From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754530AbbFTAjY (ORCPT ); Fri, 19 Jun 2015 20:39:24 -0400 Received: from www.sr71.net ([198.145.64.142]:48293 "EHLO blackbird.sr71.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753324AbbFTAjS (ORCPT ); Fri, 19 Jun 2015 20:39:18 -0400 Message-ID: <5584B62F.5080506@sr71.net> Date: Fri, 19 Jun 2015 17:39:11 -0700 From: Dave Hansen User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.7.0 MIME-Version: 1.0 To: Andi Kleen CC: dave.hansen@linux.intel.com, akpm@linux-foundation.org, jack@suse.cz, viro@zeniv.linux.org.uk, eparis@redhat.com, john@johnmccutchan.com, rlove@rlove.org, tim.c.chen@linux.intel.com, linux-kernel@vger.kernel.org, paulmck@linux.vnet.ibm.com Subject: Re: [RFC][PATCH] fs: optimize inotify/fsnotify code for unwatched files References: <20150619215025.4F689817@viggo.jf.intel.com> <20150619233306.GT25760@tassilo.jf.intel.com> In-Reply-To: <20150619233306.GT25760@tassilo.jf.intel.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 06/19/2015 04:33 PM, Andi Kleen wrote: >> > I *think* we can avoid taking the srcu_read_lock() for the >> > common case where there are no actual marks on the file >> > being modified *or* the vfsmount. > What is so expensive in it? Just the memory barrier in it? The profiling doesn't hit on the mfence directly, but I assume that the overhead is coming from there. The "mov 0x8(%rdi),%rcx" is identical before and after the barrier, but it appears much more expensive _after_. That makes no sense unless the barrier is the thing causing it. Here's how the annotation mode of 'perf top' breaks it down: > │ ffffffff810fb480 : > │ nop > │ mov (%rdi),%rax > 0.58 │ push %rbp > │ incl %gs:0x7ef0f488(%rip) > 1.73 │ mov %rsp,%rbp > │ and $0x1,%eax > │ movslq %eax,%rdx > 0.58 │ mov 0x8(%rdi),%rcx > │ incq %gs:(%rcx,%rdx,8) > │ mfence > 69.94 │ add $0x2,%rdx > 7.51 │ mov 0x8(%rdi),%rcx > 4.05 │ incq %gs:(%rcx,%rdx,8) > 13.87 │ decl %gs:0x7ef0f45f(%rip) > │ pop %rbp > 1.73 │ ← retq > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in Please read the FAQ at http://www.tux.org/lkml/