linux-kernel - Re: [PATCH v1] mm/contpte: Optimize loop to reduce redundant operations

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Message-ID: <027cc666-a562-46fa-bca5-1122ea00ec0e@arm.com>
Date: Mon, 7 Apr 2025 21:49:21 +0530
From: Dev Jain <dev.jain@....com>
To: Lance Yang <ioworker0@...il.com>, Xavier <xavier_qy@....com>
Cc: akpm@...ux-foundation.org, baohua@...nel.org, catalin.marinas@....com,
 linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
 ryan.roberts@....com, will@...nel.org
Subject: Re: [PATCH v1] mm/contpte: Optimize loop to reduce redundant
 operations


Hi Xavier,

On 07/04/25 7:01 pm, Lance Yang wrote:
> On Mon, Apr 7, 2025 at 8:56 PM Xavier <xavier_qy@....com> wrote:
>>
>>
>>
>> Hi Lance,
>>
>> Thanks for your feedback, my response is as follows.
>>
>> --
>> Thanks,
>> Xavier
>>
>>
>>
>>
>>
>> At 2025-04-07 19:29:22, "Lance Yang" <ioworker0@...il.com> wrote:
>>> Thanks for the patch. Would the following change be better?
>>>
>>> diff --git a/arch/arm64/mm/contpte.c b/arch/arm64/mm/contpte.c
>>> index 55107d27d3f8..64eb3b2fbf06 100644
>>> --- a/arch/arm64/mm/contpte.c
>>> +++ b/arch/arm64/mm/contpte.c
>>> @@ -174,6 +174,9 @@ pte_t contpte_ptep_get(pte_t *ptep, pte_t orig_pte)
>>>
>>>                if (pte_young(pte))
>>>                        orig_pte = pte_mkyoung(orig_pte);
>>> +
>>> +              if (pte_young(orig_pte) && pte_dirty(orig_pte))
>>> +                      break;
>>>        }

Quite the coincidence, I was thinking of doing exactly this some days 
back and testing it out : ) Can you do a microanalysis whether this gets 
us a benefit or not? This looks like an optimization on paper but may 
not be one after all because CONT_PTES is only 16 and a simple loop 
without extra if-conditions may just be faster.

>>>
>>>        return orig_pte;
>>> --
>>>
>>> We can check the orig_pte flags directly instead of using extra boolean
>>> variables, which gives us an early-exit when both dirty and young flags
>>> are set.
>> Your way of writing the code is indeed more concise. However, I think
>>   using boolean variables might be more efficient. Although it introduces
>>   additional variables, comparing boolean values is likely to be more
>>   efficient than checking bit settings.
>>
>>>
>>> Also, is this optimization really needed for the common case?
>> This function is on a high-frequency execution path. During debugging,
>>   I found that in most cases, the first few pages are already marked as
>>   both dirty and young. But currently, the program still has to complete
>>   the entire loop of 16 ptep iterations, which seriously reduces the efficiency.
> 
> Hmm... agreed that this patch helps when early PTEs are dirty/young, but
> for late-ones-only cases, it only introduces overhead with no benefit, IIUC.
> 
> So, let's wait for folks to take a look ;)
> 
> Thanks,
> Lance
> 
>>>
>>> Thanks,
>>> Lance
>