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

[David Hildenbrand <david@redhat.com>]

On 20.12.23 10:51, 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@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.
>>
>>
>> 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.
> 
> But upon review, I've noticed the part that I think makes this difficult for
> arm64 with the contpte optimization; You are calling ptep_get() for every pte in
> the batch. While this is functionally correct, once arm64 has the contpte
> changes, its ptep_get() has to read every pte in the contpte block in order to
> gather the access and dirty bits. So if your batching function ends up wealking
> a 16 entry contpte block, that will cause 16 x 16 reads, which kills
> performance. That's why I added the arch-specific pte_batch_remaining()
> function; this allows the core-mm to skip to the end of the contpte block and
> avoid ptep_get() for the 15 tail ptes. So we end up with 16 READ_ONCE()s instead
> of 256.
> 
> I considered making a ptep_get_noyoungdirty() variant, which would avoid the bit
> gathering. But we have a similar problem in zap_pte_range() and that function
> needs the dirty bit to update the folio. So it doesn't work there. (see patch 3
> in my series).
> 
> I guess you are going to say that we should combine both approaches, so that
> your batching loop can skip forward an arch-provided number of ptes? That would
> certainly work, but feels like an orthogonal change to what I'm trying to
> achieve :). Anyway, I'll spend some time playing with it today.

You can overwrite the function or add special-casing internally, yes.

Right now, your patch is called "mm: Batch-copy PTE ranges during 
fork()" and it doesn't do any of that besides preparing for some arm64 work.

-- 
Cheers,

David / dhildenb