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    SubjectRe: [RFC] convert ftrace syscall tracer to TRACE_EVENT()
    2009/5/9 Ingo Molnar <>:
    > * Jason Baron <> wrote:
    >> Hi,
    >> I've been thinking about converting the current ftrace syscall
    >> tracer to the TRACE_EVENT() macros. There are a few issues with
    >> the current syscall tracer approach:
    >> 1) It has to be enabled for all processes and all syscalls. By
    >> moving to TRACE_EVENT(), it can be enabled/disabled per tracepoint
    >> and can also make use of the generic tracing filters, such as
    >> "trace all process for pid x"
    >> 2) Other tracers can not tie into it, since its not tracepoint
    >> based. TRACE_EVENT() fixes this.
    >> 3) data formatting. The syscall tracer I don't believe understands
    >> all the various types for output formatting. By moving to
    >> TRACE_EVENT(), we can print out a more readible syscall trace.
    >> 4) The ftrace syscall tracer needs a new arch specific code for
    >> each architecture. By converting to TRACE_EVENT() we don't need
    >> any architecutre specific code.
    >> Other issues to consider:
    >> * Maintainence. The current syscall tracer automatically picks up
    >> new syscalls. The TRACE_EVENT() will be harder to initially set
    >> up. But once its done, syscalls are obviously not added often. So
    >> I don't think this will be too bad.
    >> * Performance. The current syscall tracer adds a
    >> 'test_thread_flag()' to syscall entry/exit. The TRACE_EVENT()
    >> would add a per-syscall global to check. So they are going to have
    >> different cache profiles...however, the tracepoint infrastructure
    >> is hopefully moving to the 'immediate' value work, which will make
    >> this more highly optimized.
    >> I've also tested the patch shown below (which uses,
    >> DECLARE_TRACE(), as a preliminary proof of concept), using
    >> getpid() in a loop, and tbench, and saw very small performance
    >> differences. Obviously we would have to do more extensive testing
    >> before deciding.
    >> Patch is pretty rough, but should give a rough sense of what the
    >> DECLARE_TRACE() type patch might look like...
    > Yeah, i very much agree with the direction. (I've Cc:-ed Tom Zanussi
    > who also has expressed interest in this.)
    > I'm not sure about the implementation as you've posted it though:
    > Firstly, it adds two new tracepoints to every system call. That is
    > unnecessary - we already have the TIF flag based callbacks, and we
    > can use the existing syscall attributes table to get to tracepoints
    > - without slow down (or impacting) the fast path in any way.

    Agreed, that's unnecessary because we already hook in ptrace without
    impacting the off-case.

    > Secondly, we should reuse the information we get in SYSCALL_DEFINE,
    > to construct the TRACE_EVENT tracepoints directly - without having
    > to list all syscalls again in a separate file.

    Indeed, that's not trivial though, but feasible.
    I'm not sure we can reuse the TRACE_EVENT macro directly inside SYSCALL_DEFINE.
    The resulting macro tempest effect that would occur confuses me and I
    have troubles to imagine the result.

     \ /
      Last update: 2009-05-09 14:55    [W:0.050 / U:10.508 seconds]
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