On x86_64, when gcc instruments functions and compiles at -O0, it saves
the function's return value in register rbx before calling the trace
callback. It provides a nice opportunity to display certain useful
values (flags, booleans etc) during trace sessions. It's absolutely
not guaranteed that it will always work but it provides a considerable
help when it does so it's worth activating it. When building on a
different architecture, the value 0 is always reported as the return
value. On x86_64 with optimizations (-O), the RBX register will not
necessarily match and random values will be reported, but since it's
not the primary target it's not a problem.
Now any call to trace() in the code will automatically appear interleaved
with the call sequence and timestamped in the trace file. They appear with
a '#' on the 3rd argument (caller's pointer) in order to make them easy to
spot. If the trace functionality is not used, a dmumy weak function is used
instead so that it doesn't require to recompile every time traces are
enabled/disabled.
The trace decoder knows how to deal with these messages, detects them and
indents them similarly to the currently traced function. This can be used
to print function arguments for example.
Note that we systematically flush the log when calling trace() to ensure we
never miss important events, so this may impact performance.
The trace() function uses the same format as printf() so it should be easy
to setup during debugging sessions.
This feature relies on GCC's ability to call helpers at function entry/exit
points. We define these helpers to quickly dump the minimum info into a trace
file that can be converted to a human readable format using a script in the
contrib/trace directory. This has only been implemented in the GNU makefile
for now on as it is unsure whether it's supported on all OSes.
The feature is enabled by building with "TRACE=1". The performance impact is
huge, so this feature should only be used when debugging. To limit the loss
of performance, fprintf() has been disabled and the output is hand-crafted
and emitted using fwrite(), resulting in doubling the performance. Using the
TSC instead of gettimeofday() also doubles the performance. Around 1200 conns/s
may be achieved on a Pentium-M 1.7 GHz which leads to around 50 MB/s of traces.
The entry and exits of all functions will be dumped into a file designated
by the HAPROXY_TRACE environment variable, or by default "trace.out". If the
trace file name is empty or "/dev/null", then traces are disabled. If
opening the trace file fails, then stderr is used. If HAPROXY_TRACE_FAST is
used, then the time is taken from the global <now> variable. Last, if
HAPROXY_TRACE_TSC is used, then the machine's TSC is used instead of the
real time (almost twice as fast).
The output format is :
<sec.usec> <level> <caller_ptr> <dir> <callee_ptr>
or :
<tsc> <level> <caller_ptr> <dir> <callee_ptr>
where <dir> is '>' when entering a function and '<' when leaving.
The awk script in contrib/trace provides a nicer indented output :
6f74989e6f8 ->->-> run_poll_loop > signal_process_queue [src/haproxy.c:1097:0x804bd69] > [include/proto/signal.h:32:0x8049cd0]
6f74989eb00 run_poll_loop < signal_process_queue [src/haproxy.c:1097:0x804bd69] < [include/proto/signal.h:32:0x8049cd0]
6f74989ef44 ->->-> run_poll_loop > wake_expired_tasks [src/haproxy.c:1100:0x804bd72] > [src/task.c:123:0x8055060]
6f74989f3a6 ->->->-> wake_expired_tasks > eb32_lookup_ge [src/task.c:128:0x8055091] > [ebtree/eb32tree.c:138:0x80a8c70]
6f74989f7e9 wake_expired_tasks < eb32_lookup_ge [src/task.c:128:0x8055091] < [ebtree/eb32tree.c:138:0x80a8c70]
6f74989fc0d ->->->-> wake_expired_tasks > eb32_first [src/task.c:134:0x80550d5] > [ebtree/eb32tree.h:55:0x8054ad0]
6f7498a003d ->->->->-> eb32_first > eb_first [ebtree/eb32tree.h:56:0x8054af1] > [ebtree/ebtree.h:520:0x8054a10]
6f7498a0436 ->->->->->-> eb_first > eb_walk_down [ebtree/ebtree.h:521:0x8054a33] > [ebtree/ebtree.h:442:0x80549a0]
6f7498a0843 ->->->->->->-> eb_walk_down > eb_gettag [ebtree/ebtree.h:445:0x80549d6] > [ebtree/ebtree.h:418:0x80548e0]
6f7498a0c2b eb_walk_down < eb_gettag [ebtree/ebtree.h:445:0x80549d6] < [ebtree/ebtree.h:418:0x80548e0]
6f7498a1042 ->->->->->->-> eb_walk_down > eb_untag [ebtree/ebtree.h:447:0x80549e2] > [ebtree/ebtree.h:412:0x80548a0]
6f7498a1498 eb_walk_down < eb_untag [ebtree/ebtree.h:447:0x80549e2] < [ebtree/ebtree.h:412:0x80548a0]
6f7498a18c6 ->->->->->->-> eb_walk_down > eb_root_to_node [ebtree/ebtree.h:448:0x80549e7] > [ebtree/ebtree.h:432:0x8054960]
6f7498a1cd4 eb_walk_down < eb_root_to_node [ebtree/ebtree.h:448:0x80549e7] < [ebtree/ebtree.h:432:0x8054960]
6f7498a20c4 eb_first < eb_walk_down [ebtree/ebtree.h:521:0x8054a33] < [ebtree/ebtree.h:442:0x80549a0]
6f7498a24b4 eb32_first < eb_first [ebtree/eb32tree.h:56:0x8054af1] < [ebtree/ebtree.h:520:0x8054a10]
6f7498a289c wake_expired_tasks < eb32_first [src/task.c:134:0x80550d5] < [ebtree/eb32tree.h:55:0x8054ad0]
6f7498a2c8c run_poll_loop < wake_expired_tasks [src/haproxy.c:1100:0x804bd72] < [src/task.c:123:0x8055060]
6f7498a3095 ->->-> run_poll_loop > process_runnable_tasks [src/haproxy.c:1103:0x804bd7a] > [src/task.c:190:0x8055150]
A nice improvement would possibly consist in trying to get the function's
arguments in the stack and to dump a few more infor for some well-known
functions (eg: the session's status for process_session).
Willy Tarreau