Re: [PATCH] perf tools: Move precise_ip detection into perf_evsel__open

Arnaldo Carvalho de Melo
2019-03-15T14:35:14Z

Em Fri, Mar 15, 2019 at 01:15:46PM +0100, Jiri Olsa escreveu:
> On Thu, Mar 14, 2019 at 08:49:11AM -0700, Andi Kleen wrote:
> > > > Still seems like a hack. Even though I don't know of a case that
> > > > would break now. But it would be better to have the precise probing
> > > > in the open retry loop, instead of trying to reinvent it here.
> > >
> > > how about something like below?
> >
> > Yes looks reasonable.
>
> full patch attached,
>
> jirka
>
>
> ---
> After discussion with Andi, moving the precise_ip detection
> for maximum precise config (via :P modifier or for default
> cycles event) into perf_evsel__open code.
>
> The current detection code in perf_event_attr__set_max_precise_ip
> function is tricky, because precise_ip config is specific for
> given event and it currently checks only hw cycles.
>
> We now check for valid precise_ip value right after failing
> sys_perf_event_open for specific event, before any of the
> perf_event_attr fallback code gets executed. This way we get
> the proper config in perf_event_attr together with allowed
> precise_ip settings.
>
> We can see that code activity with -vv, like:
>
> $ perf record -vv ls
> ...
> ------------------------------------------------------------
> perf_event_attr:
> size 112
> { sample_period, sample_freq } 4000
> ...
> precise_ip 3
> sample_id_all 1
> exclude_guest 1
> mmap2 1
> comm_exec 1
> ksymbol 1
> ------------------------------------------------------------
> sys_perf_event_open: pid 9926 cpu 0 group_fd -1 flags 0x8
> sys_perf_event_open failed, error -95
> decreasing precise_ip by one (2)
> ------------------------------------------------------------
> perf_event_attr:
> size 112
> { sample_period, sample_freq } 4000
> ...
> precise_ip 2
> sample_id_all 1
> exclude_guest 1
> mmap2 1
> comm_exec 1
> ksymbol 1
> ------------------------------------------------------------
> sys_perf_event_open: pid 9926 cpu 0 group_fd -1 flags 0x8 = 4
> ...
>
> Suggested-by: Andi Kleen <ak@linux.intel.com>
> Link: http://lkml.kernel.org/n/tip-dkvxxbeg7lu74155d4jhlmc9@git.kernel.org
> Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> ---
> tools/perf/util/evlist.c | 29 ----------------
> tools/perf/util/evlist.h | 2 --
> tools/perf/util/evsel.c | 72 ++++++++++++++++++++++++++++++++--------
> 3 files changed, 59 insertions(+), 44 deletions(-)
>
> diff --git a/tools/perf/util/evlist.c b/tools/perf/util/evlist.c
> index ed20f4379956..3a243c249c29 100644
> --- a/tools/perf/util/evlist.c
> +++ b/tools/perf/util/evlist.c
> @@ -230,35 +230,6 @@ void perf_evlist__set_leader(struct perf_evlist *evlist)
> }
> }
>
> -void perf_event_attr__set_max_precise_ip(struct perf_event_attr *pattr)
> -{
> - struct perf_event_attr attr = {
> - .type = PERF_TYPE_HARDWARE,
> - .config = PERF_COUNT_HW_CPU_CYCLES,
> - .exclude_kernel = 1,
> - .precise_ip = 3,
> - };
> -
> - event_attr_init(&attr);
> -
> - /*
> - * Unnamed union member, not supported as struct member named
> - * initializer in older compilers such as gcc 4.4.7
> - */
> - attr.sample_period = 1;
> -
> - while (attr.precise_ip != 0) {
> - int fd = sys_perf_event_open(&attr, 0, -1, -1, 0);
> - if (fd != -1) {
> - close(fd);
> - break;
> - }
> - --attr.precise_ip;
> - }
> -
> - pattr->precise_ip = attr.precise_ip;
> -}
> -
> int __perf_evlist__add_default(struct perf_evlist *evlist, bool precise)
> {
> struct perf_evsel *evsel = perf_evsel__new_cycles(precise);
> diff --git a/tools/perf/util/evlist.h b/tools/perf/util/evlist.h
> index 744906dd4887..cb15e3366eba 100644
> --- a/tools/perf/util/evlist.h
> +++ b/tools/perf/util/evlist.h
> @@ -303,8 +303,6 @@ void perf_evlist__to_front(struct perf_evlist *evlist,
> void perf_evlist__set_tracking_event(struct perf_evlist *evlist,
> struct perf_evsel *tracking_evsel);
>
> -void perf_event_attr__set_max_precise_ip(struct perf_event_attr *attr);
> -
> struct perf_evsel *
> perf_evlist__find_evsel_by_str(struct perf_evlist *evlist, const char *str);
>
> diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
> index 3bbf73e979c0..f606e8a81f0e 100644
> --- a/tools/perf/util/evsel.c
> +++ b/tools/perf/util/evsel.c
> @@ -295,7 +295,6 @@ struct perf_evsel *perf_evsel__new_cycles(bool precise)
> if (!precise)
> goto new_event;
>
> - perf_event_attr__set_max_precise_ip(&attr);
> /*
> * Now let the usual logic to set up the perf_event_attr defaults
> * to kick in when we return and before perf_evsel__open() is called.
> @@ -305,6 +304,8 @@ struct perf_evsel *perf_evsel__new_cycles(bool precise)
> if (evsel == NULL)
> goto out;
>
> + evsel->precise_max = true;
> +
> /* use asprintf() because free(evsel) assumes name is allocated */
> if (asprintf(&evsel->name, "cycles%s%s%.*s",
> (attr.precise_ip || attr.exclude_kernel) ? ":" : "",
> @@ -1083,7 +1084,7 @@ void perf_evsel__config(struct perf_evsel *evsel, struct record_opts *opts,
> }
>
> if (evsel->precise_max)
> - perf_event_attr__set_max_precise_ip(attr);
> + attr->precise_ip = 3;
>
> if (opts->all_user) {
> attr->exclude_kernel = 1;
> @@ -1749,6 +1750,59 @@ static bool ignore_missing_thread(struct perf_evsel *evsel,
> return true;
> }
>
> +static void display_attr(struct perf_event_attr *attr)
> +{
> + if (verbose >= 2) {
> + fprintf(stderr, "%.60s\n", graph_dotted_line);
> + fprintf(stderr, "perf_event_attr:\n");
> + perf_event_attr__fprintf(stderr, attr, __open_attr__fprintf, NULL);
> + fprintf(stderr, "%.60s\n", graph_dotted_line);
> + }
> +}
> +
> +static int perf_event_open(struct perf_evsel *evsel,
> + pid_t pid, int cpu, int group_fd,
> + unsigned long flags)

