ftrace histogram sorting broken on BE architecures

[Sven Schnelle <svens@stackframe.org>]

Hi List,

i was looking into a ftracetest failure on s390:

# ./ftracetest test.d/trigger/trigger-hist.tc
=== Ftrace unit tests ===
[1] event trigger - test histogram trigger	[FAIL]
[2] (instance)  event trigger - test histogram trigger	[FAIL]

from the -vvv log: ++ fail 'sort param on sched_process_fork did not work'

# cat events/sched/sched_process_fork/hist

# event histogram
#
# trigger info: hist:keys=parent_pid,child_pid:vals=hitcount:sort=child_pid:size=2048 [active]
#

{ parent_pid:       1406, child_pid:       1428 } hitcount:          1
{ parent_pid:       1406, child_pid:       1430 } hitcount:          1
{ parent_pid:       1406, child_pid:       1427 } hitcount:          1
{ parent_pid:       1406, child_pid:       1432 } hitcount:          1
{ parent_pid:       1406, child_pid:       1431 } hitcount:          1
{ parent_pid:       1406, child_pid:       1429 } hitcount:          1

So the test is right, the entries are not sorted. After digging into the
ftrace code i noticed that integer values always get extended to 64 bit
in event_hist_trigger(), but cmp_entries_key() from tracing_map.c uses the
type of the field (which is a pid_t, and therefore 4 bytes).

On Little Endian this doesn't hurt, but on BE s390 this makes the compare
function compare 4 zero bytes, which is the reason why sorting doesn't
work. As a test i forced the compare function used in cmp_entries_key() to
tracing_map_cmp_s64(), which made the ftrace tests pass.

I also tested this on 64 bit parisc with the same results, so the architecture
doesn't seem make a difference (besides LE vs. BE)

Any thoughts on how to fix this? I'm not sure whether i fully understand the
ftrace maps... ;-)

Best,
Sven