From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1761116AbdAKXPa (ORCPT ); Wed, 11 Jan 2017 18:15:30 -0500 Received: from mail.linuxfoundation.org ([140.211.169.12]:36424 "EHLO mail.linuxfoundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751688AbdAKXP2 (ORCPT ); Wed, 11 Jan 2017 18:15:28 -0500 Date: Wed, 11 Jan 2017 15:15:26 -0800 From: Andrew Morton To: Jonathan Corbet Cc: Tim Chen , "Huang, Ying" , dave.hansen@intel.com, ak@linux.intel.com, aaron.lu@intel.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Hugh Dickins , Shaohua Li , Minchan Kim , Rik van Riel , Andrea Arcangeli , "Kirill A . Shutemov" , Vladimir Davydov , Johannes Weiner , Michal Hocko , Hillf Danton , Christian Borntraeger Subject: Re: [PATCH v5 2/9] mm/swap: Add cluster lock Message-Id: <20170111151526.e905b91d6f1ee9f21e6907be@linux-foundation.org> In-Reply-To: <20170111160729.23e06078@lwn.net> References: <20170111150029.29e942aa00af69f9c3c4e9b1@linux-foundation.org> <20170111160729.23e06078@lwn.net> 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, 11 Jan 2017 16:07:29 -0700 Jonathan Corbet wrote: > On Wed, 11 Jan 2017 15:00:29 -0800 > Andrew Morton wrote: > > > hm, bit_spin_lock() is a nasty thing. It is slow and it doesn't have > > all the lockdep support. > > > > Would the world end if we added a spinlock to swap_cluster_info? > > FWIW, I asked the same question in December, this is what I got: > > ... > > > > Why the roll-your-own locking and data structures here? To my naive > > > understanding, it seems like you could do something like: > > > > > > struct swap_cluster_info { > > > spinlock_t lock; > > > atomic_t count; > > > unsigned int flags; > > > }; > > > > > > Then you could use proper spinlock operations which, among other things, > > > would make the realtime folks happier. That might well help with the > > > cache-line sharing issues as well. Some of the count manipulations could > > > perhaps be done without the lock entirely; similarly, atomic bitops might > > > save you the locking for some of the flag tweaks - though I'd have to look > > > more closely to be really sure of that. > > > > > > The cost, of course, is the growth of this structure, but you've already > > > noted that the overhead isn't all that high; seems like it could be worth > > > it. > > > > Yes. The data structure you proposed is much easier to be used than the > > current one. The main concern is the RAM usage. The size of the data > > structure you proposed is about 80 bytes, while that of the current one > > is about 8 bytes. There will be one struct swap_cluster_info for every > > 1MB swap space, so for 1TB swap space, the total size will be 80M > > compared with 8M of current implementation. Where did this 80 bytes come from? That swap_cluster_info is 12 bytes and could perhaps be squeezed into 8 bytes if we can get away with a 24-bit "count". > > In the other hand, the return of the increased size is not overwhelming. > > The bit spinlock on cluster will not be heavy contended because it is a > > quite fine-grained lock. So the benefit will be little to use lockless > > operations. I guess the realtime issue isn't serious given the lock is > > not heavy contended and the operations protected by the lock is > > light-weight too.