Hi, Michal,
On Mon, Apr 28, 2014 at 10:54 PM, Michal Hocko <mho...@suse.cz> wrote:
> I really fail to see how that helps. compound_head is inlined and the
> compiler should be clever enough to optimize the code properly. I
> haven't tried that to be honest but this looks like it only adds a code
> without any good reason. And I really hate the new name as well. What
> does it suppose to mean?
the code in question is as below:
--- snipt ----
if (likely(!PageTail(page))) { <------ (1)
if (put_page_testzero(page)) {
/*
¦* By the time all refcounts have been released
¦* split_huge_page cannot run anymore from under us.
¦*/
if (PageHead(page))
__put_compound_page(page);
else
__put_single_page(page);
}
return;
}
/* __split_huge_page_refcount can run under us */
page_head = compound_head(page); <------------ (2)
--- snipt ---
if at (1) , we fail the check, this means page is *likely* a tail page.
Then at (2), yes, compoud_head(page) is inlined, it is :
--- snipt ---
static inline struct page *compound_head(struct page *page)
{
if (unlikely(PageTail(page))) { <----------- (3)
struct page *head = page->first_page;
smp_rmb();
if (likely(PageTail(page)))
return head;
}
return page;
}
--- snipt ---
here, the (3) unlikely in the case is a negative hint, because it
is *likely* a tail page. So the check (3) in this case is not good,
so I introduce a helper for this case.
Actually, I checked the assembled code, the compiler is _not_
so smart to recognize this case. It just does optimization as
the hint unlikely() told it.
Thanks,
Jianyu Zhan
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