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

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Message-ID: <20c9bd15.b2a7.1961b1e9e88.Coremail.xavier_qy@163.com>
Date: Wed, 9 Apr 2025 23:15:13 +0800 (CST)
From: Xavier  <xavier_qy@....com>
To: "Lance Yang" <ioworker0@...il.com>
Cc: "Dev Jain" <dev.jain@....com>, 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:Re: [PATCH v1] mm/contpte: Optimize loop to reduce redundant
 operations



Hi Lance,


At 2025-04-08 17:17:27, "Lance Yang" <ioworker0@...il.com> wrote:
>On Tue, Apr 8, 2025 at 12:19 AM Dev Jain <dev.jain@....com> wrote:
>>
>>
>> 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.
>
>Yeah, I was thinking the same ;)
>
>This change seems reasonable in theory. But with CONT_PTES=16, keeping
>it simple might actually be faster, IMO.

Please take a look at patch v2. I've updated the code without introducing any
additional checks. The number of checks in the new patch is definitely less than
or equal to that in the original code.

--
Thanks,
Xavier


>
>Thanks,
>Lance
>
>>
>> >>>
>> >>>        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
>> >
>>