Re: perf/tracepoints access to interpreted strings

Arnaldo Carvalho de Melo
2015-04-15T18:09:41Z

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

- Arnaldo