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

[Shaohua Li]

On Thu, Apr 03, 2014 at 01:35:37PM +0200, Ingo Molnar wrote:
> 
> * 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?

Sure, that's better, thanks!
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