Re: [PATCH -V2] mm,unmap: avoid flushing TLB in batch if PTE is inaccessible

[haoxin <xhao@xxxxxxxxxxxxxxxxx>]

在 2023/4/24 下午2:54, Huang Ying 写道:
> Hi, Andrew,
>
> The version 1 of this patch was merged in mm-unstable branch.  If you
> want to move that patch into mm-stable recently, it may be better to
> update that patch with this new version firstly.  If you want to do
> that after v6.4-rc1, I will rebase this patch and resend it after
> v6.4-rc1 is released.
>
> Hi, Amit,
>
> The patch has been changed based on comments from Xin.  I keep your
> "reviewed-by" because I think the change is trivial.  But if you think
> it's inappropriate,  I will change that.
>
> Best Regards,
> Huang, Ying
>
> ------------------------------->8------------------------------------------
> 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>
> Reviewed-by: Nadav Amit <namit@xxxxxxxxxx>
> Cc: haoxin <xhao@xxxxxxxxxxxxxxxxx>
> Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
> Cc: Hugh Dickins <hughd@xxxxxxxxxx>
> Cc: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
> Cc: David Hildenbrand <david@xxxxxxxxxx>
> ---
>   mm/rmap.c | 12 ++++++++----
>   1 file changed, 8 insertions(+), 4 deletions(-)
>
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 8632e02661ac..be19232e94f4 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -641,10 +641,14 @@ void try_to_unmap_flush_dirty(void)
>   #define TLB_FLUSH_BATCH_PENDING_LARGE			\
>   	(TLB_FLUSH_BATCH_PENDING_MASK / 2)
>   
> -static void set_tlb_ubc_flush_pending(struct mm_struct *mm, bool writable)
> +static void set_tlb_ubc_flush_pending(struct mm_struct *mm, pte_t pteval)
>   {
>   	struct tlbflush_unmap_batch *tlb_ubc = &current->tlb_ubc;
>   	int batch, nbatch;
> +	bool writable = pte_dirty(pteval);
> +
> +	if (!pte_accessible(mm, pteval))
> +		return;

LGTM

Reviewed-by: Xin Hao <xhao@xxxxxxxxxxxxxxxxx>

>   
>   	arch_tlbbatch_add_mm(&tlb_ubc->arch, mm);
>   	tlb_ubc->flush_required = true;
> @@ -731,7 +735,7 @@ void flush_tlb_batched_pending(struct mm_struct *mm)
>   	}
>   }
>   #else
> -static void set_tlb_ubc_flush_pending(struct mm_struct *mm, bool writable)
> +static void set_tlb_ubc_flush_pending(struct mm_struct *mm, pte_t pteval)
>   {
>   }
>   
> @@ -1582,7 +1586,7 @@ 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));
> +				set_tlb_ubc_flush_pending(mm, pteval);
>   			} else {
>   				pteval = ptep_clear_flush(vma, address, pvmw.pte);
>   			}
> @@ -1963,7 +1967,7 @@ 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));
> +				set_tlb_ubc_flush_pending(mm, pteval);
>   			} else {
>   				pteval = ptep_clear_flush(vma, address, pvmw.pte);
>   			}