Re: [PATCH v2] mm: migrate: record the mlocked page status to remove unnecessary lru drain

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 10/20/2023 12:48 PM, Hugh Dickins wrote:
> On Fri, 20 Oct 2023, Baolin Wang wrote:
> 
>> When doing compaction, I found the lru_add_drain() is an obvious hotspot
>> when migrating pages. The distribution of this hotspot is as follows:
>>     - 18.75% compact_zone
>>        - 17.39% migrate_pages
>>           - 13.79% migrate_pages_batch
>>              - 11.66% migrate_folio_move
>>                 - 7.02% lru_add_drain
>>                    + 7.02% lru_add_drain_cpu
>>                 + 3.00% move_to_new_folio
>>                   1.23% rmap_walk
>>              + 1.92% migrate_folio_unmap
>>           + 3.20% migrate_pages_sync
>>        + 0.90% isolate_migratepages
>>
>> The lru_add_drain() was added by commit c3096e6782b7 ("mm/migrate:
>> __unmap_and_move() push good newpage to LRU") to drain the newpage to LRU
>> immediately, to help to build up the correct newpage->mlock_count in
>> remove_migration_ptes() for mlocked pages. However, if there are no mlocked
>> pages are migrating, then we can avoid this lru drain operation, especailly
>> for the heavy concurrent scenarios.
>>
>> So we can record the source pages' mlocked status in migrate_folio_unmap(),
>> and only drain the lru list when the mlocked status is set in migrate_folio_move().
>> In addition, the page was already isolated from lru when migrating, so checking
>> the mlocked status is stable by folio_test_mlocked() in migrate_folio_unmap().
>>
>> After this patch, I can see the hotpot of the lru_add_drain() is gone:
>>     - 9.41% migrate_pages_batch
>>        - 6.15% migrate_folio_move
>>           - 3.64% move_to_new_folio
>>              + 1.80% migrate_folio_extra
>>              + 1.70% buffer_migrate_folio
>>           + 1.41% rmap_walk
>>           + 0.62% folio_add_lru
>>        + 3.07% migrate_folio_unmap
>>
>> Meanwhile, the compaction latency shows some improvements when running
>> thpscale:
>>                              base                   patched
>> Amean     fault-both-1      1131.22 (   0.00%)     1112.55 *   1.65%*
>> Amean     fault-both-3      2489.75 (   0.00%)     2324.15 *   6.65%*
>> Amean     fault-both-5      3257.37 (   0.00%)     3183.18 *   2.28%*
>> Amean     fault-both-7      4257.99 (   0.00%)     4079.04 *   4.20%*
>> Amean     fault-both-12     6614.02 (   0.00%)     6075.60 *   8.14%*
>> Amean     fault-both-18    10607.78 (   0.00%)     8978.86 *  15.36%*
>> Amean     fault-both-24    14911.65 (   0.00%)    11619.55 *  22.08%*
>> Amean     fault-both-30    14954.67 (   0.00%)    14925.66 *   0.19%*
>> Amean     fault-both-32    16654.87 (   0.00%)    15580.31 *   6.45%*
>>
> 
> Seems a sensible change with good results (I'll conceal how little of
> the stats I understand, I expect everyone else understands them: in my
> naivety, I'm mainly curious why rmap_walk's 1.23% didn't get a + on it).

TBH, I also don't know why the rmap_walk didn't get a + on it, let me 
check it again.

>> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
>> ---
>> Chages from v1:
>>   - Use separate flags in __migrate_folio_record() to avoid to pack flags
>> in each call site per Ying.
>> ---
>>   mm/migrate.c | 47 +++++++++++++++++++++++++++++++++++------------
>>   1 file changed, 35 insertions(+), 12 deletions(-)
>>
>> diff --git a/mm/migrate.c b/mm/migrate.c
>> index 125194f5af0f..fac96139dbba 100644
>> --- a/mm/migrate.c
>> +++ b/mm/migrate.c
>> @@ -1027,22 +1027,39 @@ union migration_ptr {
>>   	struct anon_vma *anon_vma;
>>   	struct address_space *mapping;
>>   };
>> +
>> +enum {
>> +	PAGE_WAS_MAPPED = 1 << 0,
>> +	PAGE_WAS_MLOCKED = 1 << 1,
>> +};
>> +
> 
> I was whispering to myself "I bet someone will suggest BIT()";
> and indeed that someone has turned out to be Huang, Ying.

Sure.

> 
>>   static void __migrate_folio_record(struct folio *dst,
>> -				   unsigned long page_was_mapped,
>> +				   unsigned int page_was_mapped,
>> +				   unsigned int page_was_mlocked,
>>   				   struct anon_vma *anon_vma)
>>   {
>>   	union migration_ptr ptr = { .anon_vma = anon_vma };
>> +	unsigned long page_flags = 0;
> 
> Huang, Ying preferred a different name, me too: old_page_state?

OK, sounds better to me.

> 
>> +
>> +	if (page_was_mapped)
>> +		page_flags |= PAGE_WAS_MAPPED;
>> +	if (page_was_mlocked)
>> +		page_flags |= PAGE_WAS_MLOCKED;
> 
> What's annoying me about the patch is all this mix of page_was_mapped and
> page_was_mlocked variables, then the old_page_state bits.  Can't it be
> done with PAGE_WAS_ bits in old_page_state throughout, without any
> page_was_mapped and page_was_mlocked variables?

Yes, good point. Let me try it. Thanks for your comments.