From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-oi0-f72.google.com (mail-oi0-f72.google.com [209.85.218.72]) by kanga.kvack.org (Postfix) with ESMTP id B6CC66B0038 for ; Thu, 30 Nov 2017 15:49:05 -0500 (EST) Received: by mail-oi0-f72.google.com with SMTP id w78so3323216oiw.6 for ; Thu, 30 Nov 2017 12:49:05 -0800 (PST) Received: from mx1.redhat.com (mx1.redhat.com. [209.132.183.28]) by mx.google.com with ESMTPS id p23si1860571oth.295.2017.11.30.12.49.04 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 30 Nov 2017 12:49:04 -0800 (PST) Subject: Re: [PATCH] list_lru: Prefetch neighboring list entries before acquiring lock 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> <20171130124736.e60c75d120b74314c049c02b@linux-foundation.org> From: Waiman Long Message-ID: <377e9162-3d26-f553-c945-458738ebacf5@redhat.com> Date: Thu, 30 Nov 2017 15:49:03 -0500 MIME-Version: 1.0 In-Reply-To: <20171130124736.e60c75d120b74314c049c02b@linux-foundation.org> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Content-Language: en-US Sender: owner-linux-mm@kvack.org List-ID: To: Andrew Morton Cc: Dave Chinner , Vladimir Davydov , Johannes Weiner , linux-kernel@vger.kernel.org, linux-mm@kvack.org On 11/30/2017 03:47 PM, Andrew Morton wrote: > 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--; > _ > Yes, that look good to me. Thanks for fixing that. Cheers, Longman -- 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