[PATCH v4 1/2] perf: Reveal PMU type in fdinfo

[PATCH v4 1/2] perf: Reveal PMU type in fdinfo

[Chun-Tse Shao]

It gives useful info on knowing which PMUs are reserved by this process.
Also add config which would be useful.
Testing cycles:

  $ ./perf stat -e cycles &
  $ cat /proc/`pidof perf`/fdinfo/3
  pos:    0
  flags:  02000002
  mnt_id: 16
  ino:    3081
  perf_event_attr.type:   0
  perf_event_attr.config: 0

Testing L1-dcache-load-misses:

  $ ./perf stat -e L1-dcache-load-misses &
  $ cat /proc/`pidof perf`/fdinfo/3
  pos:    0
  flags:  02000002
  mnt_id: 16
  ino:    1072
  perf_event_attr.type:   3
  perf_event_attr.config: 65536

Reviewed-by: Ian Rogers <irogers@google.com>
Signed-off-by: Chun-Tse Shao <ctshao@google.com>
---
v4:
  Removed the first patch in v3 since it is merged.
  Retested to make sure the patch still work.

v3: lore.kernel.org/20241106003007.2112584-2-ctshao@google.com

 kernel/events/core.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 95e703891b24..bad563692d63 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -56,6 +56,7 @@
 #include <linux/buildid.h>
 #include <linux/task_work.h>
 #include <linux/percpu-rwsem.h>
+#include <linux/seq_file.h>

 #include "internal.h"

@@ -7077,6 +7078,14 @@ static int perf_fasync(int fd, struct file *filp, int on)
 	return 0;
 }

+static void perf_show_fdinfo(struct seq_file *m, struct file *f)
+{
+	struct perf_event *event = f->private_data;
+
+	seq_printf(m, "perf_event_attr.type:\t%u\n", event->orig_type);
+	seq_printf(m, "perf_event_attr.config:\t%llu\n", (unsigned long long)event->attr.config);
+}
+
 static const struct file_operations perf_fops = {
 	.release		= perf_release,
 	.read			= perf_read,
@@ -7085,6 +7094,7 @@ static const struct file_operations perf_fops = {
 	.compat_ioctl		= perf_compat_ioctl,
 	.mmap			= perf_mmap,
 	.fasync			= perf_fasync,
+	.show_fdinfo		= perf_show_fdinfo,
 };

 /*
--
2.49.0.1204.g71687c7c1d-goog

[PATCH v4 2/2] perf evsel: Find process with busy PMUs for EBUSY

[Chun-Tse Shao]

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@google.com>
Signed-off-by: Chun-Tse Shao <ctshao@google.com>
---
v4:
  Removed the first patch in v3 since it is merged.
  Retested to make sure the patch still work.

v3: lore.kernel.org/20241106003007.2112584-3-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 d55482f094bf..fa281ea15a25 100644
--- a/tools/perf/util/evsel.c
+++ b/tools/perf/util/evsel.c
@@ -3619,7 +3619,8 @@ 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;
@@ -3660,29 +3661,61 @@ static int dump_perf_event_processes(char *msg, size_t size)
 				continue;
 			/* Take care as readlink doesn't null terminate the string. */
 			if (!strncmp(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)
-					continue;
-				cmdline_size = read(cmdline_fd, buf, sizeof(buf) - 1);
-				close(cmdline_fd);
-				if (cmdline_size < 0)
+				int fdinfo_fd;
+				ssize_t fdinfo_size;
+				char *line;
+				u32 perf_event_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);
+				fdinfo_size = read(fdinfo_fd, buf, sizeof(buf) - 1);
+				if (fdinfo_size < 0)
 					continue;
-				buf[cmdline_size] = '\0';
-				for (ssize_t i = 0; i < cmdline_size; i++) {
-					if (buf[i] == '\0')
-						buf[i] = ' ';
+				buf[fdinfo_size] = '\0';
+
+				line = strtok(buf, "\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, 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 ||
+				    perf_event_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] = ' ';
+					}
+
+					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);
@@ -3799,7 +3832,9 @@ 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)
--
2.49.0.1204.g71687c7c1d-goog

Re: [PATCH v4 1/2] perf: Reveal PMU type in fdinfo

[Ian Rogers]

On Tue, Jun 3, 2025 at 11:16 AM Chun-Tse Shao <ctshao@google.com> wrote:
>
> It gives useful info on knowing which PMUs are reserved by this process.
> Also add config which would be useful.
> Testing cycles:
>
>   $ ./perf stat -e cycles &
>   $ cat /proc/`pidof perf`/fdinfo/3
>   pos:    0
>   flags:  02000002
>   mnt_id: 16
>   ino:    3081
>   perf_event_attr.type:   0
>   perf_event_attr.config: 0
>
> Testing L1-dcache-load-misses:
>
>   $ ./perf stat -e L1-dcache-load-misses &
>   $ cat /proc/`pidof perf`/fdinfo/3
>   pos:    0
>   flags:  02000002
>   mnt_id: 16
>   ino:    1072
>   perf_event_attr.type:   3
>   perf_event_attr.config: 65536
>
> Reviewed-by: Ian Rogers <irogers@google.com>
> Signed-off-by: Chun-Tse Shao <ctshao@google.com>

Ping.

