Re: perf/tracepoints access to interpreted strings

Steven Rostedt
2015-04-20T20:46:20Z

On Wed, 15 Apr 2015 15:09:27 -0300
Arnaldo Carvalho de Melo <acme@kernel.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