linux-kernel - Re: perf/tracepoints access to interpreted strings

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Date:	Mon, 20 Apr 2015 16:46:16 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Arnaldo Carvalho de Melo <acme@...nel.org>
Cc:	David Ahern <dsahern@...il.com>,
	Namhyung Kim <namhyung@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: perf/tracepoints access to interpreted strings

On Wed, 15 Apr 2015 15:09:27 -0300
Arnaldo Carvalho de Melo <acme@...nel.org> wrote:

> Em Wed, Apr 15, 2015 at 10:20:08AM -0600, David Ahern escreveu:
> > I was hoping you could provide points on how to get access to an interpreted
> > field in a tracepoint within perf.
>  
> > This is an example of the tracepoint:
>  
> > # cat /sys/kernel/debug/tracing/events/irq/softirq_exit/format
> > name: softirq_exit
> > ID: 99
> > format:
> > 	field:unsigned short common_type;	offset:0;	size:2;	signed:0;
> > 	field:unsigned char common_flags;	offset:2;	size:1;	signed:0;
> > 	field:unsigned char common_preempt_count;	offset:3;	size:1;	signed:0;
> > 	field:int common_pid;	offset:4;	size:4;	signed:1;
> > 	field:int common_padding;	offset:8;	size:4;	signed:1;
> > 
> > 	field:unsigned int vec;	offset:12;	size:4;	signed:0;
> > 
> > print fmt: "vec=%u [action=%s]", REC->vec, __print_symbolic(REC->vec, {
> > HI_SOFTIRQ, "HI" }, { TIMER_SOFTIRQ, "TIMER" }, { NET_TX_SOFTIRQ, "NET_TX"
> > }, { NET_RX_SOFTIRQ, "NET_RX" }, { BLOCK_SOFTIRQ, "BLOCK" }, {
> > BLOCK_IOPOLL_SOFTIRQ, "BLOCK_IOPOLL" }, { TASKLET_SOFTIRQ, "TASKLET" }, {
> > SCHED_SOFTIRQ, "SCHED" }, { HRTIMER_SOFTIRQ, "HRTIMER" }, { RCU_SOFTIRQ,
> > "RCU" })
>  
> > I would like to programmatically extract the action string. 'perf script'
> > prints the samples fine which suggests libtraceevent extracts the
> > information somehow.
>  
> > Can you provide a suggestion -- something along the lines of
> > perf_evsel__intval() or perf_evsel__rawptr()?
> 
> We'll gonna have to parse the "print fmt" thing and look for entries
> surrounded by [], then match it with the list of parameters, so that we
> can end up with a:
> 
> 	const char *perf_evsel__enum_entry(struct perf_evsel *evsel,
> 					   struct perf_sample *sample,
> 					   const char *enum_name,
> 					   int value);
> 
> That would return one of "TIMER", "NET_TX", etc, that is, 0,
> 1, N.
> 
> If it is strictly an enum, i.e. no holes and just by looking at the
> "format" file above I don't see how it could have holes, albeit enums
> may have, we can as well have this:
> 
> 	const char *perf_evsel__enum(struct perf_evsel *evsel,
> 				     struct perf_sample *sample,
> 				     const char *enum_name);
> 
> That would return an array of strings that you could directly access,
> indexing using some of the fields.
> 
> I.e. internally we would see the tracepoint format file as:
> 
>  	field:enum action vec;	offset:12;	size:4;	signed:0;
> 
> 	enum: action: TIMER, NET_TX, NET_RX, BLOCK, BLOCK_IOPOLL, TASKLET, SCHED, HRTIMER, RCU
> 

Note, with the new TRACE_DEFINE_ENUM() that was already added to
Linus's tree, that print_fmt now looks like:

print fmt: "vec=%u [action=%s]", REC->vec, __print_symbolic(REC->vec,
 { 0, "HI" }, { 1, "TIMER" }, { 2, "NET_TX" }, { 3, "NET_RX" }, { 4, "BLOCK" },
 { 5, "BLOCK_IOPOLL" }, { 6, "TASKLET" }, { 7, "SCHED" }, { 8, "HRTIMER" },
 { 9, "RCU" })


-- Steve
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