Re: [PATCH] mm,unmap: avoid flushing TLB in batch if PTE is inaccessible

[Huang, Ying]

haoxin <xhao@xxxxxxxxxxxxxxxxx> writes:

> ( 2023/4/10 H3:52, Huang Ying S:
>> 0Day/LKP reported a performance regression for commit
>> 7e12beb8ca2a ("migrate_pages: batch flushing TLB"). In the commit, the
>> TLB flushing during page migration is batched.  So, in
>> try_to_migrate_one(), ptep_clear_flush() is replaced with
>> set_tlb_ubc_flush_pending().  In further investigation, it is found
>> that the TLB flushing can be avoided in ptep_clear_flush() if the PTE
>> is inaccessible.  In fact, we can optimize in similar way for the
>> batched TLB flushing too to improve the performance.
>>
>> So in this patch, we check pte_accessible() before
>> set_tlb_ubc_flush_pending() in try_to_unmap/migrate_one().  Tests show
>> that the benchmark score of the anon-cow-rand-mt test case of
>> vm-scalability test suite can improve up to 2.1% with the patch on a
>> Intel server machine.  The TLB flushing IPI can reduce up to 44.3%.
>>
>> Link: https://lore.kernel.org/oe-lkp/202303192325.ecbaf968-yujie.liu@xxxxxxxxx
>> Link: https://lore.kernel.org/oe-lkp/ab92aaddf1b52ede15e2c608696c36765a2602c1.camel@xxxxxxxxx/
>> Fixes: 7e12beb8ca2a ("migrate_pages: batch flushing TLB")
>> Reported-by: kernel test robot <yujie.liu@xxxxxxxxx>
>> Signed-off-by: "Huang, Ying" <ying.huang@xxxxxxxxx>
>> Cc: Nadav Amit <namit@xxxxxxxxxx>
>> Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
>> Cc: Hugh Dickins <hughd@xxxxxxxxxx>
>> Cc: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
>> Cc: David Hildenbrand <david@xxxxxxxxxx>
>> ---
>>   mm/rmap.c | 6 ++++--
>>   1 file changed, 4 insertions(+), 2 deletions(-)
>>
>> diff --git a/mm/rmap.c b/mm/rmap.c
>> index 8632e02661ac..3c7c43642d7c 100644
>> --- a/mm/rmap.c
>> +++ b/mm/rmap.c
>> @@ -1582,7 +1582,8 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
>>   				 */
>>   				pteval = ptep_get_and_clear(mm, address, pvmw.pte);
>>   -				set_tlb_ubc_flush_pending(mm,
>> pte_dirty(pteval));
>> +				if (pte_accessible(mm, pteval))
>> +					set_tlb_ubc_flush_pending(mm, pte_dirty(pteval));
>>   			} else {
>>   				pteval = ptep_clear_flush(vma, address, pvmw.pte);
>>   			}
>> @@ -1963,7 +1964,8 @@ static bool try_to_migrate_one(struct folio *folio, struct vm_area_struct *vma,
>>   				 */
>>   				pteval = ptep_get_and_clear(mm, address, pvmw.pte);
>>   -				set_tlb_ubc_flush_pending(mm,
>> pte_dirty(pteval));
>> +				if (pte_accessible(mm, pteval))
>> +					set_tlb_ubc_flush_pending(mm, pte_dirty(pteval));
>
> Just a advice, can you put pte_accessible() into
> set_tlb_ubc_flush_pendin(), just like ptep_clear_flush(); so that we
> no need to add if (pte_accessible()) in per place
>
> where call set_tlb_ubc_flush_pending();

Sounds reasonable for me, will do that in the next version.  Thanks for
suggestion.

Best Regards,
Huang, Ying

>>   			} else {
>>   				pteval = ptep_clear_flush(vma, address, pvmw.pte);
>>   			}