Thanks,
Ian

> ---
> v4:
>   Removed the first patch in v3 since it is merged.
>   Retested to make sure the patch still work.
>
> v3: lore.kernel.org/20241106003007.2112584-2-ctshao@google.com
>
>  kernel/events/core.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
>
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 95e703891b24..bad563692d63 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -56,6 +56,7 @@
>  #include <linux/buildid.h>
>  #include <linux/task_work.h>
>  #include <linux/percpu-rwsem.h>
> +#include <linux/seq_file.h>
>
>  #include "internal.h"
>
> @@ -7077,6 +7078,14 @@ static int perf_fasync(int fd, struct file *filp, int on)
>         return 0;
>  }
>
> +static void perf_show_fdinfo(struct seq_file *m, struct file *f)
> +{
> +       struct perf_event *event = f->private_data;
> +
> +       seq_printf(m, "perf_event_attr.type:\t%u\n", event->orig_type);
> +       seq_printf(m, "perf_event_attr.config:\t%llu\n", (unsigned long long)event->attr.config);
> +}
> +
>  static const struct file_operations perf_fops = {
>         .release                = perf_release,
>         .read                   = perf_read,
> @@ -7085,6 +7094,7 @@ static const struct file_operations perf_fops = {
>         .compat_ioctl           = perf_compat_ioctl,
>         .mmap                   = perf_mmap,
>         .fasync                 = perf_fasync,
> +       .show_fdinfo            = perf_show_fdinfo,
>  };
>
>  /*
> --
> 2.49.0.1204.g71687c7c1d-goog
>

Re: [PATCH v4 1/2] perf: Reveal PMU type in fdinfo

[Ian Rogers]

On Tue, Jun 24, 2025 at 8:06 PM Ian Rogers <irogers@google.com> wrote:
>
> On Tue, Jun 3, 2025 at 11:16 AM Chun-Tse Shao <ctshao@google.com> wrote:
> >
> > It gives useful info on knowing which PMUs are reserved by this process.
> > Also add config which would be useful.
> > Testing cycles:
> >
> >   $ ./perf stat -e cycles &
> >   $ cat /proc/`pidof perf`/fdinfo/3
> >   pos:    0
> >   flags:  02000002
> >   mnt_id: 16
> >   ino:    3081
> >   perf_event_attr.type:   0
> >   perf_event_attr.config: 0
> >
> > Testing L1-dcache-load-misses:
> >
> >   $ ./perf stat -e L1-dcache-load-misses &
> >   $ cat /proc/`pidof perf`/fdinfo/3
> >   pos:    0
> >   flags:  02000002
> >   mnt_id: 16
> >   ino:    1072
> >   perf_event_attr.type:   3
> >   perf_event_attr.config: 65536
> >
> > Reviewed-by: Ian Rogers <irogers@google.com>
> > Signed-off-by: Chun-Tse Shao <ctshao@google.com>
>
> Ping.
>
> Thanks,
> Ian

Hi Peter and Ingo,

This small kernel change enables being able to better diagnose which
processes are using perf events for what. In the series it is used in
perf to blame EBUSY for PMUs like intel-pt on which processes are
using that PMU.
Are there issues that need resolving before the patch can be
considered for inclusion?

The original patch was sent in October 2024:
https://lore.kernel.org/lkml/20241031223948.4179222-2-ctshao@google.com/

Thanks,
Ian

> > ---
> > v4:
> >   Removed the first patch in v3 since it is merged.
> >   Retested to make sure the patch still work.
> >
> > v3: lore.kernel.org/20241106003007.2112584-2-ctshao@google.com
> >
> >  kernel/events/core.c | 10 ++++++++++
> >  1 file changed, 10 insertions(+)
> >
> > diff --git a/kernel/events/core.c b/kernel/events/core.c
> > index 95e703891b24..bad563692d63 100644
> > --- a/kernel/events/core.c
> > +++ b/kernel/events/core.c
> > @@ -56,6 +56,7 @@
> >  #include <linux/buildid.h>
> >  #include <linux/task_work.h>
> >  #include <linux/percpu-rwsem.h>
> > +#include <linux/seq_file.h>
> >
> >  #include "internal.h"
> >
> > @@ -7077,6 +7078,14 @@ static int perf_fasync(int fd, struct file *filp, int on)
> >         return 0;
> >  }
> >
> > +static void perf_show_fdinfo(struct seq_file *m, struct file *f)
> > +{
> > +       struct perf_event *event = f->private_data;
> > +
> > +       seq_printf(m, "perf_event_attr.type:\t%u\n", event->orig_type);
> > +       seq_printf(m, "perf_event_attr.config:\t%llu\n", (unsigned long long)event->attr.config);
> > +}
> > +
> >  static const struct file_operations perf_fops = {
> >         .release                = perf_release,
> >         .read                   = perf_read,
> > @@ -7085,6 +7094,7 @@ static const struct file_operations perf_fops = {
> >         .compat_ioctl           = perf_compat_ioctl,
> >         .mmap                   = perf_mmap,
> >         .fasync                 = perf_fasync,
> > +       .show_fdinfo            = perf_show_fdinfo,
> >  };
> >
> >  /*
> > --
> > 2.49.0.1204.g71687c7c1d-goog
> >