappings.
>>>>>>>>
>>>>>>>> To ensure the arm64 is performant once implemented, this change is very
>>>>>>>> careful to only call ptep_get() once per pte batch.
>>>>>>>>
>>>>>>>> The following microbenchmark results demonstate that there is no
>>>>>>>> significant performance change after this patch. Fork is called in a
>>>>>>>> tight loop in a process with 1G of populated memory and the time for the
>>>>>>>> function to execute is measured. 100 iterations per run, 8 runs
>>>>>>>> performed on both Apple M2 (VM) and Ampere Altra (bare metal). Tests
>>>>>>>> performed for case where 1G memory is comprised of order-0 folios and
>>>>>>>> case where comprised of pte-mapped order-9 folios. Negative is faster,
>>>>>>>> positive is slower, compared to baseline upon which the series is based:
>>>>>>>>
>>>>>>>> | Apple M2 VM   | order-0 (pte-map) | order-9 (pte-map) |
>>>>>>>> | fork          |-------------------|-------------------|
>>>>>>>> | microbench    |    mean |   stdev |    mean |   stdev |
>>>>>>>> |---------------|---------|---------|---------|---------|
>>>>>>>> | baseline      |    0.0% |    1.1% |    0.0% |    1.2% |
>>>>>>>> | after-change  |   -1.0% |    2.0% |   -0.1% |    1.1% |
>>>>>>>>
>>>>>>>> | Ampere Altra  | order-0 (pte-map) | order-9 (pte-map) |
>>>>>>>> | fork          |-------------------|-------------------|
>>>>>>>> | microbench    |    mean |   stdev |    mean |   stdev |
>>>>>>>> |---------------|---------|---------|---------|---------|
>>>>>>>> | baseline      |    0.0% |    1.0% |    0.0% |    0.1% |
>>>>>>>> | after-change  |   -0.1% |    1.2% |   -0.1% |    0.1% |
>>>>>>>>
>>>>>>>> Tested-by: John Hubbard <jhubbard at nvidia.com>
>>>>>>>> Reviewed-by: Alistair Popple <apopple at nvidia.com>
>>>>>>>> Signed-off-by: Ryan Roberts <ryan.roberts at arm.com>
>>>>>>>> ---
>>>>>>>>      include/linux/pgtable.h | 80 +++++++++++++++++++++++++++++++++++
>>>>>>>>      mm/memory.c             | 92 ++++++++++++++++++++++++++---------------
>>>>>>>>      2 files changed, 139 insertions(+), 33 deletions(-)
>>>>>>>>
>>>>>>>> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
>>>>>>>> index af7639c3b0a3..db93fb81465a 100644
>>>>>>>> --- a/include/linux/pgtable.h
>>>>>>>> +++ b/include/linux/pgtable.h
>>>>>>>> @@ -205,6 +205,27 @@ static inline int pmd_young(pmd_t pmd)
>>>>>>>>      #define arch_flush_lazy_mmu_mode()    do {} while (0)
>>>>>>>>      #endif
>>>>>>>>      +#ifndef pte_batch_remaining
>>>>>>>> +/**
>>>>>>>> + * pte_batch_remaining - Number of pages from addr to next batch boundary.
>>>>>>>> + * @pte: Page table entry for the first page.
>>>>>>>> + * @addr: Address of the first page.
>>>>>>>> + * @end: Batch ceiling (e.g. end of vma).
>>>>>>>> + *
>>>>>>>> + * Some architectures (arm64) can efficiently modify a contiguous batch of
>>>>>>>> ptes.
>>>>>>>> + * In such cases, this function returns the remaining number of pages to
>>>>>>>> the end
>>>>>>>> + * of the current batch, as defined by addr. This can be useful when
>>>>>>>> iterating
>>>>>>>> + * over ptes.
>>>>>>>> + *
>>>>>>>> + * May be overridden by the architecture, else batch size is always 1.
>>>>>>>> + */
>>>>>>>> +static inline unsigned int pte_batch_remaining(pte_t pte, unsigned long
>>>>>>>> addr,
>>>>>>>> +                        unsigned long end)
>>>>>>>> +{
>>>>>>>> +    return 1;
>>>>>>>> +}
>>>>>>>> +#endif
>>>>>>>
>>>>>>> It's a shame we now lose the optimization for all other archtiectures.
>>>>>>>
>>>>>>> Was there no way to have some basic batching mechanism that doesn't require
>>>>>>> arch
>>>>>>> specifics?
>>>>>>
>>>>>> I tried a bunch of things but ultimately the way I've done it was the only way
>>>>>> to reduce the order-0 fork regression to 0.
>>>>>>
>>>>>> My original v3 posting was costing 5% extra and even my first attempt at an
>>>>>> arch-specific version that didn't resolve to a compile-time constant 1 still
>>>>>> cost an extra 3%.
>>>>>>
>>>>>>
>>>>>>>
>>>>>>> I'd have thought that something very basic would have worked like:
>>>>>>>
>>>>>>> * Check if PTE is the same when setting the PFN to 0.
>>>>>>> * Check that PFN is consecutive
>>>>>>> * Check that all PFNs belong to the same folio
>>>>>>
>>>>>> I haven't tried this exact approach, but I'd be surprised if I can get the
>>>>>> regression under 4% with this. Further along the series I spent a lot of time
>>>>>> having to fiddle with the arm64 implementation; every conditional and every
>>>>>> memory read (even when in cache) was a problem. There is just so little in the
>>>>>> inner loop that every instruction matters. (At least on Ampere Altra and Apple
>>>>>> M2).
>>>>>>
>>>>>> Of course if you're willing to pay that 4-5% for order-0 then the benefit to
>>>>>> order-9 is around 10% in my measurements. Personally though, I'd prefer to play
>>>>>> safe and ensure the common order-0 case doesn't regress, as you previously
>>>>>> suggested.
>>>>>>
>>>>>
>>>>> I just hacked something up, on top of my beloved rmap cleanup/batching series. I
>>>>> implemented very generic and simple batching for large folios (all PTE bits
>>>>> except the PFN have to match).
>>>>>
>>>>> Some very quick testing (don't trust each last % ) on Intel(R) Xeon(R) Silver
>>>>> 4210R CPU.
>>>>>
>>>>> order-0: 0.014210 -> 0.013969
>>>>>
>>>>> -> Around 1.7 % faster
>>>>>
>>>>> order-9: 0.014373 -> 0.009149
>>>>>
>>>>> -> Around 36.3 % faster
>>>>
>>>> Well I guess that shows me :)
>>>>
>>>> I'll do a review and run the tests on my HW to see if it concurs.
>>>
>>>
>>> I pushed a simple compile fixup (we need pte_next_pfn()).
>>
>> I've just been trying to compile and noticed this. Will take a look at your update.
> 
> Took a look; there will still be arch work needed; arm64 doesn't define
> PFN_PTE_SHIFT because it defines set_ptes(). I'm not sure if there are other
> arches that also don't define PFN_PTE_SHIFT (or pte_next_pfn()) if the math is
> more complex) - it will need an audit.
> 

Right, likely many that have their own set_ptes() implementation right now.

-- 
Cheers,

David / dhildenb