From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-11.1 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,MENTIONS_GIT_HOSTING,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 2C2C5C4707F for ; Thu, 27 May 2021 06:50:25 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 08A2E613D3 for ; Thu, 27 May 2021 06:50:25 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229737AbhE0Gv4 (ORCPT ); Thu, 27 May 2021 02:51:56 -0400 Received: from mail.kernel.org ([198.145.29.99]:33722 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229635AbhE0Gvy (ORCPT ); Thu, 27 May 2021 02:51:54 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id C01EC60FF3; Thu, 27 May 2021 06:50:20 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1622098221; bh=H0EQHOrXpzLNcQ0iwSBYSRvs8jQccJRm2PXUOu/X8Yw=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=nytTHl4IEOUqz9ZlgYkdoEjadwrRbnO25x0BW6yFP/d3HWW19vDCYKsmQFiH1D9Ue NxxRk8sG58KWbWG1Hbr8AGvW2Gva5NM+h7SR0XhZ/+ROXnvZYv2I5fHpCtvVkOsHfX Q/zzmg2hJTyt1K1AFkk8tIjPZFiMB+Nbn7dxKd9Y= Date: Thu, 27 May 2021 08:50:18 +0200 From: Greg Kroah-Hartman To: Ian Kent Cc: Fox Chen , Tejun Heo , Al Viro , Eric Sandeen , Brice Goglin , Rick Lindsley , David Howells , Miklos Szeredi , Marcelo Tosatti , linux-fsdevel , Kernel Mailing List Subject: Re: [PATCH v4 0/5] kernfs: proposed locking and concurrency improvement Message-ID: References: <4eae44395ad321d05f47571b58fe3fe2413b6b36.camel@themaw.net> <6df8d572c6e7a20fab3df13a64f28c1a69648c9f.camel@themaw.net> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <6df8d572c6e7a20fab3df13a64f28c1a69648c9f.camel@themaw.net> Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On Thu, May 27, 2021 at 09:23:06AM +0800, Ian Kent wrote: > On Mon, 2021-05-17 at 09:32 +0800, Ian Kent wrote: > > On Fri, 2021-05-14 at 10:34 +0800, Fox Chen wrote: > > > On Fri, May 14, 2021 at 9:34 AM Ian Kent wrote: > > > > > > > > On Thu, 2021-05-13 at 23:37 +0800, Fox Chen wrote: > > > > > Hi Ian > > > > > > > > > > On Thu, May 13, 2021 at 10:10 PM Ian Kent > > > > > wrote: > > > > > > > > > > > > On Wed, 2021-05-12 at 16:54 +0800, Fox Chen wrote: > > > > > > > On Wed, May 12, 2021 at 4:47 PM Fox Chen > > > > > > > > > > > > > > wrote: > > > > > > > > > > > > > > > > Hi, > > > > > > > > > > > > > > > > I ran it on my benchmark ( > > > > > > > > https://github.com/foxhlchen/sysfs_benchmark). > > > > > > > > > > > > > > > > machine: aws c5 (Intel Xeon with 96 logical cores) > > > > > > > > kernel: v5.12 > > > > > > > > benchmark: create 96 threads and bind them to each core > > > > > > > > then > > > > > > > > run > > > > > > > > open+read+close on a sysfs file simultaneously for 1000 > > > > > > > > times. > > > > > > > > result: > > > > > > > > Without the patchset, an open+read+close operation takes > > > > > > > > 550- > > > > > > > > 570 > > > > > > > > us, > > > > > > > > perf shows significant time(>40%) spending on mutex_lock. > > > > > > > > After applying it, it takes 410-440 us for that operation > > > > > > > > and > > > > > > > > perf > > > > > > > > shows only ~4% time on mutex_lock. > > > > > > > > > > > > > > > > It's weird, I don't see a huge performance boost compared > > > > > > > > to > > > > > > > > v2, > > > > > > > > even > > > > > > > > > > > > > > I meant I don't see a huge performance boost here and it's > > > > > > > way > > > > > > > worse > > > > > > > than v2. > > > > > > > IIRC, for v2 fastest one only takes 40us > > > > > > > > > > > > Thanks Fox, > > > > > > > > > > > > I'll have a look at those reports but this is puzzling. > > > > > > > > > > > > Perhaps the added overhead of the check if an update is > > > > > > needed is taking more than expected and more than just > > > > > > taking the lock and being done with it. Then there's > > > > > > the v2 series ... I'll see if I can dig out your reports > > > > > > on those too. > > > > > > > > > > Apologies, I was mistaken, it's compared to V3, not V2.  