From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751023AbdK3Uri (ORCPT ); Thu, 30 Nov 2017 15:47:38 -0500 Received: from mail.linuxfoundation.org ([140.211.169.12]:54192 "EHLO mail.linuxfoundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750708AbdK3Urh (ORCPT ); Thu, 30 Nov 2017 15:47:37 -0500 Date: Thu, 30 Nov 2017 12:47:36 -0800 From: Andrew Morton To: Waiman Long Cc: Dave Chinner , Vladimir Davydov , Johannes Weiner , linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH] list_lru: Prefetch neighboring list entries before acquiring lock Message-Id: <20171130124736.e60c75d120b74314c049c02b@linux-foundation.org> In-Reply-To: <209d1aea-2951-9d4f-5638-8bc037a6676c@redhat.com> References: <1511965054-6328-1-git-send-email-longman@redhat.com> <20171129135319.ab078fbed566be8fc90c92ec@linux-foundation.org> <20171130004252.GR4094@dastard> <209d1aea-2951-9d4f-5638-8bc037a6676c@redhat.com> X-Mailer: Sylpheed 3.4.1 (GTK+ 2.24.23; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 30 Nov 2017 08:54:04 -0500 Waiman Long wrote: > > And, from that perspective, the racy shortcut in the proposed patch > > is wrong, too. Prefetch is fine, but in general shortcutting list > > empty checks outside the internal lock isn't. > > For the record, I add one more list_empty() check at the beginning of > list_lru_del() in the patch for 2 purpose: > 1. it allows the code to bail out early. > 2. It make sure the cacheline of the list_head entry itself is loaded. > > Other than that, I only add a likely() qualifier to the existing > list_empty() check within the lock critical region. But it sounds like Dave thinks that unlocked check should be removed? How does this adendum look? From: Andrew Morton Subject: list_lru-prefetch-neighboring-list-entries-before-acquiring-lock-fix include prefetch.h, remove unlocked list_empty() test, per Dave Cc: Dave Chinner Cc: Johannes Weiner Cc: Vladimir Davydov Cc: Waiman Long Signed-off-by: Andrew Morton --- mm/list_lru.c | 5 ++--- 1 file changed, 2 insertions(+), 3 deletions(-) diff -puN mm/list_lru.c~list_lru-prefetch-neighboring-list-entries-before-acquiring-lock-fix mm/list_lru.c --- a/mm/list_lru.c~list_lru-prefetch-neighboring-list-entries-before-acquiring-lock-fix +++ a/mm/list_lru.c @@ -8,6 +8,7 @@ #include #include #include +#include #include #include #include @@ -135,13 +136,11 @@ bool list_lru_del(struct list_lru *lru, /* * Prefetch the neighboring list entries to reduce lock hold time. */ - if (unlikely(list_empty(item))) - return false; prefetchw(item->prev); prefetchw(item->next); spin_lock(&nlru->lock); - if (likely(!list_empty(item))) { + if (!list_empty(item)) { l = list_lru_from_kmem(nlru, item); list_del_init(item); l->nr_items--; _