On Tue, 6 Mar 2018 16:02:20 +0100
Christophe LEROY <christophe.le...@c-s.fr> wrote:

> Le 06/03/2018 à 14:25, Nicholas Piggin a écrit :
> > The number of high slices a process might use now depends on its
> > address space size, and what allocation address it has requested.
> > 
> > This patch uses that limit throughout call chains where possible,
> > rather than use the fixed SLICE_NUM_HIGH for bitmap operations.
> > This saves some cost for processes that don't use very large address
> > spaces.
> > 
> > Perormance numbers aren't changed significantly, this may change
> > with larger address spaces or different mmap access patterns that
> > require more slice mask building.
> > 
> > Signed-off-by: Nicholas Piggin <npig...@gmail.com>
> > ---
> >   arch/powerpc/mm/slice.c | 75 
> > +++++++++++++++++++++++++++++--------------------
> >   1 file changed, 45 insertions(+), 30 deletions(-)
> > 
> > diff --git a/arch/powerpc/mm/slice.c b/arch/powerpc/mm/slice.c
> > index 086c31b8b982..507d17e2cfcd 100644
> > --- a/arch/powerpc/mm/slice.c
> > +++ b/arch/powerpc/mm/slice.c
> > @@ -61,14 +61,12 @@ static void slice_print_mask(const char *label, const 
> > struct slice_mask *mask) {
> >   #endif
> >   
> >   static void slice_range_to_mask(unsigned long start, unsigned long len,
> > -                           struct slice_mask *ret)
> > +                           struct slice_mask *ret,
> > +                           unsigned long high_slices)
> >   {
> >     unsigned long end = start + len - 1;
> >   
> >     ret->low_slices = 0;
> > -   if (SLICE_NUM_HIGH)
> > -           bitmap_zero(ret->high_slices, SLICE_NUM_HIGH);
> > -
> >     if (start < SLICE_LOW_TOP) {
> >             unsigned long mend = min(end,
> >                                      (unsigned long)(SLICE_LOW_TOP - 1));
> > @@ -77,6 +75,10 @@ static void slice_range_to_mask(unsigned long start, 
> > unsigned long len,
> >                     - (1u << GET_LOW_SLICE_INDEX(start));
> >     }
> >   
> > +   if (!SLICE_NUM_HIGH)
> > +           return;
> > +
> > +   bitmap_zero(ret->high_slices, high_slices);  
> 
> In include/linux/bitmap.h, it is said:
> 
>   * Note that nbits should be always a compile time evaluable constant.
>   * Otherwise many inlines will generate horrible code.
> 
> Not sure that's true, but it is written ...

Good question, I'll check that.

> >   static inline void slice_or_mask(struct slice_mask *dst,
> >                                     const struct slice_mask *src1,
> > -                                   const struct slice_mask *src2)
> > +                                   const struct slice_mask *src2,
> > +                                   unsigned long high_slices)
> >   {
> >     dst->low_slices = src1->low_slices | src2->low_slices;
> >     if (!SLICE_NUM_HIGH)
> >             return;
> > -   bitmap_or(dst->high_slices, src1->high_slices, src2->high_slices, 
> > SLICE_NUM_HIGH);
> > +   bitmap_or(dst->high_slices, src1->high_slices, src2->high_slices,
> > +                   high_slices);  
> 
> Why a new line here, this line is shorter than before.
> Or that was forgotten in a previous patch ?

Yeah it was previously a longer line. I will fix those.

> > @@ -643,17 +652,17 @@ unsigned long slice_get_unmapped_area(unsigned long 
> > addr, unsigned long len,
> >     if (addr == -ENOMEM)
> >             return -ENOMEM;
> >   
> > -   slice_range_to_mask(addr, len, &potential_mask);
> > +   slice_range_to_mask(addr, len, &potential_mask, high_slices);
> >     slice_dbg(" found potential area at 0x%lx\n", addr);
> >     slice_print_mask(" mask", &potential_mask);
> >   
> >    convert:
> > -   slice_andnot_mask(&potential_mask, &potential_mask, &good_mask);
> > +   slice_andnot_mask(&potential_mask, &potential_mask, &good_mask, 
> > high_slices);
> >     if (compat_maskp && !fixed)
> > -           slice_andnot_mask(&potential_mask, &potential_mask, 
> > compat_maskp);
> > +           slice_andnot_mask(&potential_mask, &potential_mask, 
> > compat_maskp, high_slices);
> >     if (potential_mask.low_slices ||
> >             (SLICE_NUM_HIGH &&
> > -            !bitmap_empty(potential_mask.high_slices, SLICE_NUM_HIGH))) {
> > +            !bitmap_empty(potential_mask.high_slices, high_slices))) {  
> 
> Are we sure high_slices is not nul here when SLICE_NUM_HIGH is not nul ?

On 64/s it should be for 64-bit processes, but perhaps not 32. I have
to look into that, so another good catch.

Perhaps I will leave this patch off the series for now because I didn't
measure much difference. Aneesh wants to expand address space even more,
so I might revisit after his patches go in, to see if the optimistation
becomes worthwhile.

Thanks,
Nick

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