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SubjectRe: [PATCH 3/4] tracing: Add action comparisons when testing matching hist triggers
Hi Tom,

On Thu, 05 Apr 2018 18:34:13 -0500
Tom Zanussi <> wrote:

> Hi Masami,
> On Thu, 2018-04-05 at 12:50 +0900, Masami Hiramatsu wrote:
> [...]
> > Can you print out the error with which event we should see? e.g.
> >
> > ERROR: Variable already defined at sched_wakeup: ts0
> >
> How about printing the event name along with the last command, for any
> error? :
> ERROR: Variable already defined: ts0
> Last command: [sched:sched_wakeup] keys=pid:ts0=common_timestamp.usecs if comm=="cyclictest"

Hmm, is the Last command shows the last command on sched_wakeup ? or sched_switch??

> Before:
> # echo 'hist:keys=pid:ts0=common_timestamp.usecs if comm=="cyclictest"' >> /sys/kernel/debug/tracing/events/sched/sched_wakeup/trigger
> # echo '!hist:keys=next_pid:ts0=common_timestamp.usecs if comm=="cyclictest"' >> /sys/kernel/debug/tracing/events/sched/sched_switch/trigger
> And other commands making us think we cleared everything out so the
> below error is a surprise
> # echo 'hist:keys=pid:ts0=common_timestamp.usecs if comm=="cyclictest"' >> /sys/kernel/debug/tracing/events/sched/sched_wakeup/trigger
> -su: echo: write error: Invalid argument

No, my senario is different.

Your senario tries
1) define ts0 on sched_wakeup
2) remove ts0 from sched_switch (but silently failed)
3) re-define ts0 on sched_wakeup and get an error

In this case, user can dump sched_wakeup/trigger and see there is already ts0 defined.

My senario is a bit different
1) define ts0 on sched_wakeup
2) remove ts0 from sched_switch (but silently failed)
3) re-define ts0 on *sched_switch* and get an error

The 3rd operation failed on "sched_switch" not on "sched_wakeup". In this case we will totally lost where the ts0 defined.
That's why I have asked you to show "where the ts0 is defined" at error line.

Anyway, I think it is a good chance to introduce <tracefs>/error_log file, since we have too many non-critical errors on operations. I feel that checking hist file by errors on trigger file is not a bit intuitive.

# cat /sys/kernel/debug/tracing/error_log
ERROR(events/sched/sched_switch/trigger): Variable already defined: ts0@sched:sched_wakeup
Command: keys=pid:ts0=common_timestamp.usecs if comm=="cyclictest"

This can be used from probe events too :)

ERROR(kprobe_events): Unsupported type: uint8
Command: p vfs_read arg1=%di:uint8

Any thought?

Thank you,

Masami Hiramatsu <>

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