| 1 | |
| 2 | |
| 3 | Mathieu Desnoyers |
| 4 | |
| 5 | March 2006 |
| 6 | |
| 7 | Here is a simpler, but slower, version of user space tracing design. |
| 8 | |
| 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). |
| 13 | |
| 14 | |
| 15 | So the design is simple : |
| 16 | |
| 17 | Two system calls : |
| 18 | |
| 19 | One system call for event logging |
| 20 | |
| 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 |
| 64 | |
| 65 | |
| 66 | |
| 67 | |
| 68 | |
| 69 | |