Re: [PATCH v2 2/2] mm: Compute first_set_pte to eliminate evaluating redundant ranges

Ryan Roberts
2024-09-19T16:56:00Z

On 19/09/2024 09:40, Dev Jain wrote:
>
> On 9/19/24 07:04, Barry Song wrote:
>> On Mon, Sep 16, 2024 at 11:08 PM Dev Jain <dev.jain@arm.com> wrote:
>>> For an mTHP allocation, we need to check, for every order, whether for
>>> that order, we have enough number of contiguous PTEs empty. Instead of
>>> iterating the while loop for every order, use some information, which
>>> is the first set PTE found, from the previous iteration to eliminate
>>> some cases. The key to understanding the correctness of the patch
>>> is that the ranges we want to examine form a strictly decreasing
>>> sequence of nested intervals.
>> Could we include some benchmark data here, as suggested by Ryan in this thread?
>>
>> https://lore.kernel.org/linux-mm/58f91a56-890a-45d0-8b1f-47c4c70c9600@arm.com/
>
> Can you please verify and get some numbers for the following program,
> because if I am doing this correctly, it would be a regression :)
> https://www.codedump.xyz/cpp/Zuvf8FwvRPH21UO2

Some brief comments on the test code:

- You don't need to enable/disable the top-level control. Regardless, I don't
think this breaks the benchmark.

- I think you have an off-by-1 in your for loop condition:

for (unsigned long i = 1; (i * border) < size; ++i) {

I think this needs to be:

for (unsigned long i = 1; (i * border) <= size; ++i) {

It just means that the final 32K block will get a single 32K mapping.

- You're measuring the whole program; including mmap/munmap and
enabling/disabling mTHP. It would be much better to just measure the loop that
writes to each page after mTHP is enabled.

I modified the code to iterate for 10 seconds and on each iteration, measure
only the time spent in the interesting loop. Running on Apple M2 VM:

Before the change:

ubuntu@ubuntuvm:~/scan-pte$ sudo ./scan-pte
Average: 0.070028 seconds per GB (iterations=98)
ubuntu@ubuntuvm:~/scan-pte$ sudo ./scan-pte
Average: 0.068495 seconds per GB (iterations=96)
ubuntu@ubuntuvm:~/scan-pte$ sudo ./scan-pte
Average: 0.070207 seconds per GB (iterations=93)

After the change:

ubuntu@ubuntuvm:~/scan-pte$ sudo ./scan-pte
Average: 0.076923 seconds per GB (iterations=88)
ubuntu@ubuntuvm:~/scan-pte$ sudo ./scan-pte
Average: 0.072206 seconds per GB (iterations=96)
ubuntu@ubuntuvm:~/scan-pte$ sudo ./scan-pte
Average: 0.072397 seconds per GB (iterations=89)

So this looks pretty clearly slower to me. So suggest we shouldn't take this patch.

>
> The program does this: disable THP completely -> mmap 1G VMA -> touch the last
> page of a 32K sized boundary. That is, 0th till 32K/4K - 2 pages are
> empty, while the 32K/4K - 1'th page is touched, and so on -> madvise
> the entire VMA -> enable all THPs except 2M -> touch all pages.
>
> Therefore, we have 0 - 6 PTEs empty, 7th is filled, and so on. Eventually,
> kernel will fall down to finding 4 contiguous PTEs empty and allocate
> 4K * 4 = 16K mTHP.
>
> The result without the patches:
>
> real: 8.250s
> user: 0.941s
> sys: 7.077s
>
> real: 8.175s
> user: 0.939s
> sys: 7.021s
>
> With the patches:
>
> real: 8.584s
> user: 1.089s
> sys: 7.234s
>
> real: 8.429s
> user: 0.954s
> sys: 7.220s

What HW did you measure this on? I'm guessing this is measuring multiple
iterations, otherwise it looks extremely slow. If you were measuring on FVP, for
example, that would not give representative performance numbers.

Thanks,
Ryan

>
> You can change the #iterations in the for loop to magnify this,
> and the current code surprisingly wins.
>
>
>>
>>> Suggested-by: Ryan Roberts <ryan.roberts@arm.com>
>>> Signed-off-by: Dev Jain <dev.jain@arm.com>
>>> ---
>>>   mm/memory.c | 20 ++++++++++++++++++--
>>>   1 file changed, 18 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/mm/memory.c b/mm/memory.c
>>> index 8bb1236de93c..e81c6abe09ce 100644
>>> --- a/mm/memory.c
>>> +++ b/mm/memory.c
>>> @@ -4633,10 +4633,11 @@ static struct folio *alloc_anon_folio(struct vm_fault
>>> *vmf)
>>>   {
>>>          struct vm_area_struct *vma = vmf->vma;
>>>   #ifdef CONFIG_TRANSPARENT_HUGEPAGE
>>> +       pte_t *first_set_pte = NULL, *align_pte, *pte;
>>>          unsigned long orders;
>>>          struct folio *folio;
>>>          unsigned long addr;
>>> -       pte_t *pte;
>>> +       int max_empty;
>>>          gfp_t gfp;
>>>          int order;
>>>
>>> @@ -4671,8 +4672,23 @@ static struct folio *alloc_anon_folio(struct vm_fault
>>> *vmf)
>>>          order = highest_order(orders);
>>>          while (orders) {
>>>                  addr = ALIGN_DOWN(vmf->address, PAGE_SIZE << order);
>>> -               if (pte_range_none(pte + pte_index(addr), 1 << order) == 1 <<
>>> order)
>>> +               align_pte = pte + pte_index(addr);
>>> +
>>> +               /* Range to be scanned known to be empty */
>>> +               if (align_pte + (1 << order) <= first_set_pte)
>>> +                       break;
>>> +
>>> +               /* Range to be scanned contains first_set_pte */
>>> +               if (align_pte <= first_set_pte)
>>> +                       goto repeat;
>>> +
>>> +               /* align_pte > first_set_pte, so need to check properly */
>>> +               max_empty = pte_range_none(align_pte, 1 << order);
>>> +               if (max_empty == 1 << order)
>>>                          break;
>>> +
>>> +               first_set_pte = align_pte + max_empty;
>>> +repeat:
>>>                  order = next_order(&orders, order);
>>>          }
>>>
>>> --
>>> 2.30.2
>>>
>> Thanks
>> barry