The > > > > > previous > > > > > benchmark report is here. > > > > > https://lore.kernel.org/linux-fsdevel/CAC2o3DKNc=sL2n8291Dpiyb0bRHaX=nd33ogvO_LkJqpBj-YmA@mail.gmail.com/ > > > > > > > > Are all these tests using a single file name in the > > > > open/read/close > > > > loop? > > > > > > Yes,  because It's easy to implement yet enough to trigger the > > > mutex_lock. > > > > > > And you are right It's not a real-life pattern, but on the bright > > > side, it proves there is no original mutex_lock problem anymore. :) > > > > I've been looking at your reports and they are quite interesting. > > > > > > > > > That being the case the per-object inode lock will behave like a > > > > mutex and once contention occurs any speed benefits of a spinlock > > > > over a mutex (or rwsem) will disappear. > > > > > > > > In this case changing from a write lock to a read lock in those > > > > functions and adding the inode mutex will do nothing but add the > > > > overhead of taking the read lock. And similarly adding the update > > > > check function also just adds overhead and, as we see, once > > > > contention starts it has a cumulative effect that's often not > > > > linear. > > > > > > > > The whole idea of a read lock/per-object spin lock was to reduce > > > > the possibility of contention for paths other than the same path > > > > while not impacting same path accesses too much for an overall > > > > gain. Based on this I'm thinking the update check function is > > > > probably not worth keeping, it just adds unnecessary churn and > > > > has a negative impact for same file contention access patterns. > > > > The reports indicate (to me anyway) that the slowdown isn't > > due to kernfs. It looks more like kernfs is now putting pressure > > on the VFS, mostly on the file table lock but it looks like > > there's a mild amount of contention on a few other locks as well > > now. > > > > That's a whole different problem and those file table handling > > functions don't appear to have any obvious problems so they are > > doing what they have to do and that can't be avoided. > > > > That's definitely out of scope for these changes. > > > > And, as you'd expect, once any appreciable amount of contention > > happens our measurements go out the window, certainly with > > respect to kernfs. > > > > It also doesn't change my option that checking if an inode > > attribute update is needed in kernfs isn't useful since, IIUC > > that file table lock contention would result even if you were > > using different paths. > > > > So I'll drop that patch from the series. > > It will probably not make any difference, but for completeness > and after some thought, I felt I should post what I think is > happening with the benchmark. > > The benchmark application runs a pthreads thread for each CPU > and goes into a tight open/read/close loop to demonstrate the > contention that can occur on the kernfs mutex as the number of > CPUs grows. > > But that tight open/read/close loop causes contention on the VFS > file table because the pthreads threads share the process file > table. > > So the poor performance is actually evidence the last patch is > doing what it's meant to do rather than evidence of a regression > with the series. > > The benchmark is putting pressure on the process file table on > some hardware configurations but those critical sections are small > and there's really nothing obvious that can be done to improve the > file table locking. > > It is however important to remember that the benckmark application > access pattern is not a normal access pattern by a long way. > > So I don't see the need for a new revision of the series with the > last patch dropped. > > If there's a change of heart and the series was to be merged I'll > leave whether to include this last patch to the discretion of the > maintainer as the bulk of the improvement comes from the earlier > patches anyway. Can you please resubmit the series, it is long-gone from my review queue. thanks, greg k-h