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SubjectRe: Unified tracing buffer


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/<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

Ah, I thought we were going to have:

/debugfs/tracing/buffers/<name>/<buffer crap>

and each tracer have

/debugfs/tracing/<name>/<trace command crap>

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/<name>/buffers/enabed <--- binary value
>
> /debugfs/tracing/<name>/<event1>
> /debugfs/tracing/<name>/<event2>
> etc ...

I wonder if we should make this another sub dir:

/debugfs/tracing/buffers/events/<event-name>


> 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



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