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Date: Mon, 28 Apr 2014 23:53:28 +0800 From: Jianyu Zhan <nasa4836@...il.com> To: Michal Hocko <mhocko@...e.cz> Cc: Andrew Morton <akpm@...ux-foundation.org>, kirill.shutemov@...ux.intel.com, Rik van Riel <riel@...hat.com>, Jiang Liu <liuj97@...il.com>, peterz@...radead.org, Johannes Weiner <hannes@...xchg.org>, Mel Gorman <mgorman@...e.de>, Andrea Arcangeli <aarcange@...hat.com>, sasha.levin@...cle.com, liwanp@...ux.vnet.ibm.com, khalid.aziz@...cle.com, "linux-mm@...ck.org" <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org> Subject: Re: [PATCH RFC 2/2] mm: introdule compound_head_by_tail() Hi, Michal, On Mon, Apr 28, 2014 at 10:54 PM, Michal Hocko <mhocko@...e.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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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