NPTL Trace Tool

Adding trace points to NPTL functions

PTT must run inside the NPTL, that is to say, PTT used an instrumented NPTL, generated from the original one by adding trace points in strategic places.

A trace point is made of one or several events. Gathering events allows to reduce the number of calls to the trace mechanim, and so not to interrupt the traced program too much.

Here is an example, written in pseudo-code, of the pthread_barrier_wait NPTL routine with associated trace points:

PTHREAD_BARRIER_WAIT (pthread_barrier_t *barrier) { result = 0 BARRIER_WAIT_IN, BARRIER_LOCK_REQUIRE, THREAD_STATE_WAIT TRACE (TR_BARRIER_8, barrier->lock) wait for barrier to become free and take it THREAD_STATE_WAKE, BARRIER_LOCK_TAKEN TRACE (TR_BARRIER_2, barrier->lock) decrement the number of waited threads BARRIER_LEFT_DEC TRACE (TR_BARRIER_9, barrier->left) if all threads already entered the barrier wake up all sleeping threads result = PTHREAD_BARRIER_SERIAL_THREAD (everything all right) else BARRIER_LOCK_FREE TRACE (TR_BARRIER_3, barrier->lock) free barrier THREAD_STATE_WAIT_BARRIER TRACE (TR_BARRIER_14) put the thread which has just arrived in blocked state THREAD_STATE_WAKE_BARRIER TRACE (TR_BARRIER_15) endif increment left if all threads left the barrier BARRIER_LEFT_INC, BARRIER_LOCK_FREE TRACE (TR_BARRIER_10, barrier->left) free barrier else BARRIER_LEFT_INC TRACE (TR_BARRIER_11, barrier->left) endif BARRIER_WAIT_OUT TRACE (TR_BARRIER_12, result) return result }

We see that some trace points correspond to one event (TR_BARRIER_9 -> BARRIER_LEFT_DEC), and others gather several events (TR_BARRIER_8 -> BARRIER_WAIT_IN, BARRIER_LOCK_REQUIRE, THREAD_STATE_WAIT).

Trace point names are made of 3 parts: the TR prefix, the name of the object (from the routine name) and an ID. We obtain TR_OBJECT_ID. In the pseudo code, TRACE (...) represents the call to the trace function with its arguments. The first argument is the trace point name. Other arguments are values like address or id which describe events with the most useful information.

Trace points list

Here is the list of all trace points defined by PTT with the associated events: