Re: [PATCH 3/3] perf evsel: Find process with busy PMUs for EBUSY

[Ian Rogers <irogers@google.com>]

On Thu, Oct 31, 2024 at 3:39 PM Chun-Tse Shao <ctshao@google.com> wrote:
>
> It parses fdinfo with PMU type, comparing with the event which failed to
> open, and report the processes causing EBUSY error.
>
> ```
> Testing cycles and intel_pt//
> $ ./perf stat -e cycles &
> [1] 55569
> $ ./perf stat -e intel_pt// &
> [2] 55683
> $ ./perf stat -e intel_pt//
> Error:
> The PMU intel_pt counters are busy and in use by another process.
> Possible processes:
> 55683 ./perf stat -e intel_pt//
> ```
> Only perf with intel_pt was reported.

I think this is a very nice addition. It is a shame there is a race
with the existing process exiting, between the perf_event_open and the
/proc scanning. A PMU may return EBUSY just because say perf list is
probing features on the PMU so we probably have some extra retries for
EBUSY too.

> Signed-off-by: Chun-Tse Shao <ctshao@google.com>
> ---
>  tools/perf/util/evsel.c | 79 +++++++++++++++++++++++++++++------------
>  1 file changed, 57 insertions(+), 22 deletions(-)
>
> diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
> index 9a5b6a6f8d2e5..d2f7c19e023ec 100644
> --- a/tools/perf/util/evsel.c
> +++ b/tools/perf/util/evsel.c
> @@ -3286,7 +3286,8 @@ static bool find_process(const char *name)
>         return ret ? false : true;
>  }
>
> -static int dump_perf_event_processes(char *msg, size_t size)
> +static int dump_perf_event_processes(const struct perf_event_attr *failed_attr,
> +                                    char *msg, size_t size)
>  {
>         DIR *proc_dir;
>         struct dirent *proc_entry;
> @@ -3327,29 +3328,61 @@ static int dump_perf_event_processes(char *msg, size_t size)
>                                 continue;
>                         /* Take care as readlink doesn't null terminate the string. */
>                         if (!strncmp(path, "anon_inode:[perf_event]", link_size)) {
> -                               int cmdline_fd;
> -                               ssize_t cmdline_size;
> -
> -                               scnprintf(path, sizeof(path), "%s/cmdline", proc_entry->d_name);
> -                               cmdline_fd = openat(dirfd(proc_dir), path, O_RDONLY);
> -                               if (cmdline_fd == -1)
> -                                       continue;
> -                               cmdline_size = read(cmdline_fd, path, sizeof(path) - 1);
> -                               close(cmdline_fd);
> -                               if (cmdline_size < 0)
> +                               int fdinfo_fd;
> +                               ssize_t fdinfo_size;
> +                               char *line;
> +                               u32 perf_event_type = PERF_TYPE_MAX;

PERF_TYPE_MAX is beyond the pre-defined perf PMU types at 6 but PMU
drivers loaded by the kernel may use this number - I think task-clock
may use this PMU type number but it shouldn't return EBUSY. Anyway, I
think -1 would be a better marker to use here and in the corresponding
check below.

Thanks,
Ian

> +
> +                               /* Let's check the PMU type reserved by this process */
> +                               scnprintf(path, sizeof(path), "%s/fdinfo/%s",
> +                                         proc_entry->d_name, fd_entry->d_name);
> +                               fdinfo_fd = openat(dirfd(proc_dir), path, O_RDONLY);
> +                               fdinfo_size = read(fdinfo_fd, path, sizeof(path) - 1);
> +                               if (fdinfo_size < 0)
>                                         continue;
> -                               path[cmdline_size] = '\0';
> -                               for (ssize_t i = 0; i < cmdline_size; i++) {
> -                                       if (path[i] == '\0')
> -                                               path[i] = ' ';
> +                               path[fdinfo_size] = '\0';
> +
> +                               line = strtok(path, "\n");
> +                               while (line != NULL) {
> +                                       if (sscanf(line,
> +                                                  "perf_event-attr.type:\t%u",
> +                                                  &perf_event_type) == 1)
> +                                               break;
> +                                       line = strtok(NULL, "\n");
>                                 }
>
> -                               if (printed == 0)
> -                                       printed += scnprintf(msg, size, "Possible processes:\n");
> -
> -                               printed += scnprintf(msg + printed, size - printed,
> -                                               "%s %s\n", proc_entry->d_name, path);
> -                               break;
> +                               /* Report the process which reserves the conflicted PMU. */
> +                               /* If fdinfo does not contain PMU type, report it too. */
> +                               if (perf_event_type == failed_attr->type ||
> +                                   perf_event_type == PERF_TYPE_MAX) {
> +                                       int cmdline_fd;
> +                                       ssize_t cmdline_size;
> +
> +                                       scnprintf(path, sizeof(path),
> +                                                 "%s/cmdline",
> +                                                 proc_entry->d_name);
> +                                       cmdline_fd = openat(dirfd(proc_dir), path, O_RDONLY);
> +                                       if (cmdline_fd == -1)
> +                                               continue;
> +                                       cmdline_size = read(cmdline_fd, path, sizeof(path) - 1);
> +                                       close(cmdline_fd);
> +                                       if (cmdline_size < 0)
> +                                               continue;
> +                                       path[cmdline_size] = '\0';
> +                                       for (ssize_t i = 0; i < cmdline_size; i++) {
> +                                               if (path[i] == '\0')
> +                                                       path[i] = ' ';
> +                                       }
> +
> +                                       if (printed == 0)
> +                                               printed += scnprintf(
> +                                                       msg, size,
> +                                                       "Possible processes:\n");
> +
> +                                       printed += scnprintf(msg + printed, size - printed,
> +                                                       "%s %s\n", proc_entry->d_name, path);
> +                                       break;
> +                               }
>                         }
>                 }
>                 closedir(fd_dir);
> @@ -3458,7 +3491,9 @@ int evsel__open_strerror(struct evsel *evsel, struct target *target,
>                         msg, size,
>                         "The PMU %s counters are busy and in use by another process.\n",
>                         evsel->pmu ? evsel->pmu->name : "");
> -               return printed + dump_perf_event_processes(msg + printed, size - printed);
> +               return printed + dump_perf_event_processes(&evsel->core.attr,
> +                                                          msg + printed,
> +                                                          size - printed);
>                 break;
>         case EINVAL:
>                 if (evsel->core.attr.sample_type & PERF_SAMPLE_CODE_PAGE_SIZE && perf_missing_features.code_page_size)
> --
> 2.47.0.163.g1226f6d8fa-goog
>