Messages in this thread |  | | Date | Thu, 10 Apr 2025 13:06:31 +1000 | | Subject | Re: [PATCH v2 1/1] mm/contpte: Optimize loop to reduce redundant operations | | From | Gavin Shan <> |
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Hi Xavier,
On 4/10/25 12:53 PM, Xavier wrote: > At 2025-04-10 08:58:46, "Gavin Shan" <gshan@redhat.com> wrote: >> >> On 4/10/25 1:10 AM, Xavier wrote: >>> >>> Thank you for carefully reviewing this patch and raising your questions. >>> I'll try to explain and answer them below. >>> >> >> Not a problem :) >> >>> >>> At 2025-04-09 12:09:48, "Gavin Shan" <gshan@redhat.com> wrote: >>>> Hi Xavier, >>>> >>>> On 4/8/25 6:58 PM, Xavier wrote: >>>>> This commit optimizes the contpte_ptep_get function by adding early >>>>> termination logic. It checks if the dirty and young bits of orig_pte >>>>> are already set and skips redundant bit-setting operations during >>>>> the loop. This reduces unnecessary iterations and improves performance. >>>>> >>>>> Signed-off-by: Xavier <xavier_qy@163.com> >>>>> --- >>>>> arch/arm64/mm/contpte.c | 22 ++++++++++++++++++++-- >>>>> 1 file changed, 20 insertions(+), 2 deletions(-) >>>>> >>>>> diff --git a/arch/arm64/mm/contpte.c b/arch/arm64/mm/contpte.c >>>>> index bcac4f55f9c1..034d153d7d19 100644 >>>>> --- a/arch/arm64/mm/contpte.c >>>>> +++ b/arch/arm64/mm/contpte.c >>>>> @@ -152,6 +152,18 @@ void __contpte_try_unfold(struct mm_struct *mm, unsigned long addr, >>>>> } >>>>> EXPORT_SYMBOL_GPL(__contpte_try_unfold); >>>>> >>>> >>>> I'm wandering how it can work. More details are given below. >>>> >>>>> +#define CHECK_CONTPTE_FLAG(start, ptep, orig_pte, flag) \ >>>>> + do { \ >>>>> + int _start = start; \ >>>>> + pte_t *_ptep = ptep; \ >>>>> + while (_start++ < CONT_PTES) { \ >>>>> + if (pte_##flag(__ptep_get(_ptep++))) { \ >>>>> + orig_pte = pte_mk##flag(orig_pte); \ >>>>> + break; \ >>>>> + } \ >>>>> + } \ >>>>> + } while (0) >>>>> + >> >> nit: the variable _start and _ptep can be dropped since the caller is going to >> bail after CHECK_CONTPTE_FLAG(). However, I'm wandering it's going to introduce >> burden to contpte_ptep_get() for its readability, much more complex than I >> thought. > > Not adding these two variables in the current code has no impact. The main purpose > of adding them is to improve maintainability and prevent the external code from > continuing operations unknowingly after the macro has modified the variables. > >> >>>> >>>> CONT_PTES is 16 with the assumption of 4KB base page size. CHECK_CONTPTE_FLAG() >>>> collects the flag for ptep[start, CONT_PTES]. >>>> >>>>> pte_t contpte_ptep_get(pte_t *ptep, pte_t orig_pte) >>>>> { >>>>> /* >>>>> @@ -169,11 +181,17 @@ pte_t contpte_ptep_get(pte_t *ptep, pte_t orig_pte) >>>>> for (i = 0; i < CONT_PTES; i++, ptep++) { >>>>> pte = __ptep_get(ptep); >>>>> >>>>> - if (pte_dirty(pte)) >>>>> + if (pte_dirty(pte)) { >>>>> orig_pte = pte_mkdirty(orig_pte); >>>>> + CHECK_CONTPTE_FLAG(i, ptep, orig_pte, young); >>>>> + break; >>>>> + } >>>>> >>>>> - if (pte_young(pte)) >>>>> + if (pte_young(pte)) { >>>>> orig_pte = pte_mkyoung(orig_pte); >>>>> + CHECK_CONTPTE_FLAG(i, ptep, orig_pte, dirty); >>>>> + break; >>>>> + } >>>>> } >>>>> >>>>> return orig_pte; >>>> >>>> There are two issues as I can see: (1) The loop stops on any dirty or young flag. Another >>>> flag can be missed when one flag is seen. For example, when ptep[0] has both dirty/young >>>> flag, only the dirty flag is collected. (2) CHECK_CONTPTE_FLAG() iterates ptep[i, CONT_PTES], >>>> conflicting to the outer loop, which iterates ptep[0, CONT_PTES]. >>> >>> No flags will be missed. The outer loop is used to check for the first flag, >>> which could be either the dirty or young flag. >>> Once this flag (let's assume it's the dirty flag) is found in the i-th PTE, >>> the dirty flag of orig_pte will be set, and the code will immediately enter >>> the inner loop, namely CHECK_CONTPTE_FLAG. This inner loop will continue >>> to check only for the young flag starting from the i-th position, and we needn't >>> concern about the dirty flag anymore. >>> If CHECK_CONTPTE_FLAG finds the young flag in the j-th PTE, the young flag >>> of orig_pte will be set. At this point, both the young and dirty flags of >>> orig_pte have been set, and there's no need for further loop judgments, so >>> the both the inner and outer loops can be exited directly. This approach >>> reduces unnecessary repeated traversals and judgments. >>> >> >> Thanks for the explanation. I missed that the subsequent young bit is collected >> on pte_dirty(). Similarly, the subsequent dirty bit is collected on pte_young(). >> Now I can see all (dirty | young) bits are collected with a lost. >> >>>> >>>> Besides, I also doubt how much performance can be gained by bailing early on (dirty | young). >>>> However, it's avoided to cross the L1D cache line boundary if possible. With 4KB base page >>>> size, 128 bytes are needed for ptep[CONT_PTES], equal to two cache lines. If we can bail >>>> early with luck, we don't have to step into another cache line. Note that extra checks needs >>>> more CPU cycles. >>> >>> Compared to the previous function, this code doesn't add any extra checks. >>> Even in the worst-case scenario, where neither a dirty nor a young flag is >>> found among the 16 PTEs, the number of checks is the same as in the original >>> function. If any flag is found earlier, the optimized patch will reduce the >>> number of subsequent checks for that flag compared to the original code. >>> >> >> There are 32 checks in the original code (assuming we have 4kb base page size >> and CONT_PTES == 16) and the number of checks is fixed. With your patch applied, >> the number becomes 32 + (number from CHECK_CONTPTE_FLAG) if all PTEs have >> pte_dirty() and none of them has pte_young(), if I don't miss anything here. >> > > If all PTEs are dirty and none are young, the code will enter CHECK_CONTPTE_FLAG > when it encounters the first dirty PTE. Inside CHECK_CONTPTE_FLAG, it only checks > for the young bit (16 - 0) times and then exits all loops. In this case, the total number > of checks is 1 + 16, which is less than the original 32. >
You're correct. The outer loop is going to stop on pte_dirty() or pte_young(). Well, you can see the code becomes hard to be understood, at least to me :)
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Thanks, Gavin
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