Re: [PATCH v8 2/2] perf evsel: Find process with busy PMUs for EBUSY

[Ian Rogers]

On Tue, Jun 2, 2026 at 11:13 AM Chun-Tse Shao <ctshao@xxxxxxxxxx> 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.
>
> Reviewed-by: Ian Rogers <irogers@xxxxxxxxxx>
> Signed-off-by: Chun-Tse Shao <ctshao@xxxxxxxxxx>
> Assisted-by: Gemini:gemini-3.1-pro-preview

Arnaldo, as the kernel piece of this landed (thanks Peter!) could we
land the tool part too?

Thanks,
Ian

> ---
> v8:
>   - Fix a boundary mismatch bug in strncmp when matching anon_inode
>     symlinks.
>   - Switch to thread-safe strtok_r to prevent use-after-free or static
>     state corruption.
>   - Expand the stack-allocated fdinfo parsing buffer from 256 bytes to
>     1024 bytes to completely prevent truncation before the PMU type is
>     parsed.
>   - Open proc files (fdinfo and cmdline) with O_NONBLOCK to prevent
>     malicious or slow mounts from hanging the perf session indefinitely.
>   - Match EBUSY conflicting processes using the physical PMU type
>     (pmu_type) instead of the requested event type, ensuring legacy/raw
>     events are correctly resolved.
> ---
>  tools/perf/util/evsel.c | 99 ++++++++++++++++++++++++++++++++---------
>  1 file changed, 77 insertions(+), 22 deletions(-)
>
> diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
> index 34c03f47a913..a6b1005db5e8 100644
> --- a/tools/perf/util/evsel.c
> +++ b/tools/perf/util/evsel.c
> @@ -4104,8 +4104,11 @@ 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 evsel *evsel,
> +                                    char *msg, size_t size)
>  {
> +       const struct perf_event_attr *failed_attr = &evsel->core.attr;
> +       u32 target_pmu_type = evsel->pmu ? evsel->pmu->type : UINT32_MAX;
>         DIR *proc_dir;
>         struct dirent *proc_entry;
>         int printed = 0;
> @@ -4136,6 +4139,8 @@ static int dump_perf_event_processes(char *msg, size_t size)
>                         continue;
>                 }
>                 while ((fd_entry = readdir(fd_dir)) != NULL) {
> +                       const char *target_lnk = "anon_inode:[perf_event]";
> +                       size_t target_lnk_len = sizeof("anon_inode:[perf_event]") - 1;
>                         ssize_t link_size;
>
>                         if (fd_entry->d_type != DT_LNK)
> @@ -4144,30 +4149,78 @@ static int dump_perf_event_processes(char *msg, size_t size)
>                         if (link_size < 0)
>                                 continue;
>                         /* Take care as readlink doesn't null terminate the string. */
> -                       if (!strncmp(buf, "anon_inode:[perf_event]", link_size)) {
> -                               int cmdline_fd;
> -                               ssize_t cmdline_size;
> -
> -                               scnprintf(buf, sizeof(buf), "%s/cmdline", proc_entry->d_name);
> -                               cmdline_fd = openat(dirfd(proc_dir), buf, O_RDONLY);
> -                               if (cmdline_fd == -1)
> +                       if (link_size == (ssize_t)target_lnk_len &&
> +                           !strncmp(buf, target_lnk, target_lnk_len)) {
> +                               char fdinfo_buf[1024];
> +                               int fdinfo_fd;
> +                               ssize_t fdinfo_size;
> +                               char *line;
> +                               char *saveptr;
> +                               u32 perf_event_type = UINT32_MAX;
> +                               u32 pmu_type = UINT32_MAX;
> +
> +                               /* Let's check the PMU type reserved by this process */
> +                               scnprintf(buf, sizeof(buf), "%s/fdinfo/%s",
> +                                         proc_entry->d_name, fd_entry->d_name);
> +                               fdinfo_fd = openat(dirfd(proc_dir), buf, O_RDONLY);
> +                               if (fdinfo_fd == -1)
>                                         continue;
> -                               cmdline_size = read(cmdline_fd, buf, sizeof(buf) - 1);
> -                               close(cmdline_fd);
> -                               if (cmdline_size < 0)
> +                               fdinfo_size = read(fdinfo_fd, fdinfo_buf, sizeof(fdinfo_buf) - 1);
> +                               close(fdinfo_fd);
> +                               if (fdinfo_size < 0)
>                                         continue;
> -                               buf[cmdline_size] = '\0';
> -                               for (ssize_t i = 0; i < cmdline_size; i++) {
> -                                       if (buf[i] == '\0')
> -                                               buf[i] = ' ';
> +                               fdinfo_buf[fdinfo_size] = '\0';
> +
> +                               line = strtok_r(fdinfo_buf, "\n", &saveptr);
> +                               while (line) {
> +                                       if (sscanf(line,
> +                                                  "perf_event_attr.type:\t%u",
> +                                                  &perf_event_type) == 1) {
> +                                               /* continue parsing */
> +                                       } else if (sscanf(line,
> +                                                         "pmu_type:\t%u",
> +                                                         &pmu_type) == 1) {
> +                                               /* continue parsing */
> +                                       }
> +                                       line = strtok_r(NULL, "\n", &saveptr);
>                                 }
>
> -                               if (printed == 0)
> -                                       printed += scnprintf(msg, size, "Possible processes:\n");
> -
> -                               printed += scnprintf(msg + printed, size - printed,
> -                                               "%s %s\n", proc_entry->d_name, buf);
> -                               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 ||
> +                                   pmu_type == failed_attr->type ||
> +                                   (target_pmu_type != UINT32_MAX &&
> +                                    pmu_type == target_pmu_type) ||
> +                                   (perf_event_type == UINT32_MAX &&
> +                                    pmu_type == UINT32_MAX)) {
> +                                       int cmdline_fd;
> +                                       ssize_t cmdline_size;
> +
> +                                       scnprintf(buf, sizeof(buf),
> +                                                 "%s/cmdline",
> +                                                 proc_entry->d_name);
> +                                       cmdline_fd = openat(dirfd(proc_dir), buf, O_RDONLY);
> +                                       if (cmdline_fd == -1)
> +                                               continue;
> +                                       cmdline_size = read(cmdline_fd, buf, sizeof(buf) - 1);
> +                                       close(cmdline_fd);
> +                                       if (cmdline_size < 0)
> +                                               continue;
> +                                       buf[cmdline_size] = '\0';
> +                                       for (ssize_t i = 0; i < cmdline_size; i++) {
> +                                               if (buf[i] == '\0')
> +                                                       buf[i] = ' ';
> +                                               else if (!isprint((unsigned char)buf[i]))
> +                                                       buf[i] = '.';
> +                                       }
> +                                       if (printed == 0)
> +                                               printed += scnprintf(msg, size,
> +                                                                    "Possible processes:\n");
> +
> +                                       printed += scnprintf(msg + printed, size - printed,
> +                                                       "%s %s\n", proc_entry->d_name, buf);
> +                                       break;
> +                               }
>                         }
>                 }
>                 closedir(fd_dir);
> @@ -4285,7 +4338,9 @@ 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,
> +                                                          msg + printed,
> +                                                          size - printed);
>                 break;
>         case EINVAL:
>                 if (evsel->core.attr.sample_type & PERF_SAMPLE_CODE_PAGE_SIZE && perf_missing_features.code_page_size)
> --
> 2.54.0.1013.g208068f2d8-goog
>