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

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Date:   Thu, 20 Apr 2023 15:44:03 +0800
From:   haoxin <xhao@...ux.alibaba.com>
To:     Huang Ying <ying.huang@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        kernel test robot <yujie.liu@...el.com>,
        Nadav Amit <namit@...are.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Hugh Dickins <hughd@...gle.com>,
        Matthew Wilcox <willy@...radead.org>,
        David Hildenbrand <david@...hat.com>
Subject: Re: [PATCH] mm,unmap: avoid flushing TLB in batch if PTE is
 inaccessible


在 2023/4/10 下午3:52, Huang Ying 写道:
> 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@intel.com
> Link: https://lore.kernel.org/oe-lkp/ab92aaddf1b52ede15e2c608696c36765a2602c1.camel@intel.com/
> Fixes: 7e12beb8ca2a ("migrate_pages: batch flushing TLB")
> Reported-by: kernel test robot <yujie.liu@...el.com>
> Signed-off-by: "Huang, Ying" <ying.huang@...el.com>
> Cc: Nadav Amit <namit@...are.com>
> Cc: Mel Gorman <mgorman@...hsingularity.net>
> Cc: Hugh Dickins <hughd@...gle.com>
> Cc: Matthew Wilcox (Oracle) <willy@...radead.org>
> Cc: David Hildenbrand <david@...hat.com>
> ---
>   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();

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