perf pmu: Use relative path for sysfs scan

The PMU information is in the kernel sysfs so it needs to scan the
directory to get the whole information like event aliases, formats and
so on.  During the traversal, it opens a lot of files and directories
like below:

  dir = opendir("/sys/bus/event_source/devices");
  while (dentry = readdir(dir)) {
    char buf[PATH_MAX];

    snprintf(buf, sizeof(buf), "%s/%s",
             "/sys/bus/event_source/devices", dentry->d_name);
    fd = open(buf, O_RDONLY);
    ...
  }

But this is not good since it needs to copy the string to build the
absolute pathname, and it makes redundant pathname walk (from the /sys)
unnecessarily.  We can use openat(2) to open the file in the given
directory.  While it's not a problem ususally, it can be a problem when
the kernel has contentions on the sysfs.

Add a couple of new helper to return the file descriptor of PMU
directory so that it can use it with relative paths.

 * perf_pmu__event_source_devices_fd()
   - returns a fd for the PMU root ("/sys/bus/event_source/devices")

 * perf_pmu__pathname_fd()
   - returns a fd for "<pmu>/<file>" under the PMU root

Now the above code can be converted something like below:

  dirfd = perf_pmu__event_source_devices_fd();
  dir = fdopendir(dirfd);
  while (dentry = readdir(dir)) {
    fd = openat(dirfd, dentry->d_name, O_RDONLY);
    ...
  }

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Acked-by: Ian Rogers <irogers@google.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Leo Yan <leo.yan@linaro.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/20230331202949.810326-2-namhyung@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

1 files changed, 8 insertions, 1 deletions

diff --git a/tools/perf/tests/pmu.c b/tools/perf/tests/pmu.c
index 8507bd615e97..3cf25f883df7 100644
--- a/tools/perf/tests/pmu.c
+++ b/tools/perf/tests/pmu.c
@@ -3,6 +3,7 @@
 #include "pmu.h"
 #include "tests.h"
 #include <errno.h>
+#include <fcntl.h>
 #include <stdio.h>
 #include <linux/kernel.h>
 #include <linux/limits.h>
@@ -149,10 +150,16 @@ static int test__pmu(struct test_suite *test __maybe_unused, int subtest __maybe
 
 	do {
 		struct perf_event_attr attr;
+		int fd;
 
 		memset(&attr, 0, sizeof(attr));
 
-		ret = perf_pmu__format_parse(format, &formats);
+		fd = open(format, O_DIRECTORY);
+		if (fd < 0) {
+			ret = fd;
+			break;
+		}
+		ret = perf_pmu__format_parse(fd, &formats);
 		if (ret)
 			break;