From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.2 (2018-09-13) on archive.lwn.net X-Spam-Level: X-Spam-Status: No, score=-5.7 required=5.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,RCVD_IN_DNSWL_HI,SPF_HELO_NONE,SPF_NONE autolearn=ham autolearn_force=no version=3.4.2 Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by archive.lwn.net (Postfix) with ESMTP id A89327D90D for ; Fri, 15 Nov 2019 13:30:04 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727427AbfKONaE (ORCPT ); Fri, 15 Nov 2019 08:30:04 -0500 Received: from mail.kernel.org ([198.145.29.99]:57702 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727421AbfKONaE (ORCPT ); Fri, 15 Nov 2019 08:30:04 -0500 Received: from gandalf.local.home (cpe-66-24-58-225.stny.res.rr.com [66.24.58.225]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 2422C206CC; Fri, 15 Nov 2019 13:30:02 +0000 (UTC) Date: Fri, 15 Nov 2019 08:30:00 -0500 From: Steven Rostedt To: "Frank A. Cancio Bello" Cc: Joel Fernandes , Ingo Molnar , Jonathan Corbet , linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, saiprakash.ranjan@codeaurora.org Subject: Re: [RFC 1/2] docs: ftrace: Clarify the RAM impact of buffer_size_kb Message-ID: <20191115083000.76f89785@gandalf.local.home> In-Reply-To: <20191115042428.6xxiqbzhgoko6vyk@ubuntu1804-desktop> References: <0e4a803c3e24140172855748b4a275c31920e208.1573661658.git.frank@generalsoftwareinc.com> <20191113113730.213ddd72@gandalf.local.home> <20191114202059.GC186056@google.com> <20191114163639.4727e3ed@gandalf.local.home> <20191115042428.6xxiqbzhgoko6vyk@ubuntu1804-desktop> X-Mailer: Claws Mail 3.17.3 (GTK+ 2.24.32; x86_64-pc-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-doc-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-doc@vger.kernel.org On Thu, 14 Nov 2019 23:24:28 -0500 "Frank A. Cancio Bello" wrote: > On Thu, Nov 14, 2019 at 04:36:39PM -0500, Steven Rostedt wrote: > > On Thu, 14 Nov 2019 15:20:59 -0500 > > Joel Fernandes wrote: > > > > > On Wed, Nov 13, 2019 at 11:37:30AM -0500, Steven Rostedt wrote: > > > > On Wed, 13 Nov 2019 11:32:36 -0500 > > > > "Frank A. Cancio Bello" wrote: > > > [snip] > > > > > + > > > > > + The number of pages allocated for each CPU buffer may not > > > > > + be the same than the round up of the division: > > > > > + buffer_size_kb / PAGE_SIZE. This is because part of each page is > > > > > + used to store a page header with metadata. E.g. with > > > > > + buffer_size_kb=4096 (kilobytes), a PAGE_SIZE=4096 bytes and a > > > > > + BUF_PAGE_HDR_SIZE=16 bytes (BUF_PAGE_HDR_SIZE is the size of the > > > > > + page header with metadata) the number of pages allocated for each > > > > > + CPU buffer is 1029, not 1024. The formula for calculating the > > > > > + number of pages allocated for each CPU buffer is the round up of: > > > > > + buffer_size_kb / (PAGE_SIZE - BUF_PAGE_HDR_SIZE). > > > > > > > > I have no problem with this patch, but the concern of documenting the > > > > implementation here, which will most likely not be updated if the > > > > implementation is ever changed, which is why I was vague to begin with. > > > > > > > > But it may never be changed as that code has been like that for a > > > > decade now. > > > > > > Agreed. To give some context, Frank is an outreachy intern I am working with and > > > one of his starter tasks was to understand the ring buffer's basics. I asked > > > him to send a patch since I thought he mentioned there was an error in the > > > documnentation. It looks like all that was missing is some explanation which > > > the deleted text in brackets above should already cover. > > > > > Not exactly in my opinion ;) The deleted text was not the problem. I > just deleted it because with the added text it turns to be redundant. > > The issue that I found with the documentation (maybe just to my > newbie's eyes) is in this part: > > "The trace buffers are allocated in pages (blocks of memory that the > kernel uses for allocation, usually 4 KB in size). If the last page > allocated has room for more bytes than requested, the rest of the > page will be used, making the actual allocation bigger than requested > or shown." > > For me that "suggests" the interpretation that the number of pages > allocated in the current implementation correspond with the round > integer division of buffer_size_kb / PAGE_SIZE, which is inaccurate > (for 5 pages in the example that I mentioned). If you would like, you could reword that to something more accurate, but still not detailing the implementation. > Understood and agreed. It is funny that what I spotted as "a problem" > was precisely an incomplete description of the implementation (the > sentences that I quoted above). What do you think about removing > those two sentences? I wouldn't remove them, just reword them to something you find more accurate. -- Steve