Re: [RFD tracing] Tracing ABI Work Plan

[Mathieu Desnoyers]

* Masami Hiramatsu (masami.hiramatsu.pt@xxxxxxxxxxx) wrote:
> Hi,
> 
> (2010/11/11 9:46), Mathieu Desnoyers wrote:
> > A) New ABI for user-space
> > 
> > This new ABI will provide the features long-awaited by tracing users. We've
> > already started this discussion by listing the requirements and acknowledging
> > them. It is now time to start discussing the ABI. Upon disagreement on answering
> > a specific requirement, two questions will arise:
> > 
> > 1. How much trouble it really is to take care of this requirement. If the answer
> >    is "not much", then we simply take care of it.
> > 2. If it really is a big deal to take care of a requirement at the ABI level,
> >    then we will have to discuss the use-cases.
> > 
> > Once we are on the same page with respect to these requirements, we can come up
> > with an ABI proposal for:
> > 
> > - Tracing control
> > - Trace format
> > 
> > 
> > B) Internal Instrumentation API
> > 
> > I propose to standardize the instrumentation mechanisms (Tracepoints, Function
> > Tracer, Dynamic Probes, System Call Tracing, etc), so they can be used by
> > Ftrace, Perf, and by the new ABI without requiring to build all three tracer
> > ABI code-bases in the kernel. This involves modularizing the instrumentation
> > sources, taking the following aspects into account:
> > 
> > - They should be stand-alone objects, which can be built without a full tracer
> >   enabled.
> > - They should offer a "registration/unregistration" API to tracers, so tracers
> >   can register a callback along with a private data pointer (this would fit
> >   with the "multiple concurrent tracing session" requirement).
> > - They should call these callbacks passing the private data pointer when the
> >   instrumentation is hit.
> > - They should provide a mechanism to list the available instrumentation (when it
> >   makes sense) and active instrumentation. E.g., it makes sense for tracepoints
> >   to list the available tracepoints, but it only makes sense for dynamic probes
> >   to list the enabled probes.
> > 
> > Masami Hiramatsu and Frederic Weisbecker already showed interest in undertaking
> > this task.
> 
> Actually, I didn't talked about what API should be provided internally.
> (Yeah, I know LTTng handler want that. However, there is no "external" handler
>  _inside_ linux kernel tree)

My target here is not LTTng. My goal is to get the ball rolling for the improved
ABI. If we make sure all instrumentation sources provide a clean API to Ftrace,
Perf, and eventually the new ABI, then it makes it easier to transition from one
ABI to another; we would not have to change the "whole world", but rather just
to switch to the new ABI when it is deemed ready.

> Instead, I and Frederic talked shortly about something like user interface
> for events. (so it would be more close to A, about controlling)

Yep, this too makes sense.

> As Thomas said, eventually kernel internal tracer should simply provide
> "events tracing" functionality. User tools will analyze it and it's not
> kernel's business. I agree with his opinion.

Right.

> From above viewpoint, currently only trace-events(tracepoint-based events)
> and dynamic-events (kprobe-based events) are providing same interface for
> users. And, for example, perf's PMU events or ftrace's mcount events aren't
> shown up under debugfs/tracing/events. IMHO, all events provided by kernel
> should be there, so that user tools can read the format and control those
> events same way.

We should decide if we keep this stuff under /debugfs or move it elsewhere. This
is part of the ABI after all. Independently of where this ends up, the
operations we need to perform are:

- For each instrumentation source (tracepoints, function tracing, dynamic
  probes, PMC, ...)
  - List available instrumentation
    - Makes sense for tracepoints and PMC, but function graph tracer and dynamic
      probes might skip this part.
  - List activated instrumentation
  - Control interface
    - Activate/deactivate instrumentation, on a per trace session basis
      - Note: the concept of "trace session" is currently inexisting in both
        perf and ftrace. We'll have to think of something in terms of ABI here.
      - Note2: each instrumentation source will expects its own sets of
        parameters to specify the instrumentation to control.
      - Note3: Handling of instrumentation in dynamically loadable modules
        (which applies also to dynamic probes) might require that we allow the
        control interface to activate a tracepoint or dynamic probe for a trace
        session (e.g. by name) before the instrumentation point is listed as
        available instrumentation. The goal is to deal with modules dynamically
        loaded and dynamic instrumentation dynamically added while the trace is
        being recorded; without requiring any user knowledge about
        module-specific parameters whatsoever.

> For this purpose, I'd like to expand trace-event/dynamic-event framework to
> those events. It seems that some PMU events can be treated as trace-events,
> mcount and other parametric events can be treated as dynamic-events.
> 
> Anyway, those stuffs can be done without new-ring-buffer-ABI things.
> I'll just expand dyn-events a bit far from here :-)

Steven wanted to clean up his debugfs event description files, so this would fit
well with this effort, and is indeed an ABI change. One way to do it is to keep
the old files around and come up with a new hierarchy for the "cleaned up"
files, along with the new features you want to add.

Also, we might want to consider moving the debugfs event description files to a
slightly different format (see my metadata proposal). It expands a bit on the
current information, and allows us to deal with bitfields much more elegantly.
However this is also an ABI change.

Thanks,

Mathieu

> 
> Best Regards,
> 
> -- 
> Masami HIRAMATSU
> 2nd Dept. Linux Technology Center
> Hitachi, Ltd., Systems Development Laboratory
> E-mail: masami.hiramatsu.pt@xxxxxxxxxxx

-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com
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