linux-kernel - Re: [PATCH v2 10/14] tracing: Document trace

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Date:   Wed, 16 May 2018 09:55:34 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Tom Zanussi <tom.zanussi@...ux.intel.com>
Cc:     linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Clark Williams <williams@...hat.com>,
        Karim Yaghmour <karim.yaghmour@...rsys.com>,
        Brendan Gregg <bgregg@...flix.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        Yann Ylavic <ylavic.dev@...il.com>,
        linux-rt-users@...r.kernel.org
Subject: Re: [PATCH v2 10/14] tracing: Document trace_marker triggers

On Mon, 14 May 2018 16:47:07 -0500
Tom Zanussi <tom.zanussi@...ux.intel.com> wrote:

> Hi Steve,
> 
> This is a nice new event feature - thanks for doing it!  Some minor typo
> comments below..

Can I get an Acked-by from you?



> > +3. User space creating a trigger
> > +--------------------------------
> > +
> > +Writing into /sys/kernel/tracing/trace_marker writes into the ftrace
> > +ring buffer. This can also act like an event, by writing into the trigger
> > +file located in /sys/kernel/tracing/events/ftrace/print/
> > +
> > +Modifying cyclictest to write into the trace_marker file before it sleeps
> > +and after it wakes up, something like this:
> > +
> > +static void traceputs(char *str)
> > +{
> > +	/* tracemark_fd is the trace_marker file descripto */
> >   
> 
> Should be 'descriptor'?

Fixed.



> > + # ./cyclictest -p80 -d0 -i250 -n -a -t --tracemark -b 1000
> > +
> > + -p80  : run threads at priority 80
> > + -d0   : have all threads run at the same interval
> > + -i250 : start the interval at 250 microseconds (all threads will do this)
> > + -n    : sleep with nanosleep
> > + -a    : affine all threads to a separate CPU
> > + -t    : one thread per available CPU
> > + --tracemark : enable trace mark writing
> > + -b 1000 : stop if any latency is greater than 1000 microseconds
> > +
> > +Note, the -b 1000 is used just to make --tracemark available.
> > +
> > +The we can see the histogram created by this with:
> > +  
> 
> 'Then we can..

Fixed


> > +The difference this time is that instead of using the trace_marker to start
> > +the latency, the sched_waking event is used, matching the common_pid for the
> > +trace_marker write with the pid that is being worken by sched_waking.
> >   
> 
> s/worken/woken ?

Fixed. (It's kind of like Borken)

> 
> > 
> > +
> > +After running cyclictest again with the same parameters, we now have:
> > +
> > + # cat events/synthetic/latency/hist
> > +# event histogram
> > +#
> > +# trigger info: hist:keys=lat,common_pid:vals=hitcount:sort=lat:size=2048 [active]
> > +#
> > +
> > +{ lat:          7, common_pid:       2302 } hitcount:        640
> >   
> 
> snip
> 
> > 
> > +{ lat:         61, common_pid:       2302 } hitcount:          1
> > +{ lat:        110, common_pid:       2302 } hitcount:          1
> > +
> > +Totals:
> > +    Hits: 89565
> > +    Entries: 158
> > +    Dropped: 0
> > +
> > +This doesn't tell us any information about how late cyclictest may have
> > +worken up, but it does show us a nice histogram of how long it took from
> > +the time that cyclictest was worken to the time it made it into user space.
> >   
> 
> A couple more that don't seem to be 'worken' ;-)

I have an itchy 'r' finger.

Thanks for the review, I just updated this.

Oh, I guess I think I prefer a "Reviewed-by" tag instead of Ack (gives
a stronger meaning).

-- Steve