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    SubjectRe: Unified tracing buffer
    On Fri, Sep 19, 2008 at 02:33:42PM -0700, 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.

    With due apologies for pitching-in late, I thought I'd bring visibility to
    the two new interfaces - namely relay_printk() and relay_dump() - now a part
    of -mm tree (since 2.6.27-rc5-mm1) are meant to address such needs;
    although not completely in its present form but quite substantially.
    (Refer: Documentation/filesystems/relay.txt). As far as re-usability is
    concerned, many parts of this interface are directly adopted from
    SystemTap's runtime. Blktrace had been made to work using these interfaces
    ( reducing about ~130 lines of code from the
    blktrace related files.

    With more effort, say additions such as a)ability to specify custom
    names for files b)ability to create user-defined control files (in
    addition to what comes default) will make it usable along with tracers
    such as ftrace (ref: (and is something that I
    intended to work upon).

    While relay_printk() interface brings a high-level abstract interface
    over 'relay' by masking all the setup/tear-down details and the ability
    to use per-CPU buffers; relay_dump() is its equivalent that performs
    binary dumping through debugfs interface (a requirement for the unified
    tracing buffer, as I learn from the email). Also the use of default
    file-names, debugfs output path results in huge reduction of setup code
    required by the end-user along with the ability to override the defaults
    if required in a special case. Examples of the resulting code-brevity can
    be seen at samples/relay/*.c in 2.6.27-rc5-mm1 tree.

    I am quite sure that with minimal changes to infrastructure underlying
    beneath these two interfaces, we can meet out most of the requirements
    stated above; and am open for suggestions.

    Kindly let me know what the community thinks about the same.


    > 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.
    > 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.
    > -------
    > 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.
    > Circular buffer per cpu, protected by per-cpu spinlock_irq
    > Word aligned records.
    > Variable record length, header will start with length record.
    > Timestamps in fixed timebase, monotonically increasing (across all CPUs)
    > ---------------
    > 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)
    > OUTPUT
    > ------
    > Data will be output via debugfs, and provide the following output streams:
    > /debugfs/tracing/<name>/buffers/text
    > clear text stream (will merge the per-cpu streams via insertion
    > sort, and use the print functions)
    > /debugfs/tracing/<name>/buffers/binary[cpu_number]
    > per-cpu binary data
    > -------
    > Sysfs style tree under debugfs
    > /debugfs/tracing/<name>/buffers/enabed <--- binary value
    > /debugfs/tracing/<name>/<event1>
    > /debugfs/tracing/<name>/<event2>
    > etc ...
    > provides a way to enable/disable events, see what's available, and
    > what's enabled.
    > -------------------
    > No way to unregister buffers and events.
    > Will provide an unregister_buffer and unregister_event call
    > 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 ;-)
    > --
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