move all projects into the trunk directory

[lttv.git] / ltt / branches / poly / doc / developer / lttng-userspace-tracing-lite.txt

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-
-
-Mathieu Desnoyers
-
-March 2006
-
-Here is a simpler, but slower, version of user space tracing design.
-
-It will be useful on architectures where doing a system call is mandatory, for
-example when CPUs are lacking a 64 bits TSC : it must be worked around by doing
-a system call to the the time or the synthetic MSB of the TSC (yes, disabling
-preemption is at least required).
-
-
-So the design is simple :
-
-Two system calls :
-
-One system call for event logging
-
-
-ltt_trace_generic
-args :
-fac_id
-event_id
-data pointer
-data size
-
-ltt_register_generic
-args:
-struct pointer (in)
-fac_id pointer (out)
-
-#define NAME_MAX 4096
-
-struct :
-char name[NAME_MAX]
-u32 num_events
-u32 checksum
-u32 alignment
-u32 int_size
-u32 long_size
-u32 pointer_size
-u32 size_t_size
-
-
-If a facility is registered twice, the same handle is used.
-       For a facility to be the exact same, it must share _every_ element of the
-       struct (even type sizes).
-Potential problem : If a facility is registered, it is never unregistered.
-
-Now.. not being able to free facilities when they are not used is not fun. So
-how can we know every process that has registered a facility have finished using
-it ? If we know that, we might do a cleanup when _all_ the traces are destroyed:
-this is way better than a reboot.
-
-A solution might be to keep a reference count to the fac_id : it would be
-incremented upon registration and decremented when a process exits. To do that,
-a process must keep an array of fac ids it uses. 0 is unset.
-
-CONFIG option :
-
-CONFIG_LTT_USERSPACE_GENERIC
-
-
-
-
-
-