Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753855AbcCZSuz (ORCPT ); Sat, 26 Mar 2016 14:50:55 -0400 Received: from mail-ig0-f174.google.com ([209.85.213.174]:32887 "EHLO mail-ig0-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751009AbcCZSuw (ORCPT ); Sat, 26 Mar 2016 14:50:52 -0400 Date: Sat, 26 Mar 2016 13:50:49 -0500 From: Eric Biggers To: linux-mm@kvack.org Cc: linux-kernel@vger.kernel.org, kirill.shutemov@linux.intel.com Subject: Bloat caused by unnecessary calls to compound_head()? Message-ID: <20160326185049.GA4257@zzz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1031 Lines: 23 Hi, I noticed that after the recent "page-flags" patchset, there are an excessive number of calls to compound_head() in certain places. For example, the frequently executed mark_page_accessed() function already starts out by calling compound_head(), but then each time it tests a page flag afterwards, there is an extra, seemingly unnecessary, call to compound_head(). This causes a series of instructions like the following to appear no fewer than 10 times throughout the function: ffffffff81119db4: 48 8b 53 20 mov 0x20(%rbx),%rdx ffffffff81119db8: 48 8d 42 ff lea -0x1(%rdx),%rax ffffffff81119dbc: 83 e2 01 and $0x1,%edx ffffffff81119dbf: 48 0f 44 c3 cmove %rbx,%rax ffffffff81119dc3: 48 8b 00 mov (%rax),%rax Part of the problem, I suppose, is that the compiler doesn't know that the pages can't be linked more than one level deep. Is this a known tradeoff, and have any possible solutions been considered? Eric