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SubjectRe: follow_page() performance regressions
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On Tue, 1 Jul 2008, Linus Torvalds wrote:
>
> I suspect it could be any of:
>
> - actual code generation differences in follow_page() (some SH person
> needs to check that)
>
> - some cache issue on SH. ZERO_PAGE is a single page at a fixed virtual
> address, while it used to populate the page tables with individual
> pages. Normally, this should be *better* for caching, but maybe there
> is some conflict? What kind of caches does SH have (virtually indexed?)
>
> - hackbench relying on follow_page() to populate the page tables, which
> it no longer does for anonymous areas (using ZERO_PAGE directly
> instead).
>
> Again, normally this would speed things up (fewer TLB misses etc), but
> if it then causes a new page fault that used to have been covered by
> follow_page(), who knows?
>
> What does hackbench actually do? Anybody?

I am reporter of this problem.
Sorry, my testing was not enough. This problem depends our platform or maybe SH architecture.

When I changed that the code of |goto no_page| and |goto bad_page| to both |goto unlock| at follow_page(), I did not notice that the code of |bad_page:| and |no_page:| was lost by optimizer. In the result, only the size of the text code of kernel became small 0x40 bytes. I think that the cause of this problem is in any of our platform or in the replacement of SH cache.


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