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

From: haoxin <xhao@linux.alibaba.com>
To: Huang Ying <ying.huang@intel.com>,
	Andrew Morton <akpm@linux-foundation.org>
Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org,
	kernel test robot <yujie.liu@intel.com>,
	Nadav Amit <namit@vmware.com>,
	Mel Gorman <mgorman@techsingularity.net>,
	Hugh Dickins <hughd@google.com>,
	Matthew Wilcox <willy@infradead.org>,
	David Hildenbrand <david@redhat.com>
Subject: Re: [PATCH] mm,unmap: avoid flushing TLB in batch if PTE is inaccessible
Date: Thu, 20 Apr 2023 15:44:03 +0800	[thread overview]
Message-ID: <97e79078-69e8-e387-9e77-a4d741eace4e@linux.alibaba.com> (raw)
In-Reply-To: <20230410075224.827740-1-ying.huang@intel.com>


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@intel.com>
> Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
> Cc: Nadav Amit <namit@vmware.com>
> Cc: Mel Gorman <mgorman@techsingularity.net>
> Cc: Hugh Dickins <hughd@google.com>
> Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
> Cc: David Hildenbrand <david@redhat.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);
>   			}