doc/developer/lttng-userspace-tracing-lite.txt

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   3 Mathieu Desnoyers
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   5 March 2006
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   7 Here is a simpler, but slower, version of user space tracing design.
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   9 It will be useful on architectures where doing a system call is mandatory, for
  10 example when CPUs are lacking a 64 bits TSC : it must be worked around by doing
  11 a system call to the the time or the synthetic MSB of the TSC (yes, disabling
  12 preemption is at least required).
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  14
  15 So the design is simple :
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  17 Two system calls :
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  19 One system call for event logging
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  21
  22 ltt_trace_generic
  23 args :
  24 fac_id
  25 event_id
  26 data pointer
  27 data size
  28
  29 ltt_register_generic
  30 args:
  31 struct pointer (in)
  32 fac_id pointer (out)
  33
  34 #define NAME_MAX 4096
  35
  36 struct :
  37 char name[NAME_MAX]
  38 u32 num_events
  39 u32 checksum
  40 u32 alignment
  41 u32 int_size
  42 u32 long_size
  43 u32 pointer_size
  44 u32 size_t_size
  45
  46
  47 If a facility is registered twice, the same handle is used.
  48         For a facility to be the exact same, it must share _every_ element of the
  49         struct (even type sizes).
  50 Potential problem : If a facility is registered, it is never unregistered.
  51
  52 Now.. not being able to free facilities when they are not used is not fun. So
  53 how can we know every process that has registered a facility have finished using
  54 it ? If we know that, we might do a cleanup when _all_ the traces are destroyed:
  55 this is way better than a reboot.
  56
  57 A solution might be to keep a reference count to the fac_id : it would be
  58 incremented upon registration and decremented when a process exits. To do that,
  59 a process must keep an array of fac ids it uses. 0 is unset.
  60
  61 CONFIG option :
  62
  63 CONFIG_LTT_USERSPACE_GENERIC
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