Re: [PATCH] mempolicy: Optimize queue_folios_pte_range by PTE batching

[Dev Jain <dev.jain@arm.com>]

On 15/04/25 5:29 pm, David Hildenbrand wrote:
> On 15.04.25 13:47, Dev Jain wrote:
>>
>>
>> On 15/04/25 3:47 pm, David Hildenbrand wrote:
>>> On 11.04.25 10:13, Dev Jain wrote:
>>>> After the check for queue_folio_required(), the code only cares 
>>>> about the
>>>> folio in the for loop, i.e the PTEs are redundant. Therefore, optimize
>>>> this
>>>> loop by skipping over a PTE batch mapping the same folio.
>>>>
>>>> Signed-off-by: Dev Jain <dev.jain@arm.com>
>>>> ---
>>>> Unfortunately I have only build tested this since my test 
>>>> environment is
>>>> broken.
>>>>
>>>>    mm/mempolicy.c | 12 +++++++++++-
>>>>    1 file changed, 11 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
>>>> index b28a1e6ae096..b019524da8a2 100644
>>>> --- a/mm/mempolicy.c
>>>> +++ b/mm/mempolicy.c
>>>> @@ -573,6 +573,9 @@ static int queue_folios_pte_range(pmd_t *pmd,
>>>> unsigned long addr,
>>>>        pte_t *pte, *mapped_pte;
>>>>        pte_t ptent;
>>>>        spinlock_t *ptl;
>>>> +    int max_nr;
>>>> +    const fpb_t fpb_flags = FPB_IGNORE_DIRTY | FPB_IGNORE_SOFT_DIRTY;
>>>> +    int nr = 1;
>>>
>>> Try sticking to reverse xmas tree, please. (not completely the case
>>> here, but fpb_flags can easily be moved all he way to the top)
>>
>> I thought that the initializations were to be kept at the bottom.
> 
> Not that I am aware of.
> 
>> Asking for future patches, should I put all declarations in reverse-xmas
>> fashion (even those which I don't intend to touch w.r.t the patch
>> logic), or do I do that for only my additions?
> 
> We try to stay as close to reverse-xmas tree as possible. It's not 
> always possible (e.g., dependent assignments), but fpb_flags in this 
> case here can easily go all the way to the top.

Sure.

> 
> ...
> 
>>
>>>
>>>   >       ptl = pmd_trans_huge_lock(pmd, vma);>       if (ptl) {
>>>> @@ -586,7 +589,8 @@ static int queue_folios_pte_range(pmd_t *pmd,
>>>> unsigned long addr,
>>>>            walk->action = ACTION_AGAIN;
>>>>            return 0;
>>>>        }
>>>   > -    for (; addr != end; pte++, addr += PAGE_SIZE) {> +    for (;
>>> addr != end; pte += nr, addr += nr * PAGE_SIZE) {
>>>> +        nr = 1;
>>>>            ptent = ptep_get(pte);
>>>>            if (pte_none(ptent))
>>>>                continue;
>>>> @@ -607,6 +611,11 @@ static int queue_folios_pte_range(pmd_t *pmd,
>>>> unsigned long addr,
>>>>            if (!queue_folio_required(folio, qp))
>>>>                continue;
>>>>            if (folio_test_large(folio)) {
>>>> +            max_nr = (end - addr) >> PAGE_SHIFT;
>>>> +            if (max_nr != 1)
>>>> +                nr = folio_pte_batch(folio, addr, pte, ptent,
>>>> +                             max_nr, fpb_flags,
>>>> +                             NULL, NULL, NULL);
>>>
>>> We should probably do that immediately after we verified that
>>> vm_normal_folio() have us something reasonable.
>>
>> But shouldn't we keep the small folio case separate to avoid the
>> overhead of folio_pte_batch()?
> 
> Yes, just do something like
> 
> if (folio_test_large(folio) && end - addr > 1)
>      nr = folio_pte_batch(folio, addr, pte, ptent, end - addr,
>                   max_nr, fpb_flags, ...);
> 
> before the folio_test_reserved().
> 
> Then you'd also skip the all ptes if !queue_folio_required.

Ah got you, thanks.

> 
>>
>>>
>>>>                /*
>>>>                 * A large folio can only be isolated from LRU once,
>>>>                 * but may be mapped by many PTEs (and Copy-On-Write may
>>>> @@ -633,6 +642,7 @@ static int queue_folios_pte_range(pmd_t *pmd,
>>>> unsigned long addr,
>>>>                qp->nr_failed++;
>>>>                if (strictly_unmovable(flags))
>>>>                    break;
>>>> +            qp->nr_failed += nr - 1;
>>>
>>> Can't we do qp->nr_failed += nr; above?
>>
>> I did not dive deep into the significance of nr_failed, but I did that
>> to keep the code, before and after the change, equivalent:
> 
> And I question exactly that.
> 
> If we hit strictly_unmovable(flags), we end up returning "-EIO" from
> queue_folios_pte_range().
> 
> And staring at queue_pages_range(), we ignore nr_failed if 
> walk_page_range() returned an error.
> 
> So looks like we can just add everything in one shot, independent of 
> strictly_unmovable()?

Looks good to me this way. I'll change it, thanks.

>