The patch is ok, but I think the naming of this function is too generic,
so I'm renaming it to:

static int perf_evsel__open_adjust_precise_ip(struct perf_evsel *evsel,
pid_t pid, int cpu, int group_fd,
unsigned long flags)

Ok?

The perf_evsel__open() code is already complex with that fallback
mechanism, this is just one more way of fallbacking when asking the
kernel for something that may fail.

In fact what happens if the precise_ip that is being asked _is_
supported but sys_perf_event_open() fails because some other
perf_event_attr attribute that is set is not supported?

I see, it gets it back restored to what the user asked so that the
standard fallback is tried, ok, I'll apply with just the rename for this
function,

Thanks,

- Arnaldo

> +{
> + int precise_ip = evsel->attr.precise_ip;
> + int fd;
> +
> + while (1) {
> + pr_debug2("sys_perf_event_open: pid %d cpu %d group_fd %d flags %#lx",
> + pid, cpu, group_fd, flags);
> +
> + fd = sys_perf_event_open(&evsel->attr, pid, cpu, group_fd, flags);
> + if (fd >= 0)
> + break;
> +
> + /*
> + * Do quick precise_ip fallback if:
> + * - there is precise_ip set in perf_event_attr
> + * - maximum precise is requested
> + * - sys_perf_event_open failed with ENOTSUP error,
> + * which is associated with wrong precise_ip
> + */
> + if (!precise_ip || !evsel->precise_max || (errno != ENOTSUP))
> + break;
> +
> + /*
> + * We tried all the precise_ip values, and it's
> + * still failing, so leave it to standard fallback.
> + */
> + if (!evsel->attr.precise_ip) {
> + evsel->attr.precise_ip = precise_ip;
> + break;
> + }
> +
> + pr_debug2("\nsys_perf_event_open failed, error %d\n", -ENOTSUP);
> + evsel->attr.precise_ip--;
> + pr_debug2("decreasing precise_ip by one (%d)\n", evsel->attr.precise_ip);
> + display_attr(&evsel->attr);
> + }
> +
> + return fd;
> +}
> +
> int perf_evsel__open(struct perf_evsel *evsel, struct cpu_map *cpus,
> struct thread_map *threads)
> {
> @@ -1824,12 +1878,7 @@ int perf_evsel__open(struct perf_evsel *evsel, struct cpu_map *cpus,
> if (perf_missing_features.sample_id_all)
> evsel->attr.sample_id_all = 0;
>
> - if (verbose >= 2) {
> - fprintf(stderr, "%.60s\n", graph_dotted_line);
> - fprintf(stderr, "perf_event_attr:\n");
> - perf_event_attr__fprintf(stderr, &evsel->attr, __open_attr__fprintf, NULL);
> - fprintf(stderr, "%.60s\n", graph_dotted_line);
> - }
> + display_attr(&evsel->attr);
>
> for (cpu = 0; cpu < cpus->nr; cpu++) {
>
> @@ -1841,13 +1890,10 @@ int perf_evsel__open(struct perf_evsel *evsel, struct cpu_map *cpus,
>
> group_fd = get_group_fd(evsel, cpu, thread);
> retry_open:
> - pr_debug2("sys_perf_event_open: pid %d cpu %d group_fd %d flags %#lx",
> - pid, cpus->map[cpu], group_fd, flags);
> -
> test_attr__ready();
>
> - fd = sys_perf_event_open(&evsel->attr, pid, cpus->map[cpu],
> - group_fd, flags);
> + fd = perf_event_open(evsel, pid, cpus->map[cpu],
> + group_fd, flags);
>
> FD(evsel, cpu, thread) = fd;
>
> --
> 2.17.2

--

- Arnaldo