Re: [PATCH] mm, thp: move invariant bug check out of loop in __split_huge_page_map

Kirill A. Shutemov
2014-06-17T07:24:32Z

On Mon, Jun 16, 2014 at 11:45:42PM -0400, Waiman Long wrote:
> On 06/16/2014 04:59 PM, Kirill A. Shutemov wrote:
> >On Mon, Jun 16, 2014 at 11:49:34PM +0300, Kirill A. Shutemov wrote:
> >>On Mon, Jun 16, 2014 at 03:35:48PM -0400, Waiman Long wrote:
> >>>In the __split_huge_page_map() function, the check for
> >>>page_mapcount(page) is invariant within the for loop. Because of the
> >>>fact that the macro is implemented using atomic_read(), the redundant
> >>>check cannot be optimized away by the compiler leading to unnecessary
> >>>read to the page structure.
> >And atomic_read() is *not* atomic operation. It's implemented as
> >dereferencing though cast to volatile, which suppress compiler
> >optimization, but doesn't affect what CPU can do with the variable.
> >
> >So I doubt difference will be measurable anywhere.
> >
>
> Because it is treated as an volatile object, the compiler will have to
> reread the value of the relevant page structure field in every iteration of
> the loop (512 for x86) when pmd_write(*pmd) is true. I saw some slight
> improvement (about 2%) of a microbench that I wrote to break up 1000 THPs
> with 1000 forked processes.

Then bring patch with performance data.