Re: [v5 09/15] sparc64: optimized struct page zeroing

[Pasha Tatashin]

Hi Sam,

Thank you for looking at this. I will update patch description, and as 
you suggested replace memset() via static assert in next iteration.

Pasha

On 08/04/2017 01:37 AM, Sam Ravnborg wrote:
> Hi Pavel.
>
> On Thu, Aug 03, 2017 at 05:23:47PM -0400, Pavel Tatashin wrote:
> > Add an optimized mm_zero_struct_page(), so struct page's are zeroed without
> > calling memset(). We do eight regular stores, thus avoid cost of membar.
>
> The commit message does no longer reflect the implementation,
> and should be updated.
>
> > Signed-off-by: Pavel Tatashin <pasha.tatashin@xxxxxxxxxx>
> > Reviewed-by: Steven Sistare <steven.sistare@xxxxxxxxxx>
> > Reviewed-by: Daniel Jordan <daniel.m.jordan@xxxxxxxxxx>
> > Reviewed-by: Bob Picco <bob.picco@xxxxxxxxxx>
> > ---
> >   arch/sparc/include/asm/pgtable_64.h | 32 ++++++++++++++++++++++++++++++++
> >   1 file changed, 32 insertions(+)
> >
> > diff --git a/arch/sparc/include/asm/pgtable_64.h b/arch/sparc/include/asm/pgtable_64.h
> > index 6fbd931f0570..be47537e84c5 100644
> > --- a/arch/sparc/include/asm/pgtable_64.h
> > +++ b/arch/sparc/include/asm/pgtable_64.h
> > @@ -230,6 +230,38 @@ extern unsigned long _PAGE_ALL_SZ_BITS;
> >   extern struct page *mem_map_zero;
> >   #define ZERO_PAGE(vaddr)	(mem_map_zero)
> >   
> > +/* This macro must be updated when the size of struct page grows above 80
> > + * or reduces below 64.
> > + * The idea that compiler optimizes out switch() statement, and only
> > + * leaves clrx instructions or memset() call.
> > + */
> > +#define	mm_zero_struct_page(pp) do {					\
> > +	unsigned long *_pp = (void *)(pp);				\
> > +									\
> > +	/* Check that struct page is 8-byte aligned */			\
> > +	BUILD_BUG_ON(sizeof(struct page) & 7);				\
> Would also be good to catch if sizeof > 80 so we do not silently
> migrate to the suboptimal version (silent at build time).
> Can you at build time catch if size is no any of: 64, 72, 80
> and simplify the below a little?
>
> 	Sam