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?

Changed the comments and added ACK of Johannes, so you can pick up directly.
 
Subject: x86: clearing access bit don't flush tlb

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.

Update comments by Ingo.

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>
Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
---
 arch/x86/mm/pgtable.c |   21 ++++++++++++++-------
 1 file changed, 14 insertions(+), 7 deletions(-)

Index: linux/arch/x86/mm/pgtable.c
===================================================================
--- linux.orig/arch/x86/mm/pgtable.c	2014-04-07 08:36:02.843221074 +0800
+++ linux/arch/x86/mm/pgtable.c	2014-04-07 08:37:26.438170140 +0800
@@ -399,13 +399,20 @@ 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;
+	/*
+         * 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. ]
+         */
+	return ptep_test_and_clear_young(vma, address, ptep);
 }
 
 #ifdef CONFIG_TRANSPARENT_HUGEPAGE
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