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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 8A7C4C6FD18 for ; Wed, 29 Mar 2023 12:47:50 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229941AbjC2Mrt (ORCPT ); Wed, 29 Mar 2023 08:47:49 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:55904 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229944AbjC2Mrq (ORCPT ); Wed, 29 Mar 2023 08:47:46 -0400 Received: from dfw.source.kernel.org (dfw.source.kernel.org [139.178.84.217]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 31D5540D9; Wed, 29 Mar 2023 05:47:44 -0700 (PDT) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by dfw.source.kernel.org (Postfix) with ESMTPS id C893061C83; Wed, 29 Mar 2023 12:47:43 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 7056EC433D2; Wed, 29 Mar 2023 12:47:42 +0000 (UTC) Date: Wed, 29 Mar 2023 08:47:35 -0400 From: Steven Rostedt To: Vincent Donnefort Cc: mhiramat@kernel.org, linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org, kernel-team@android.com Subject: Re: [PATCH v2 1/2] ring-buffer: Introducing ring-buffer mapping functions Message-ID: <20230329084735.6c4a9229@rorschach.local.home> In-Reply-To: References: <20230322102244.3239740-1-vdonnefort@google.com> <20230322102244.3239740-2-vdonnefort@google.com> <20230328224411.0d69e272@gandalf.local.home> <20230329070353.1e1b443b@gandalf.local.home> X-Mailer: Claws Mail 3.17.8 (GTK+ 2.24.33; x86_64-pc-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-trace-kernel@vger.kernel.org On Wed, 29 Mar 2023 13:23:01 +0100 Vincent Donnefort wrote: > On Wed, Mar 29, 2023 at 07:03:53AM -0400, Steven Rostedt wrote: > > On Wed, 29 Mar 2023 10:19:44 +0100 > > Vincent Donnefort wrote: > > > > > > I've been playing with this a bit, and I'm thinking, do we need the > > > > data_pages[] array on the meta page? > > > > > > > > I noticed that I'm not even using it. > > > > > > > > Currently, we need to do a ioctl every time we finish with the reader page, > > > > and that updates the reader_page in the meta data to point to the next page > > > > to read. When do we need to look at the data_start section? > > > > > > This is for non-consuming read, to get all the pages in order. > > > > Yeah, I was trying to see how a non consuming read would work, and was > > having issues figuring that out without the tail page being updated. > > Would the userspace really need to know where is the tail page? It can just stop > whenever it finds out a page doesn't have any events, and make sure it does not > loop once back to the head? I'm trying to come up with a possible algorithm that doesn't need ioctls. It would need to know if the writer moved or not. Probably need a counter that gets incremented every time the writer goes to a new page. Having the tail page was just a convenient way to know where the end is. > > > > > > > > > If we remove this section we would lose this ability ... but we'd also simplify > > > the code by a good order of magnitude (don't need the update ioctl anymore, no > > > need to keep those pages in order and everything can fit a 0-order meta-page). > > > And the non-consuming read doesn't bring much to the user over the pipe version. > > > > > > This will although impact our hypervisor tracing which will only be able to > > > expose trace_pipe interfaces. But I don't think it is a problem, all userspace > > > tools only relying on consuming read anyway. > > > > > > So if you're happy dropping this support, let's get rid of it. > > > > I don't really want to get rid of it, but perhaps break it up where we > > don't have it in the first release, but add it in a second one. That will > > also make sure that we can expand the API if necessary (one reason I wanted > > the "data_start" in the first place). > > > > Let's drop it for now, but be able to add it later, an have the current > > structure be: > > Ok, I will prepare a V3 accordingly. > > > > > struct ring_buffer_meta_page_header { > > #if __BITS_PER_LONG == 64 > > __u64 entries; > > __u64 overrun; > > #else > > __u32 entries; > > __u32 overrun; > > #endif > > __u32 pages_touched; > > __u32 meta_page_size; > > __u32 reader_page; /* page ID for the reader page */ > > __u32 nr_data_pages; /* doesn't take into account the reader_page */ > > }; > > > > BTW, shouldn't the nr_data_pages take into account the reader page? As it > > is part of the array we traverse isn't it? > > It depends if the reader page has ever been swapped out. If yes, the reader > would have to start from reader_page and then switch to the data_pages. > Which sounds like a fiddly interface for the userspace. > > So yeah, consuming-read only feels like a better start. > I agree. I'd like to get something in that can be extended, but simple enough that it's not too much of a barrier wrt getting the API correct. -- Steve