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

[Ryan Roberts]

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@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.
>>>>
>>>
>>> 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.


> 
> 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
> 
>