Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S966957AbaFQWix (ORCPT ); Tue, 17 Jun 2014 18:38:53 -0400 Received: from g4t3426.houston.hp.com ([15.201.208.54]:54098 "EHLO g4t3426.houston.hp.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S966205AbaFQWiV (ORCPT ); Tue, 17 Jun 2014 18:38:21 -0400 From: Waiman Long To: Andrew Morton , Mel Gorman , Rik van Riel , Ingo Molnar , Peter Zijlstra , "Kirill A. Shutemov" Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, Scott J Norton , Waiman Long Subject: [PATCH v2 1/2] mm, thp: move invariant bug check out of loop in __split_huge_page_map Date: Tue, 17 Jun 2014 18:37:58 -0400 Message-Id: <1403044679-9993-2-git-send-email-Waiman.Long@hp.com> X-Mailer: git-send-email 1.7.1 In-Reply-To: <1403044679-9993-1-git-send-email-Waiman.Long@hp.com> References: <1403044679-9993-1-git-send-email-Waiman.Long@hp.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. This patch moves the invariant bug check out of the loop so that it will be done only once. On a 3.16-rc1 based kernel, the execution time of a microbenchmark that broke up 1000 transparent huge pages using munmap() had an execution time of 38,245us and 38,548us with and without the patch respectively. The performance gain is about 1%. Signed-off-by: Waiman Long --- mm/huge_memory.c | 4 ++-- 1 files changed, 2 insertions(+), 2 deletions(-) diff --git a/mm/huge_memory.c b/mm/huge_memory.c index e60837d..be84c71 100644 --- a/mm/huge_memory.c +++ b/mm/huge_memory.c @@ -1744,6 +1744,8 @@ static int __split_huge_page_map(struct page *page, if (pmd) { pgtable = pgtable_trans_huge_withdraw(mm, pmd); pmd_populate(mm, &_pmd, pgtable); + if (pmd_write(*pmd)) + BUG_ON(page_mapcount(page) != 1); haddr = address; for (i = 0; i < HPAGE_PMD_NR; i++, haddr += PAGE_SIZE) { @@ -1753,8 +1755,6 @@ static int __split_huge_page_map(struct page *page, entry = maybe_mkwrite(pte_mkdirty(entry), vma); if (!pmd_write(*pmd)) entry = pte_wrprotect(entry); - else - BUG_ON(page_mapcount(page) != 1); if (!pmd_young(*pmd)) entry = pte_mkold(entry); if (pmd_numa(*pmd)) -- 1.7.1 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/