Re: [RFC] perf evlist: Warn if event group has mixed sw/hw events

[Stephane Eranian]

On Mon, Oct 26, 2020 at 7:19 AM Namhyung Kim <namhyung@xxxxxxxxxx> wrote:
>
> I found that order of events in a group impacts performance during the
> open.  If a group has a software event as a leader and has other
> hardware events, the lead needs to be moved to a hardware context.
> This includes RCU synchronization which takes about 20 msec on my
> system.  And this is just for a single group, so total time increases
> in proportion to the number of event groups and the number of cpus.
>
> On my 36 cpu system, opening 3 groups system-wide takes more than 2
> seconds.  You can see and compare it easily with the following:
>
>   $ time ./perf stat -a -e '{cs,cycles},{cs,cycles},{cs,cycles}' sleep 1
>   ...
>        1.006333430 seconds time elapsed
>
>   real  0m3.969s
>   user  0m0.089s
>   sys   0m0.074s
>
>   $ time ./perf stat -a -e '{cycles,cs},{cycles,cs},{cycles,cs}' sleep 1
>   ...
>        1.006755292 seconds time elapsed
>
>   real  0m1.144s
>   user  0m0.067s
>   sys   0m0.083s
>
> This patch just added a warning before running it.  I'd really want to
> fix the kernel if possible but don't have a good idea.  Thoughts?
>
This is a problem for us. This has caused problems on our systems with
perf command taking much longer than expected and firing timeouts.

The cost of perf_event_open() should not be so dependent on the order
of the events in a group. The penalty incurred by synchronize_rcu()
is very large and likely does not scale too well. Scalability may not
only be impacted by the number of CPUs of the machine. I am not an
expert
at RCU but it seems it exposes perf_event_open() to penalties caused
by other subsystem operations. I am wondering if there would be a
different way of handling the change of group type that would avoid
the high cost of synchronize_rcu().


