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

[Ryan Roberts]

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