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

[Ryan Roberts <ryan.roberts@arm.com>]

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@nvidia.com>
>>>> Reviewed-by: Alistair Popple <apopple@nvidia.com>
>>>> Signed-off-by: Ryan Roberts <ryan.roberts@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.

> 
> 
> But it's likely buggy, so don't trust the numbers just yet. If they actually
> hold up, we should probably do something like that ahead of time, before all the
> arm-specific cont-pte work.
> 
> I suspect you can easily extend that by arch hooks where reasonable.
> 
> The (3) patches on top of the rmap cleanups can be found at:
> 
>     https://github.com/davidhildenbrand/linux/tree/fork-batching
>