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

[David Hildenbrand]

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()).

Note that we should probably handle "ptep_set_wrprotects" rather like set_ptes:

#ifndef wrprotect_ptes
static inline void wrprotect_ptes(struct mm_struct *mm, unsigned long addr,
                pte_t *ptep, unsigned int nr)
{
        for (;;) {
                ptep_set_wrprotect(mm, addr, ptep);
                if (--nr == 0)
                        break;
                ptep++;
                addr += PAGE_SIZE;
        }
}
#endif


-- 
Cheers,

David / dhildenb