[tor-bugs] #32910 [Core Tor/Tor]: trace: Add tracepoints and userspace tracer support

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#32910: trace: Add tracepoints and userspace tracer support
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     Reporter:  dgoulet       |      Owner:  dgoulet
         Type:  enhancement   |     Status:  assigned
     Priority:  Medium        |  Milestone:  Tor: 0.4.4.x-final
    Component:  Core Tor/Tor  |    Version:
     Severity:  Normal        |   Keywords:  tracing
Actual Points:                |  Parent ID:
       Points:  3             |   Reviewer:
      Sponsor:                |
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 Tor-dev email: https://lists.torproject.org/pipermail/tor-
 dev/2019-December/014111.html

 These are what needs to be done but they sorta need to be together to make
 sense thus this one ticket:

 a. Add a series of tracepoints in tor code base. I propose to start with
 circuit and cell level tracepoints for now.

 b. Add USDT (User Statically-Defined Tracing) probes support which is used
 by SystemTap, DTrace and perf.

 c. Add LTTng support which if enable also emits USDT.

 d. Integrate all this to our build system.

 About(d), the consensus among the network team is that it should NEVER be
 enabled in production and should be a configure switch.

 I believe if we add on top a torrc option, it might not be that useful in
 the end considering the configure switch but mainly it will degrade
 performance since the check needs to be at runtime for every tracepoint.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/32910>
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