From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752652AbYITJDo (ORCPT ); Sat, 20 Sep 2008 05:03:44 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751018AbYITJDg (ORCPT ); Sat, 20 Sep 2008 05:03:36 -0400 Received: from hrndva-omtalb.mail.rr.com ([71.74.56.122]:34175 "EHLO hrndva-omtalb.mail.rr.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751009AbYITJDf (ORCPT ); Sat, 20 Sep 2008 05:03:35 -0400 Date: Sat, 20 Sep 2008 05:03:33 -0400 (EDT) From: Steven Rostedt X-X-Sender: rostedt@gandalf.stny.rr.com To: Martin Bligh cc: Linux Kernel Mailing List , Linus Torvalds , Thomas Gleixner , Mathieu Desnoyers , od@novell.com, "Frank Ch. Eigler" Subject: Re: Unified tracing buffer In-Reply-To: <33307c790809191433w246c0283l55a57c196664ce77@mail.gmail.com> Message-ID: References: <33307c790809191433w246c0283l55a57c196664ce77@mail.gmail.com> User-Agent: Alpine 1.10 (DEB 962 2008-03-14) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Martin, First I like to express my appreciation to you for writing this up. Not only that, but being the one person from keeping us from killing each other ;-) On Fri, 19 Sep 2008, Martin Bligh wrote: > During kernel summit and Plumbers conference, Linus and others > expressed a desire for a unified > tracing buffer system for multiple tracing applications (eg ftrace, > lttng, systemtap, blktrace, etc) to use. > This provides several advantages, including the ability to interleave > data from multiple sources, > not having to learn 200 different tools, duplicated code/effort, etc. > > Several of us got together last night and tried to cut this down to > the simplest usable system > we could agree on (and nobody got hurt!). This will form version 1. Yes, we kept the chairs on the floor the whole time. > I've sketched out a few > enhancements we know that we want, but have agreed to leave these > until version 2. > The answer to most questions about the below is "yes we know, we'll > fix that in version 2" > (or 3). Simplicity was the rule ... > > Sketch of design. Enjoy flaming me. Code will follow shortly. > > > STORAGE > ------- > > We will support multiple buffers for different tracing systems, with > separate names, event id spaces. > Event ids are 16 bit, dynamically allocated. > A "one line of text" print function will be provided for each event, > or use the default (probably hex printf) > Will provide a "flight data recorder" mode, and a "spool to disk" mode. I don't remember talking about the "spool to disk" for version 1. We still want to do this? I thought we would have overwrite mode (flight data record), and a "throw all new data away when the producer fills the buffer before the consumer takes" mode. > > Circular buffer per cpu, protected by per-cpu spinlock_irq > Word aligned records. As stated in another email "8 byte aligned" words should be fine. > Variable record length, header will start with length record. > Timestamps in fixed timebase, monotonically increasing (across all CPUs) > > > INPUT_FUNCTIONS > --------------- > > allocate_buffer (name, size) > return buffer_handle > > register_event (buffer_handle, event_id, print_function) > You can pass in a requested event_id from a fixed set, and > will be given it, or an error > 0 means allocate me one dynamically > returns event_id (or -E_ERROR) > > record_event (buffer_handle, event_id, length, *buf) I was talking with Thomas about this, and we probably want (and I'm sure Mathieu and others would agree), a... event_handle = reserve_event(buffer_handle, event_id, length) as well as a.. comit_event(event_handle). Oh, and all commands should start with the namespace. ring_buffer_alloc() ring_buffer_free() ring_buffer_record_event() etc. > > > OUTPUT > ------ > > Data will be output via debugfs, and provide the following output streams: > > /debugfs/tracing//buffers/text > clear text stream (will merge the per-cpu streams via insertion > sort, and use the print functions) > > /debugfs/tracing//buffers/binary[cpu_number] > per-cpu binary data Ah, I thought we were going to have: /debugfs/tracing/buffers// and each tracer have /debugfs/tracing// This way we can easily see all the buffers in one place that are allocated without having to see a tracer name first. The reason I like the way I propose, is that a utility that needs to read all the buffers, doesn't need to go into directories that don't even have buffers. Not all tracers will allocate a buffer. > > > CONTROL > ------- > > Sysfs style tree under debugfs > > /debugfs/tracing//buffers/enabed <--- binary value > > /debugfs/tracing// > /debugfs/tracing// > etc ... I wonder if we should make this another sub dir: /debugfs/tracing/buffers/events/ > provides a way to enable/disable events, see what's available, and > what's enabled. > > > KNOWN ISSUES / PLANS > ------------------- > > No way to unregister buffers and events. > Will provide an unregister_buffer and unregister_event call I can see registering events, but shouldn't we "allocate" buffers? > > > Generating systemwide time is hard on some platforms > Yes. Time-based output provides a lot of simplicity for the user though > We won't support these platforms at first, we'll add functionality > to make it work for them later. > (plan based on tick-based ms timing, plus counter offset from that > if needed). > > Spinlock_irq is ineffecient, and doesn't support tracing in NMIs > True. We'll implement a lockless scheme later (see lttng) > > Putting a length record in every event is inefficient > True. Fixed record length with optional extensions is better, but > more complex. v2. > > Putting a full timestamp rather than an offset in every event is inefficient > See above. True, but v2. > > Relayfs already exists! use that! > People were universally not keen on that idea. Complexity, interface, etc. > We're also providing some higher level shared functions for time & > event ids. > > There's no way to decode the binary data stream > Code will be shared from the kernel to decode it, so that we can > get the compact binary > format and decode it later. That code will be kept in the kernel > tree (it's a trivial piece of C). > Version 1.1 ;-) > Sounds good, Thanks! -- Steve