Re: [PATCH rfc 1/3] mm: prepare more high-order pages to be stored on the per-cpu lists

[Kefeng Wang <wangkefeng.wang@huawei.com>]

On 2024/4/15 19:41, Baolin Wang wrote:
> 
> 
> On 2024/4/15 16:12, Kefeng Wang wrote:
>> Both the file pages and anonymous pages support large folio, high-order
>> pages except HPAGE_PMD_ORDER(PMD_SHIFT - PAGE_SHIFT) will be allocated
>> frequently which will increase the zone lock contention, allow high-order
>> pages on pcp lists could alleviate the big zone lock contention, in order
>> to allows high-orders(PAGE_ALLOC_COSTLY_ORDER, HPAGE_PMD_ORDER) to be
>> stored on the per-cpu lists, similar with PMD_ORDER pages, more lists is
>> added in struct per_cpu_pages (one list each high-order pages), also a
>> new PCP_MAX_ORDER instead of HPAGE_PMD_ORDER is added in mmzone.h.
>>
>> But as commit 44042b449872 ("mm/page_alloc: allow high-order pages to be
>> stored on the per-cpu lists") pointed, it may not win in all the scenes,
>> so this don't allow higher-order pages to be added to PCP list, the next
>> will add a control to enable or disable it.
>>
>> The struct per_cpu_pages increases in size from 256(4 cache lines) to
>> 320 bytes (5 cache lines) on arm64 with defconfig.
>>
>> Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
>> ---
>>   include/linux/mmzone.h |  4 +++-
>>   mm/page_alloc.c        | 10 +++++-----
>>   2 files changed, 8 insertions(+), 6 deletions(-)
>>
>> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
>> index c11b7cde81ef..c745e2f1a0f2 100644
>> --- a/include/linux/mmzone.h
>> +++ b/include/linux/mmzone.h
>> @@ -657,11 +657,13 @@ enum zone_watermarks {
>>    * failures.
>>    */
>>   #ifdef CONFIG_TRANSPARENT_HUGEPAGE
>> -#define NR_PCP_THP 1
>> +#define PCP_MAX_ORDER (PMD_SHIFT - PAGE_SHIFT)
>> +#define NR_PCP_THP (PCP_MAX_ORDER - PAGE_ALLOC_COSTLY_ORDER)
>>   #else
>>   #define NR_PCP_THP 0
>>   #endif
>>   #define NR_LOWORDER_PCP_LISTS (MIGRATE_PCPTYPES * 
>> (PAGE_ALLOC_COSTLY_ORDER + 1))
>> +#define HIGHORDER_PCP_LIST_INDEX (NR_LOWORDER_PCP_LISTS - 
>> (PAGE_ALLOC_COSTLY_ORDER + 1))
> 
> Thanks for starting the discussion.
> 
> I am concerned that mixing mTHPs of different migratetypes in a single 
> pcp list might lead to fragmentation issues, potentially causing 
> unmovable mTHPs to occupy movable pageblocks, which would reduce 
> compaction efficiency.
> 

Yes, this is not enabled it by default.

> But also not sure if it is suitable to add more pcp lists, maybe we can 
> just add the most commonly used mTHP as a start, for example: 64K?

Do you mean only add only one list for 64K, I think it before, but it is
not true for all cases, maybe other order is most used in different
tests, so only enable specified  high-order by a pcp_enabled sysfs, but
it is certain that we need find a case to show improvement when use the 
high-order(eg,order4 = 64K) on pcp list.