Re: [patch] x86: clearing access bit don't flush tlb

[Ingo Molnar]

* Shaohua Li <shli@xxxxxxxxxx> wrote:

> Add a few acks and resend this patch.
> 
> We use access bit to age a page at page reclaim. When clearing pte access bit,
> we could skip tlb flush in X86. The side effect is if the pte is in tlb and pte
> access bit is unset in page table, when cpu access the page again, cpu will not
> set page table pte's access bit. Next time page reclaim will think this hot
> page is yong and reclaim it wrongly, but this doesn't corrupt data.
> 
> And according to intel manual, tlb has less than 1k entries, which covers < 4M
> memory. In today's system, several giga byte memory is normal. After page
> reclaim clears pte access bit and before cpu access the page again, it's quite
> unlikely this page's pte is still in TLB. And context swich will flush tlb too.
> The chance skiping tlb flush to impact page reclaim should be very rare.
> 
> Originally (in 2.5 kernel maybe), we didn't do tlb flush after clear access bit.
> Hugh added it to fix some ARM and sparc issues. Since I only change this for
> x86, there should be no risk.
> 
> And in some workloads, TLB flush overhead is very heavy. In my simple
> multithread app with a lot of swap to several pcie SSD, removing the tlb flush
> gives about 20% ~ 30% swapout speedup.
> 
> Signed-off-by: Shaohua Li <shli@xxxxxxxxxxxx>
> Acked-by: Rik van Riel <riel@xxxxxxxxxx>
> Acked-by: Mel Gorman <mgorman@xxxxxxx>
> Acked-by: Hugh Dickins <hughd@xxxxxxxxxx>
> ---
>  arch/x86/mm/pgtable.c |   13 ++++++-------
>  1 file changed, 6 insertions(+), 7 deletions(-)
> 
> Index: linux/arch/x86/mm/pgtable.c
> ===================================================================
> --- linux.orig/arch/x86/mm/pgtable.c	2014-03-27 05:22:08.572100549 +0800
> +++ linux/arch/x86/mm/pgtable.c	2014-03-27 05:46:12.456131121 +0800
> @@ -399,13 +399,12 @@ int pmdp_test_and_clear_young(struct vm_
>  int ptep_clear_flush_young(struct vm_area_struct *vma,
>  			   unsigned long address, pte_t *ptep)
>  {
> -	int young;
> -
> -	young = ptep_test_and_clear_young(vma, address, ptep);
> -	if (young)
> -		flush_tlb_page(vma, address);
> -
> -	return young;
> +	/*
> +	 * In X86, clearing access bit without TLB flush doesn't cause data
> +	 * corruption. Doing this could cause wrong page aging and so hot pages
> +	 * are reclaimed, but the chance should be very rare.

So, beyond the spelling mistakes, I guess this explanation should also 
be a bit more explanatory - how about something like:

	/*
	 * On x86 CPUs, clearing the accessed bit without a TLB flush 
	 * doesn't cause data corruption. [ It could cause incorrect
	 * page aging and the (mistaken) reclaim of hot pages, but the
	 * chance of that should be relatively low. ]
	 *
	 * So as a performance optimization don't flush the TLB when 
	 * clearing the accessed bit, it will eventually be flushed by 
	 * a context switch or a VM operation anyway. [ In the rare 
	 * event of it not getting flushed for a long time the delay 
	 * shouldn't really matter because there's no real memory 
	 * pressure for swapout to react to. ]
	 */

Agreed?

Thanks,

	Ingo
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