libtracefs: Allow for the same event to be start and end in tracefs_sql()
When creating a synthetic event that is based off of the same event with
two different fields, it should still work. But the assumption was that
every field that has a label will have the same label, which is not
true.
For example, to create an event that measures the time a task is
blocked, it needs to start and end on the same event (sched_switch), but
just use different fields.
sqlhist -n blocked 'SELECT end.next_pid AS pid,end.next_comm AS comm,(end.TIMESTAMP_USECS - start.TIMESTAMP_USECS) AS lat
FROM sched_switch AS start JOIN sched_switch AS end ON start.prev_pid = end.next_pid WHERE start.prev_state = 2'
But this gave the following error:
Failed creating synthetic event!: Success
SELECT end.next_pid AS pid,end.next_comm AS comm,(end.TIMESTAMP_USECS - start.TIMESTAMP_USECS) AS lat FROM sched_switch AS start JOIN sched_switch AS end ON start.prev_pid = end.next_pid WHERE start.prev_state = 2
^
ERROR: 'end.next_pid'
'start.prev_pid' and 'end.next_pid' must be a field for each event: 'sched_switch' and 'sched_switch'
Because the end sched_switch would already have the label "start" and it
would not create one for "end" making the "end" not resolve correctly.
Now it gives the correct answer:
echo 's:blocked pid_t next_pid; char next_comm[16]; u64 lat;' >> /sys/kernel/tracing/dynamic_events
echo 'hist:keys=prev_pid:__arg_21065_1=next_pid,__arg_21065_2=next_comm,__arg_21065_3=common_timestamp.usecs if prev_state == 2' >> /sys/kernel/tracing/events/sched/sched_switch/trigger
echo 'hist:keys=next_pid:next_pid=$__arg_21065_1,next_comm=$__arg_21065_2,lat=common_timestamp.usecs-$__arg_21065_3:onmatch(sched.sched_switch).trace(blocked,$next_pid,$next_comm,$lat)' >> /sys/kernel/tracing/events/sched/sched_switch/trigger
Link: https://lore.kernel.org/linux-trace-devel/20220531172435.2a51eff3@rorschach.local.home
Fixes: 25446407 ("libtracefs: Added new API tracefs_sql()")
Tested-by: Harald Seiler <hws@denx.de>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
1 file changed
