Re: [patch]x86: avoid unnecessary tlb flush

[Shaohua Li]

On Sat, Aug 14, 2010 at 07:00:37AM +0800, Siddha, Suresh B wrote:
> On Fri, 2010-08-13 at 14:08 -0700, H. Peter Anvin wrote:
> > On 08/13/2010 12:29 PM, Hugh Dickins wrote:
> > > 
> > > Just added Andrea to the Cc list: he did that TLB flush in 1a44e149,
> > > I'd feel more comfortable noop-ing it on x86 if you've convinced him.
> > > 
> > > Hugh
> > 
> > Andrea is probably on his way back from LinuxCon, but looking at the
> > original patch it might be something that non-x86 architectures need,
> > but which can be optimized specifically on x86, since x86 has explicit
> > "no flush needed when going to more permissive" semantics.
> 
> Yes. I don't see a problem with the proposed patch. This is the case of
> parallel thread execution getting spurious write protection faults for
> the same page for which the pte entry is already up to date and the
> fault has already flushed the existing spurious TLB entry in the case of
> x86.
> 
> I prefer a better name for the new flush_tlb_nonprotect_page() to
> reflect the above. something like tlb_fix_spurious_fault() or something?
this name is better.


In x86, access and dirty bits are set automatically by CPU when CPU accesses
memory. When we go into the code path of below flush_tlb_fix_spurious_fault(),
we already set dirty bit for pte and don't need flush tlb. This might mean
tlb entry in some CPUs hasn't dirty bit set, but this doesn't matter. When
the CPUs do page write, they will automatically check the bit and no software
involved. 

On the other hand, flush tlb in below position is harmful. Test creates CPU
number of threads, each thread writes to a same but random address in same vma
range and we measure the total time. Under a 4 socket system, original time is
1.96s, while with the patch, the time is 0.8s. Under a 2 socket system, there is
20% time cut too. perf shows a lot of time are taking to send ipi/handle ipi for
tlb flush.

Signed-off-by: Shaohua Li <shaohua.li@xxxxxxxxx>
Acked-by: Suresh Siddha <suresh.b.siddha@xxxxxxxxx>

---
 arch/x86/include/asm/pgtable.h |    2 ++
 include/asm-generic/pgtable.h  |    4 ++++
 mm/memory.c                    |    2 +-
 3 files changed, 7 insertions(+), 1 deletion(-)

Index: linux/arch/x86/include/asm/pgtable.h
===================================================================
--- linux.orig/arch/x86/include/asm/pgtable.h	2010-08-16 09:00:02.000000000 +0800
+++ linux/arch/x86/include/asm/pgtable.h	2010-08-16 09:03:41.000000000 +0800
@@ -603,6 +603,8 @@ static inline void ptep_set_wrprotect(st
 	pte_update(mm, addr, ptep);
 }
 
+#define flush_tlb_fix_spurious_fault(vma, address)
+
 /*
  * clone_pgd_range(pgd_t *dst, pgd_t *src, int count);
  *
Index: linux/include/asm-generic/pgtable.h
===================================================================
--- linux.orig/include/asm-generic/pgtable.h	2010-08-16 09:00:02.000000000 +0800
+++ linux/include/asm-generic/pgtable.h	2010-08-16 09:03:41.000000000 +0800
@@ -129,6 +129,10 @@ static inline void ptep_set_wrprotect(st
 #define move_pte(pte, prot, old_addr, new_addr)	(pte)
 #endif
 
+#ifndef flush_tlb_fix_spurious_fault
+#define flush_tlb_fix_spurious_fault(vma, address) flush_tlb_page(vma, address)
+#endif
+
 #ifndef pgprot_noncached
 #define pgprot_noncached(prot)	(prot)
 #endif
Index: linux/mm/memory.c
===================================================================
--- linux.orig/mm/memory.c	2010-08-16 09:03:08.000000000 +0800
+++ linux/mm/memory.c	2010-08-16 09:03:41.000000000 +0800
@@ -3140,7 +3140,7 @@ static inline int handle_pte_fault(struc
 		 * with threads.
 		 */
 		if (flags & FAULT_FLAG_WRITE)
-			flush_tlb_page(vma, address);
+			flush_tlb_fix_spurious_fault(vma, address);
 	}
 unlock:
 	pte_unmap_unlock(pte, ptl);
--
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