From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753160AbdK2VxX (ORCPT ); Wed, 29 Nov 2017 16:53:23 -0500 Received: from mail.linuxfoundation.org ([140.211.169.12]:35502 "EHLO mail.linuxfoundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751770AbdK2VxV (ORCPT ); Wed, 29 Nov 2017 16:53:21 -0500 Date: Wed, 29 Nov 2017 13:53:19 -0800 From: Andrew Morton To: Waiman Long Cc: Vladimir Davydov , Johannes Weiner , Dave Chinner , linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH] list_lru: Prefetch neighboring list entries before acquiring lock Message-Id: <20171129135319.ab078fbed566be8fc90c92ec@linux-foundation.org> In-Reply-To: <1511965054-6328-1-git-send-email-longman@redhat.com> References: <1511965054-6328-1-git-send-email-longman@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 Wed, 29 Nov 2017 09:17:34 -0500 Waiman Long wrote: > The list_lru_del() function removes the given item from the LRU list. > The operation looks simple, but it involves writing into the cachelines > of the two neighboring list entries in order to get the deletion done. > That can take a while if the cachelines aren't there yet, thus > prolonging the lock hold time. > > To reduce the lock hold time, the cachelines of the two neighboring > list entries are now prefetched before acquiring the list_lru_node's > lock. > > Using a multi-threaded test program that created a large number > of dentries and then killed them, the execution time was reduced > from 38.5s to 36.6s after applying the patch on a 2-socket 36-core > 72-thread x86-64 system. Patch looks good. Can someone (Dave?) please explain why list_lru_del() supports deletion of an already list_empty(item)? This seems a rather dangerous thing to encourage. Use cases I can think of are: a) item is already reliably deleted, so why the heck was the caller calling list_lru_del() and b) item might be concurrently deleted by another thread, in which case the race loser is likely to hit a use-after-free. Is there a good use case here?