linux-kernel - Re: [PATCH 3/4] tracing: Add action comparisons when testing matching hist triggers

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Date:   Fri, 6 Apr 2018 10:53:09 +0900
From:   Masami Hiramatsu <mhiramat@...nel.org>
To:     Tom Zanussi <tom.zanussi@...ux.intel.com>
Cc:     rostedt@...dmis.org, tglx@...utronix.de, namhyung@...nel.org,
        vedang.patel@...el.com, bigeasy@...utronix.de,
        joel.opensrc@...il.com, joelaf@...gle.com,
        mathieu.desnoyers@...icios.com, baohong.liu@...el.com,
        rajvi.jingar@...el.com, julia@...com, fengguang.wu@...el.com,
        linux-kernel@...r.kernel.org, linux-rt-users@...r.kernel.org
Subject: Re: [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 <tom.zanussi@...ux.intel.com> 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@...ed:sched_wakeup
  Command: keys=pid:ts0=common_timestamp.usecs if comm=="cyclictest"

This can be used from probe events too :) 
e.g.

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

Any thought?

Thank you,



-- 
Masami Hiramatsu <mhiramat@...nel.org>