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=-1.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,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 F1B85C43381 for ; Thu, 28 Feb 2019 03:26:11 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id C429E2183F for ; Thu, 28 Feb 2019 03:26:11 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730530AbfB1D0K (ORCPT ); Wed, 27 Feb 2019 22:26:10 -0500 Received: from mga04.intel.com ([192.55.52.120]:24548 "EHLO mga04.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1730131AbfB1D0K (ORCPT ); Wed, 27 Feb 2019 22:26:10 -0500 X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False Received: from orsmga004.jf.intel.com ([10.7.209.38]) by fmsmga104.fm.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 27 Feb 2019 19:26:09 -0800 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.58,421,1544515200"; d="scan'208";a="278452682" Received: from yhuang-dev.sh.intel.com (HELO yhuang-dev) ([10.239.159.151]) by orsmga004.jf.intel.com with ESMTP; 27 Feb 2019 19:26:07 -0800 From: "Huang\, Ying" To: Waiman Long Cc: Linus Torvalds , Matthew Wilcox , "Chen\, Rong A" , "lkp\@01.org" , LKML , Andi Kleen , Dave Hansen , Tim C Chen Subject: Re: [LKP] [page cache] eb797a8ee0: vm-scalability.throughput -16.5% regression References: <20181114092242.GD18977@shao2-debian> <20181114141713.GA25731@bombadil.infradead.org> <875zt7t14h.fsf@yhuang-dev.intel.com> <1c33a91c-a436-a879-ca14-7eebcbf971c2@redhat.com> <87imx4pv5q.fsf@yhuang-dev.intel.com> <04aed7af-fe04-5639-cfe1-fe8468164897@redhat.com> Date: Thu, 28 Feb 2019 11:26:06 +0800 In-Reply-To: <04aed7af-fe04-5639-cfe1-fe8468164897@redhat.com> (Waiman Long's message of "Wed, 27 Feb 2019 21:37:41 -0500") Message-ID: <87a7igpp9t.fsf@yhuang-dev.intel.com> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/26.1 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain; charset=ascii Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Waiman Long writes: > On 02/27/2019 08:18 PM, Huang, Ying wrote: >> Waiman Long writes: >> >>> On 02/26/2019 12:30 PM, Linus Torvalds wrote: >>>> On Tue, Feb 26, 2019 at 12:17 AM Huang, Ying wrote: >>>>> As for fixing. Should we care about the cache line alignment of struct >>>>> inode? Or its size is considered more important because there may be a >>>>> huge number of struct inode in the system? >>>> Thanks for the great analysis. >>>> >>>> I suspect we _would_ like to make sure inodes are as small as >>>> possible, since they are everywhere. Also, they are usually embedded >>>> in other structures (ie "struct inode" is embedded into "struct >>>> ext4_inode_info"), and unless we force alignment (and thus possibly >>>> lots of padding), the actual alignment of 'struct inode' will vary >>>> depending on filesystem. >>>> >>>> So I would suggest we *not* do cacheline alignment, because it will >>>> result in random padding. >>>> >>>> But it sounds like maybe the solution is to make sure that the >>>> different fields of the inode can and should be packed differently? >>>> >>>> So one thing to look at is to see what fields in 'struct inode' might >>>> be best moved together, to minimize cache accesses. >>>> >>>> And in particular, if this is *only* an issue of "struct >>>> rw_semaphore", maybe we should look at the layout of *that*. In >>>> particular, I'm getting the feeling that we should put the "owner" >>>> field right next to the "count" field, because the normal >>>> non-contended path only touches those two fields. >>> That is true. Putting the two next to each other reduces the chance of >>> needing to touch 2 cachelines to acquire a rwsem. >>> >>>> Right now those two fields are pretty far from each other in 'struct >>>> rw_semaphore', which then makes the "oops they got allocated in >>>> different cachelines" much more likely. >>>> >>>> So even if 'struct inode' layout itself isn't changed, maybe just >>>> optimizing the layout of 'struct rw_semaphore' a bit for the common >>>> case might fix it all up. >>>> >>>> Waiman, I didn't check if your rewrite already possibly fixes this? >>> My current patch doesn't move the owner field, but I will add one to do >>> it. That change alone probably won't solve the regression we see here. >>> The optimistic spinner is spinning on the on_cpu flag of the task >>> structure as well as the rwsem->owner value (looking for change). The >>> lock holder only need to touch the count/owner values once at unlock. >>> However, if other hot data variables are in the same cacheline as >>> rwsem->owner, we will have cacaheline bouncing problem. So we need to >>> pad some rarely touched variables right before the rwsem in order to >>> reduce the chance of false cacheline sharing. >> Yes. And if my understanding were correct, if the rwsem is locked, the >> new rw_sem users (which calls down_write()) will write rwsem->count and >> some other fields of rwsem. This will cause cache ping-pong between >> lock holder and the new users too if the memory accessed by lock holder >> shares the same cache line with rwsem->count, thus hurt the system >> performance. For the regression reported, the rwsem holder will change >> address_space->i_mmap, if I put i_mmap and rwsem->count in the same >> cache line and rwsem->owner in a different cache line, the performance >> can improve ~8.3%. While if I put i_mmap in one cache line and all >> fields of rwsem in another different cache line, the performance can >> improve ~12.9% (in another machine, where the regression is ~14%). > > So it is better to have i_mmap and the rwsem in separate cachelines. Right? Yes. >> So I think in the heavily contended situation, we should put the fields >> accessed by rwsem holder in a different cache line of rwsem. But in >> un-contended situation, we should put the fields accessed in rwsem >> holder and rwsem in the same cache line to reduce the cache footprint. >> The requirement of un-contended and heavily contended situation is >> contradicted. > > Write to the rwsem's count mostly happens at lock and unlock times. It > is the constant spinning on owner by the optimistic waiter that is > likely to cause the most problem when its cacheline is shared with > another piece of data outside of the rwsem that is rewritten to fairly > frequently. Perhaps moving i_mmap further away from i_mmap_rwsem may help. Yes. I think rwsem->owner is more important too. rwsem->count has measurable effect too. And yes, moving i_mmap further away should help rwsem->count sharing too. Best Regards, Huang, Ying > Cheers, > Longman