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Message-ID: <3d52c5bb-60c5-4bfd-958e-737a8cc90853@linux.alibaba.com>
Date: Fri, 9 May 2025 20:25:52 +0800
From: Baolin Wang <baolin.wang@...ux.alibaba.com>
To: David Hildenbrand <david@...hat.com>, akpm@...ux-foundation.org
Cc: 21cnbao@...il.com, ryan.roberts@....com, dev.jain@....com,
ziy@...dia.com, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] mm: mincore: use pte_batch_bint() to batch process
large folios
On 2025/5/9 15:51, David Hildenbrand wrote:
> On 09.05.25 02:45, Baolin Wang wrote:
>> When I tested the mincore() syscall, I observed that it takes longer with
>> 64K mTHP enabled on my Arm64 server. The reason is the
>> mincore_pte_range()
>> still checks each PTE individually, even when the PTEs are contiguous,
>> which is not efficient.
>>
>> Thus we can use pte_batch_hint() to get the batch number of the present
>> contiguous PTEs, which can improve the performance. I tested the
>> mincore()
>> syscall with 1G anonymous memory populated with 64K mTHP, and observed an
>> obvious performance improvement:
>>
>> w/o patch w/ patch changes
>> 6022us 549us +91%
>>
>> Moreover, I also tested mincore() with disabling mTHP/THP, and did not
>> see any obvious regression for base pages.
>>
>> Signed-off-by: Baolin Wang <baolin.wang@...ux.alibaba.com>
>> ---
>> Changes from v2:
>> - Re-calculate the max_nr, per Barry.
>> Changes from v1:
>> - Change to use pte_batch_hint() to get the batch number, per Ryan.
>>
>> Note: I observed the min_t() can introduce a slight performance
>> regression
>> for base pages, so I change to add a batch size check for base pages,
>> which can resolve the performance regression issue.
>> ---
>> mm/mincore.c | 22 +++++++++++++++++-----
>> 1 file changed, 17 insertions(+), 5 deletions(-)
>>
>> diff --git a/mm/mincore.c b/mm/mincore.c
>> index 832f29f46767..42d6c9c8da86 100644
>> --- a/mm/mincore.c
>> +++ b/mm/mincore.c
>> @@ -21,6 +21,7 @@
>> #include <linux/uaccess.h>
>> #include "swap.h"
>> +#include "internal.h"
>> static int mincore_hugetlb(pte_t *pte, unsigned long hmask, unsigned
>> long addr,
>> unsigned long end, struct mm_walk *walk)
>> @@ -105,6 +106,7 @@ static int mincore_pte_range(pmd_t *pmd, unsigned
>> long addr, unsigned long end,
>> pte_t *ptep;
>> unsigned char *vec = walk->private;
>> int nr = (end - addr) >> PAGE_SHIFT;
>> + int step, i;
>> ptl = pmd_trans_huge_lock(pmd, vma);
>> if (ptl) {
>> @@ -118,16 +120,26 @@ static int mincore_pte_range(pmd_t *pmd,
>> unsigned long addr, unsigned long end,
>> walk->action = ACTION_AGAIN;
>> return 0;
>> }
>> - for (; addr != end; ptep++, addr += PAGE_SIZE) {
>> + for (; addr != end; ptep += step, addr += step * PAGE_SIZE) {
>> pte_t pte = ptep_get(ptep);
>> + step = 1;
>> /* We need to do cache lookup too for pte markers */
>> if (pte_none_mostly(pte))
>> __mincore_unmapped_range(addr, addr + PAGE_SIZE,
>> vma, vec);
>> - else if (pte_present(pte))
>> - *vec = 1;
>> - else { /* pte is a swap entry */
>> + else if (pte_present(pte)) {
>> + unsigned int batch = pte_batch_hint(ptep, pte);
>> +
>> + if (batch > 1) {
>> + unsigned int max_nr = (end - addr) >> PAGE_SHIFT;
>
> Nit: probably would have called this max_step to match step.
OK. If need respin the patch, I'll rename it.
>> +
>> + step = min_t(unsigned int, batch, max_nr);
>> + }
>> +
>> + for (i = 0; i < step; i++)
>> + vec[i] = 1;
>
> I'm surprised this micro-optimization matters that much. Probably the
Me too.
> compiler
> defers the calculation of max_nr. I am not convinced we need that level of
> micro-optimization in this code ...
>
>
> But if we're already micro-optimizing, you could have optimized out the
> loop as
> well for order-0:
>
> unsigned int batch = pte_batch_hint(ptep, pte);
>
> if (batch > 1) {
> unsigned int max_nr = (end - addr) >> PAGE_SHIFT;
>
> step = min_t(unsigned int, batch, max_nr);
> for (i = 0; i < step; i++)
> vec[i] = 1;
> } else {
> *vec = 1;
> }
I tried this method, and it had no impact on performance.
> In any case
>
> Acked-by: David Hildenbrand <david@...hat.com>
Thanks.
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