> Signed-off-by: Namhyung Kim <namhyung@xxxxxxxxxx>
> ---
>  tools/perf/builtin-record.c |  2 +
>  tools/perf/builtin-stat.c   |  2 +
>  tools/perf/builtin-top.c    |  2 +
>  tools/perf/util/evlist.c    | 78 +++++++++++++++++++++++++++++++++++++
>  tools/perf/util/evlist.h    |  1 +
>  5 files changed, 85 insertions(+)
>
> diff --git a/tools/perf/builtin-record.c b/tools/perf/builtin-record.c
> index adf311d15d3d..c0b08cacbae0 100644
> --- a/tools/perf/builtin-record.c
> +++ b/tools/perf/builtin-record.c
> @@ -912,6 +912,8 @@ static int record__open(struct record *rec)
>
>         perf_evlist__config(evlist, opts, &callchain_param);
>
> +       evlist__warn_mixed_group(evlist);
> +
>         evlist__for_each_entry(evlist, pos) {
>  try_again:
>                 if (evsel__open(pos, pos->core.cpus, pos->core.threads) < 0) {
> diff --git a/tools/perf/builtin-stat.c b/tools/perf/builtin-stat.c
> index b01af171d94f..d5d4e02bda69 100644
> --- a/tools/perf/builtin-stat.c
> +++ b/tools/perf/builtin-stat.c
> @@ -738,6 +738,8 @@ static int __run_perf_stat(int argc, const char **argv, int run_idx)
>         if (affinity__setup(&affinity) < 0)
>                 return -1;
>
> +       evlist__warn_mixed_group(evsel_list);
> +
>         evlist__for_each_cpu (evsel_list, i, cpu) {
>                 affinity__set(&affinity, cpu);
>
> diff --git a/tools/perf/builtin-top.c b/tools/perf/builtin-top.c
> index 7c64134472c7..9ad319cea948 100644
> --- a/tools/perf/builtin-top.c
> +++ b/tools/perf/builtin-top.c
> @@ -1027,6 +1027,8 @@ static int perf_top__start_counters(struct perf_top *top)
>
>         perf_evlist__config(evlist, opts, &callchain_param);
>
> +       evlist__warn_mixed_group(evlist);
> +
>         evlist__for_each_entry(evlist, counter) {
>  try_again:
>                 if (evsel__open(counter, top->evlist->core.cpus,
> diff --git a/tools/perf/util/evlist.c b/tools/perf/util/evlist.c
> index 8bdf3d2c907c..02cff39e509e 100644
> --- a/tools/perf/util/evlist.c
> +++ b/tools/perf/util/evlist.c
> @@ -28,6 +28,7 @@
>  #include <unistd.h>
>  #include <sched.h>
>  #include <stdlib.h>
> +#include <dirent.h>
>
>  #include "parse-events.h"
>  #include <subcmd/parse-options.h>
> @@ -1980,3 +1981,80 @@ struct evsel *evlist__find_evsel(struct evlist *evlist, int idx)
>         }
>         return NULL;
>  }
> +
> +static int *sw_types;
> +static int nr_sw_types;
> +
> +static void collect_software_pmu_types(void)
> +{
> +       const char *known_sw_pmu[] = {
> +               "software", "tracepoint", "breakpoint", "kprobe", "uprobe", "msr"
> +       };
> +       DIR *dir;
> +       struct dirent *d;
> +       char path[PATH_MAX];
> +       int i;
> +
> +       if (sw_types != NULL)
> +               return;
> +
> +       nr_sw_types = ARRAY_SIZE(known_sw_pmu);
> +       sw_types = calloc(nr_sw_types, sizeof(int));
> +       if (sw_types == NULL) {
> +               pr_err("Memory allocation failed!\n");
> +               return;
> +       }
> +
> +       dir = opendir("/sys/bus/event_source/devices");
> +       while ((d = readdir(dir)) != NULL) {
> +               for (i = 0; i < nr_sw_types; i++) {
> +                       if (strcmp(d->d_name, known_sw_pmu[i]))
> +                               continue;
> +
> +                       snprintf(path, sizeof(path), "%s/%s/type",
> +                                "bus/event_source/devices", d->d_name);
> +                       sysfs__read_int(path, &sw_types[i]);
> +               }
> +       }
> +       closedir(dir);
> +}
> +
> +static bool is_software_event(struct evsel *evsel)
> +{
> +       int i;
> +
> +       for (i = 0; i < nr_sw_types; i++) {
> +               if (evsel->core.attr.type == (unsigned)sw_types[i])
> +                       return true;
> +       }
> +       return false;
> +}
> +
> +void evlist__warn_mixed_group(struct evlist *evlist)
> +{
> +       struct evsel *leader, *evsel;
> +       bool warn = true;
> +
> +       collect_software_pmu_types();
> +
> +       /* Warn if an event group has a sw leader and hw siblings */
> +       evlist__for_each_entry(evlist, leader) {
> +               if (!evsel__is_group_event(leader))
> +                       continue;
> +
> +               if (!is_software_event(leader))
> +                       continue;
> +
> +               for_each_group_member(evsel, leader) {
> +                       if (is_software_event(evsel))
> +                               continue;
> +                       if (!warn)
> +                               continue;
> +
> +                       pr_warning("WARNING: Event group has mixed hw/sw events.\n"
> +                                  "This will slow down the perf_event_open syscall.\n"
> +                                  "Consider putting a hw event as a leader.\n\n");
> +                       warn = false;
> +               }
> +       }
> +}
> diff --git a/tools/perf/util/evlist.h b/tools/perf/util/evlist.h
> index e1a450322bc5..a5b0a1d03a00 100644
> --- a/tools/perf/util/evlist.h
> +++ b/tools/perf/util/evlist.h
> @@ -387,4 +387,5 @@ int evlist__ctlfd_ack(struct evlist *evlist);
>  #define EVLIST_DISABLED_MSG "Events disabled\n"
>
>  struct evsel *evlist__find_evsel(struct evlist *evlist, int idx);
> +void evlist__warn_mixed_group(struct evlist *evlist);
>  #endif /* __PERF_EVLIST_H */
> --
> 2.29.0.rc1.297.gfa9743e501-goog
>