Re: [PATCH V4 0/2] mm: FAULT_AROUND_ORDER patchset performance data for powerpc

["Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>]

Madhavan Srinivasan wrote:
> On Tuesday 20 May 2014 03:57 PM, Kirill A. Shutemov wrote:
> > Rusty Russell wrote:
> >> "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> writes:
> >>> Andrew Morton wrote:
> >>>> On Mon, 19 May 2014 16:23:07 -0700 (PDT) Hugh Dickins <hughd@google.com> wrote:
> >>>>
> >>>>> Shouldn't FAULT_AROUND_ORDER and fault_around_order be changed to be
> >>>>> the order of the fault-around size in bytes, and fault_around_pages()
> >>>>> use 1UL << (fault_around_order - PAGE_SHIFT)
> >>>>
> >>>> Yes.  And shame on me for missing it (this time!) at review.
> >>>>
> >>>> There's still time to fix this.  Patches, please.
> >>>
> >>> Here it is. Made at 3.30 AM, build tested only.
> >>
> >> Prefer on top of Maddy's patch which makes it always a variable, rather
> >> than CONFIG_DEBUG_FS.  It's got enough hair as it is.
> > 
> > Something like this?
> > 
> > From: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
> > Date: Tue, 20 May 2014 13:02:03 +0300
> > Subject: [PATCH] mm: nominate faultaround area in bytes rather then page order
> > 
> > There are evidences that faultaround feature is less relevant on
> > architectures with page size bigger then 4k. Which makes sense since
> > page fault overhead per byte of mapped area should be less there.
> > 
> > Let's rework the feature to specify faultaround area in bytes instead of
> > page order. It's 64 kilobytes for now.
> > 
> > The patch effectively disables faultaround on architectures with
> > page size >= 64k (like ppc64).
> > 
> > It's possible that some other size of faultaround area is relevant for a
> > platform. We can expose `fault_around_bytes' variable to arch-specific
> > code once such platforms will be found.
> > 
> > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> > ---
> >  mm/memory.c | 62 +++++++++++++++++++++++--------------------------------------
> >  1 file changed, 23 insertions(+), 39 deletions(-)
> > 
> > diff --git a/mm/memory.c b/mm/memory.c
> > index 037b812a9531..252b319e8cdf 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -3402,63 +3402,47 @@ void do_set_pte(struct vm_area_struct *vma, unsigned long address,
> >  	update_mmu_cache(vma, address, pte);
> >  }
> > 
> > -#define FAULT_AROUND_ORDER 4
> > +static unsigned long fault_around_bytes = 65536;
> > +
> > +static inline unsigned long fault_around_pages(void)
> > +{
> > +	return rounddown_pow_of_two(fault_around_bytes) / PAGE_SIZE;
> > +}
> > +
> > +static inline unsigned long fault_around_mask(void)
> > +{
> > +	return ~(rounddown_pow_of_two(fault_around_bytes) - 1) & PAGE_MASK;
> > +}
> > 
> > -#ifdef CONFIG_DEBUG_FS
> > -static unsigned int fault_around_order = FAULT_AROUND_ORDER;
> > 
> > -static int fault_around_order_get(void *data, u64 *val)
> > +#ifdef CONFIG_DEBUG_FS
> > +static int fault_around_bytes_get(void *data, u64 *val)
> >  {
> > -	*val = fault_around_order;
> > +	*val = fault_around_bytes;
> >  	return 0;
> >  }
> > 
> > -static int fault_around_order_set(void *data, u64 val)
> > +static int fault_around_bytes_set(void *data, u64 val)
> >  {
> 
> Kindly ignore the question if not relevant. Even though we need root
> access to alter the value, will we be fine with
> negative value?.

val is u64. or I miss something?

-- 
 Kirill A. Shutemov