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

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Date:   Tue, 14 Nov 2017 10:21:26 +0900
From:   Namhyung Kim <namhyung@...nel.org>
To:     Tom Zanussi <tom.zanussi@...ux.intel.com>
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 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 '!='..

Thanks,
Namhyung


> +
> +	return fmt;
> +}