From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wr0-f197.google.com (mail-wr0-f197.google.com [209.85.128.197]) by kanga.kvack.org (Postfix) with ESMTP id E22C46B0038 for ; Thu, 30 Nov 2017 15:47:39 -0500 (EST) Received: by mail-wr0-f197.google.com with SMTP id y23so4558263wra.16 for ; Thu, 30 Nov 2017 12:47:39 -0800 (PST) Received: from mail.linuxfoundation.org (mail.linuxfoundation.org. [140.211.169.12]) by mx.google.com with ESMTPS id k82si3526745wmf.103.2017.11.30.12.47.38 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 30 Nov 2017 12:47:38 -0800 (PST) Date: Thu, 30 Nov 2017 12:47:36 -0800 From: Andrew Morton 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> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Waiman Long Cc: Dave Chinner , Vladimir Davydov , Johannes Weiner , linux-kernel@vger.kernel.org, linux-mm@kvack.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--; _ -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org