linux-kernel - Re: [PATCH v5 24/37] tracing: Add support for 'synthetic' events

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Date:   Tue, 14 Nov 2017 13:24:56 -0600
From:   Tom Zanussi <tom.zanussi@...ux.intel.com>
To:     Namhyung Kim <namhyung@...nel.org>
Cc:     rostedt@...dmis.org, tglx@...utronix.de, mhiramat@...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, linux-kernel@...r.kernel.org,
        linux-rt-users@...r.kernel.org, kernel-team@....com
Subject: Re: [PATCH v5 24/37] tracing: Add support for 'synthetic' events

Hi Namhyung,

On Tue, 2017-11-14 at 10:21 +0900, Namhyung Kim wrote:
> Hi Tom,
> 
> On Thu, Nov 09, 2017 at 02:33:55PM -0600, Tom Zanussi wrote:
> > Synthetic events are user-defined events generated from hist trigger
> > variables saved from one or more other events.
> > 
> > To define a synthetic event, the user writes a simple specification
> > consisting of the name of the new event along with one or more
> > variables and their type(s), to the tracing/synthetic_events file.
> > 
> > For instance, the following creates a new event named 'wakeup_latency'
> > with 3 fields: lat, pid, and prio:
> > 
> >     # echo 'wakeup_latency u64 lat; pid_t pid; int prio' >> \
> >       /sys/kernel/debug/tracing/synthetic_events
> > 
> > Reading the tracing/synthetic_events file lists all the
> > currently-defined synthetic events, in this case the event we defined
> > above:
> > 
> >     # cat /sys/kernel/debug/tracing/synthetic_events
> >     wakeup_latency u64 lat; pid_t pid; int prio
> > 
> > At this point, the synthetic event is ready to use, and a histogram
> > can be defined using it:
> > 
> >     # echo 'hist:keys=pid,prio,lat.log2:sort=pid,lat' >> \
> >     /sys/kernel/debug/tracing/events/synthetic/wakeup_latency/trigger
> > 
> > The new event is created under the tracing/events/synthetic/ directory
> > and looks and behaves just like any other event:
> > 
> >     # ls /sys/kernel/debug/tracing/events/synthetic/wakeup_latency
> >       enable  filter  format  hist  id  trigger
> > 
> > Although a histogram can be defined for it, nothing will happen until
> > an action tracing that event via the trace_synth() function occurs.
> > The trace_synth() function is very similar to all the other trace_*
> > invocations spread throughout the kernel, except in this case the
> > trace_ function and its corresponding tracepoint isn't statically
> > generated but defined by the user at run-time.
> > 
> > How this can be automatically hooked up via a hist trigger 'action' is
> > discussed in a subsequent patch.
> > 
> > Signed-off-by: Tom Zanussi <tom.zanussi@...ux.intel.com>
> > ---
> 
> [SNIP]
> > +static const char *synth_field_fmt(char *type)
> > +{
> > +	const char *fmt = "%llu";
> > +
> > +	if (strcmp(type, "s64") == 0)
> > +		fmt = "%lld";
> > +	else if (strcmp(type, "u64") == 0)
> > +		fmt = "%llu";
> > +	else if (strcmp(type, "s32") == 0)
> > +		fmt = "%d";
> > +	else if (strcmp(type, "u32") == 0)
> > +		fmt = "%u";
> > +	else if (strcmp(type, "s16") == 0)
> > +		fmt = "%d";
> > +	else if (strcmp(type, "u16") == 0)
> > +		fmt = "%u";
> > +	else if (strcmp(type, "s8") == 0)
> > +		fmt = "%d";
> > +	else if (strcmp(type, "u8") == 0)
> > +		fmt = "%u";
> > +	else if (strcmp(type, "char") == 0)
> > +		fmt = "%d";
> > +	else if (strcmp(type, "unsigned char") == 0)
> > +		fmt = "%u";
> > +	else if (strcmp(type, "int") == 0)
> > +		fmt = "%d";
> > +	else if (strcmp(type, "unsigned int") == 0)
> > +		fmt = "%u";
> > +	else if (strcmp(type, "long") == 0)
> > +		fmt = "%ld";
> > +	else if (strcmp(type, "unsigned long") == 0)
> > +		fmt = "%lu";
> > +	else if (strcmp(type, "pid_t") == 0)
> > +		fmt = "%d";
> > +	else if (strstr(type, "[") == 0)
> > +		fmt = "%s";
> 
> Is it for string?  You may want to check it with '!='..
> 

It is for a string, will change (and to strchr).

Thanks,

Tom

> Thanks,
> Namhyung
> 
> 
> > +
> > +	return fmt;
> > +}