| 1 | --- |
| 2 | id: pid-tracking |
| 3 | since: 2.7 |
| 4 | --- |
| 5 | |
| 6 | It's often useful to allow only specific process IDs (PIDs) to emit |
| 7 | enabled events. For example, you may wish to record all the system |
| 8 | calls made by a given process (à la |
| 9 | <a href="http://linux.die.net/man/1/strace" class="ext">strace</a>). |
| 10 | |
| 11 | The `lttng track` and `lttng untrack` commands serve this purpose. Both |
| 12 | commands operate on a whitelist of process IDs. The `track` command |
| 13 | adds entries to this whitelist while the `untrack` command removes |
| 14 | entries. Any process having one of the PIDs in the whitelist is allowed |
| 15 | to emit [enabled](#doc-enabling-disabling-events) LTTng events. |
| 16 | |
| 17 | <div class="tip"> |
| 18 | <p> |
| 19 | <span class="t">Note:</span>The PID tracker tracks the |
| 20 | <em>numeric process IDs</em>. Should a process with a given tracked |
| 21 | ID exit and another process be given this ID, then the latter would |
| 22 | also be allowed to emit events. |
| 23 | </p> |
| 24 | </div> |
| 25 | |
| 26 | For the sake of the following examples, assume the target system has 16 |
| 27 | possible PIDs. When a [tracing session](#doc-creating-destroying-tracing-sessions) |
| 28 | is created, the whitelist contains all the possible PIDs: |
| 29 | |
| 30 | <figure class="img img-100"> |
| 31 | <img src="/images/docs27/track-all.png" alt="All PIDs are tracked"> |
| 32 | <figcaption>All PIDs are tracked</figcaption> |
| 33 | </figure> |
| 34 | |
| 35 | When the whitelist is full and the `track` command is executed to specify |
| 36 | some PIDs to track, the whitelist is first cleared, then the specific |
| 37 | PIDs are tracked. For example, after |
| 38 | |
| 39 | <pre class="term"> |
| 40 | lttng track --pid 3,4,7,10,13 |
| 41 | </pre> |
| 42 | |
| 43 | the whitelist is: |
| 44 | |
| 45 | <figure class="img img-100"> |
| 46 | <img src="/images/docs27/track-3-4-7-10-13.png" alt="PIDs 3, 4, 7, 10, and 13 are tracked"> |
| 47 | <figcaption>PIDs 3, 4, 7, 10, and 13 are tracked</figcaption> |
| 48 | </figure> |
| 49 | |
| 50 | More PIDs can be added to the whitelist afterwards: |
| 51 | |
| 52 | <pre class="term"> |
| 53 | lttng track --pid 1,15,16 |
| 54 | </pre> |
| 55 | |
| 56 | gives: |
| 57 | |
| 58 | <figure class="img img-100"> |
| 59 | <img src="/images/docs27/track-1-3-4-7-10-13-15-16.png" alt="PIDs 1, 15, and 16 are added to the whitelist"> |
| 60 | <figcaption>PIDs 1, 15, and 16 are added to the whitelist</figcaption> |
| 61 | </figure> |
| 62 | |
| 63 | The `untrack` command removes entries from the PID tracker's whitelist. |
| 64 | Given the last example, the following command: |
| 65 | |
| 66 | <pre class="term"> |
| 67 | lttng untrack --pid 3,7,10,13 |
| 68 | </pre> |
| 69 | |
| 70 | leads to this whitelist: |
| 71 | |
| 72 | <figure class="img img-100"> |
| 73 | <img src="/images/docs27/track-1-4-15-16.png" alt="PIDs 3, 7, 10, and 13 are removed from the whitelist"> |
| 74 | <figcaption>PIDs 3, 7, 10, and 13 are removed from the whitelist</figcaption> |
| 75 | </figure> |
| 76 | |
| 77 | All possible PIDs can be tracked again using the `--all` option of |
| 78 | `lttng track`: |
| 79 | |
| 80 | <pre class="term"> |
| 81 | lttng track --pid --all |
| 82 | </pre> |
| 83 | |
| 84 | gives: |
| 85 | |
| 86 | <figure class="img img-100"> |
| 87 | <img src="/images/docs27/track-all.png" alt="All PIDs are tracked"> |
| 88 | <figcaption>All PIDs are tracked</figcaption> |
| 89 | </figure> |
| 90 | |
| 91 | A very typical use case with PID tracking is starting with an empty |
| 92 | whitelist, then [starting the tracers](#doc-basic-tracing-session-control), |
| 93 | and then adding PIDs manually while tracing is active. This can be |
| 94 | accomplished by using the `--all` option of the `untrack` command |
| 95 | to clear the whitelist after a tracing session is created: |
| 96 | |
| 97 | <pre class="term"> |
| 98 | lttng untrack --pid --all |
| 99 | </pre> |
| 100 | |
| 101 | gives: |
| 102 | |
| 103 | <figure class="img img-100"> |
| 104 | <img src="/images/docs27/untrack-all.png" alt="No PIDs are tracked"> |
| 105 | <figcaption>No PIDs are tracked</figcaption> |
| 106 | </figure> |
| 107 | |
| 108 | Tracing with this whitelist configuration does not produce any event |
| 109 | because no processes are tracked. The `track` command can be used |
| 110 | as usual to track specific PIDs, for example: |
| 111 | |
| 112 | <pre class="term"> |
| 113 | lttng track --pid 6,11 |
| 114 | </pre> |
| 115 | |
| 116 | results in: |
| 117 | |
| 118 | <figure class="img img-100"> |
| 119 | <img src="/images/docs27/track-6-11.png" alt="PIDs 6 and 11 are tracked"> |
| 120 | <figcaption>PIDs 6 and 11 are tracked</figcaption> |
| 121 | </figure> |