Re: [PATCH RFC 06/12] mm/gup: Drop folio_fast_pin_allowed() in hugepd processing

Ryan Roberts
2023-12-04T12:03:49Z

On 04/12/2023 11:57, Christophe Leroy wrote:
>
>
> Le 04/12/2023 à 12:46, Ryan Roberts a écrit :
>> On 04/12/2023 11:25, Christophe Leroy wrote:
>>>
>>>
>>> Le 04/12/2023 à 12:11, Ryan Roberts a écrit :
>>>> On 03/12/2023 13:33, Christophe Leroy wrote:
>>>>>
>>>>>
>>>>> Le 30/11/2023 à 22:30, Peter Xu a écrit :
>>>>>> On Fri, Nov 24, 2023 at 11:07:51AM -0500, Peter Xu wrote:
>>>>>>> On Fri, Nov 24, 2023 at 09:06:01AM +0000, Ryan Roberts wrote:
>>>>>>>> I don't have any micro-benchmarks for GUP though, if that's your question. Is
>>>>>>>> there an easy-to-use test I can run to get some numbers? I'd be happy to try it out.
>>>>>>>
>>>>>>> Thanks Ryan. Then nothing is needed to be tested if gup is not yet touched
>>>>>>> from your side, afaict. I'll see whether I can provide some rough numbers
>>>>>>> instead in the next post (I'll probably only be able to test it in a VM,
>>>>>>> though, but hopefully that should still reflect mostly the truth).
>>>>>>
>>>>>> An update: I finished a round of 64K cont_pte test, in the slow gup micro
>>>>>> benchmark I see ~15% perf degrade with this patchset applied on a VM on top
>>>>>> of Apple M1.
>>>>>>
>>>>>> Frankly that's even less than I expected, considering not only how slow gup
>>>>>> THP used to be, but also on the fact that that's a tight loop over slow
>>>>>> gup, which in normal cases shouldn't happen: "present" ptes normally goes
>>>>>> to fast-gup, while !present goes into a fault following it. I assume
>>>>>> that's why nobody cared slow gup for THP before. I think adding cont_pte
>>>>>> support shouldn't be very hard, but that will include making cont_pte idea
>>>>>> global just for arm64 and riscv Svnapot.
>>>>>
>>>>> Is there any documentation on what cont_pte is ? I have always wondered
>>>>> if it could also fit powerpc 8xx need ?
>>>>
>>>> pte_cont() (and pte_mkcont() and pte_mknoncont()) test and manipulte the
>>>> "contiguous bit" in the arm64 PTE entries. Those helpers are arm64-specific
>>>> (AFAIK). The contiguous bit is a hint to the HW to tell it that a block of PTEs
>>>> are mapping a physically contiguous and naturally aligned piece of memory. The
>>>> HW can use this to coalesce entries in the TLB. When using 4K base pages, the
>>>> contpte size is 64K (16 PTEs). For 16K base pages, its 2M (128 PTEs) and for 64K
>>>> base pages, its 2M (32 PTEs).
>>>>
>>>>>
>>>>> On powerpc, for 16k pages, we have to define 4 consecutive PTEs. All 4
>>>>> PTE are flagged with the SPS bit telling it's a 16k pages, but for TLB
>>>>> misses the HW needs one entrie for each 4k fragment.
>>>>
>>>> From that description, it sounds like the SPS bit might be similar to arm64
>>>> contiguous bit? Although sounds like you are currently using it in a slightly
>>>> different way - telling kernel that the base page is 16K but mapping each 16K
>>>> page with 4x 4K entries (plus the SPS bit set)?
>>>
>>> Yes it's both.
>>>
>>> When the base page is 16k, there are 4x 4k entries (with SPS bit set) in
>>> the page table, and pte_t is a table of 4 'unsigned long'
>>>
>>> When the base page is 4k, there is a 16k hugepage size, which is the
>>> same 4x 4k entries with SPS bit set.
>>>
>>> So it looks similar to the contiguous bit.
>>>
>>>
>>> And by extension, the same principle is used for 512k hugepages, the bit
>>> _PAGE_HUGE is copied by the TLB miss handler into the lower bit of PS,
>>> PS being as follows:
>>> - 00 Small (4 Kbyte or 16 Kbyte)
>>> - 01 512 Kbyte
>>> - 10 Reserved
>>> - 11 8 Mbyte
>>>
>>> So as PMD size is 4M, 512k pages are 128 identical consecutive entries
>>> in the page table.
>>>
>>> I which I could have THP with 16k or 512k pages.
>>
>> Then you have come to the right place! :)
>>
>> https://lore.kernel.org/linux-mm/20231204102027.57185-1-ryan.roberts@arm.com/
>>
>
> That looks great. That series only modifies core mm/ .
> No changes needed in arch ? Will it work on powerpc without any
> change/additions to arch code ?

Yes there are also changes needed in arch; I have a separate series for arm64,
which transparently manages the contiguous bit when it sees appropriate PTEs:

https://lore.kernel.org/linux-arm-kernel/20231204105440.61448-1-ryan.roberts@arm.com/

>
> Well, I'll try it soon.
>
> Christophe