On Fri, 4 May 2012, Minchan Kim wrote:
> >> It's a overkill to align pool size with PAGE_SIZE to avoid
> >> false-sharing. This patch aligns it with just cache line size.
> >>
> >> Signed-off-by: Minchan Kim<minchan@xxxxxxxxxx>
> >> ---
> >> drivers/staging/zsmalloc/zsmalloc-main.c | 6 +++---
> >> 1 file changed, 3 insertions(+), 3 deletions(-)
> >>
> >> diff --git a/drivers/staging/zsmalloc/zsmalloc-main.c
> >> b/drivers/staging/zsmalloc/zsmalloc-main.c
> >> index 51074fa..3991b03 100644
> >> --- a/drivers/staging/zsmalloc/zsmalloc-main.c
> >> +++ b/drivers/staging/zsmalloc/zsmalloc-main.c
> >> @@ -489,14 +489,14 @@ fail:
> >>
> >> struct zs_pool *zs_create_pool(const char *name, gfp_t flags)
> >> {
> >> - int i, error, ovhd_size;
> >> + int i, error;
> >> struct zs_pool *pool;
> >>
> >> if (!name)
> >> return NULL;
> >>
> >> - ovhd_size = roundup(sizeof(*pool), PAGE_SIZE);
> >> - pool = kzalloc(ovhd_size, GFP_KERNEL);
> >> + pool = kzalloc(ALIGN(sizeof(*pool), cache_line_size()),
> >> + GFP_KERNEL);
> >
> > a basic question:
> > Is rounding off allocation size to cache_line_size enough to ensure
> > that the object is cache-line-aligned? Isn't it possible that even
> > though the object size is multiple of cache-line, it may still not be
> > properly aligned and end up sharing cache line with some other
> > read-mostly object?
>
> AFAIK, SLAB allocates object aligned cache-size so I think that problem cannot happen.
> But needs double check.
> Cced Pekka.
The kmalloc(size) function only gives you the following guarantees:
(1) The allocated object is _at least_ 'size' bytes.
(2) The returned pointer is aligned to ARCH_KMALLOC_MINALIGN.
Anything beyond that is implementation detail and probably will break if
you switch between SLAB/SLUB/SLOB.
Pekka
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