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.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

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;
 

tools/perf/util/pmu.h

@@ -211,7 +211,7 @@ void perf_pmu_error(struct list_head *list, char *name, char const *msg);
 int perf_pmu__new_format(struct list_head *list, char *name,
 			 int config, unsigned long *bits);
 void perf_pmu__set_format(unsigned long *bits, long from, long to);
-int perf_pmu__format_parse(char *dir, struct list_head *head);
+int perf_pmu__format_parse(int dirfd, struct list_head *head);
 void perf_pmu__del_formats(struct list_head *formats);
 
 struct perf_pmu *perf_pmu__scan(struct perf_pmu *pmu);
@@ -257,6 +257,8 @@ double perf_pmu__cpu_slots_per_cycle(void);
 int perf_pmu__event_source_devices_scnprintf(char *pathname, size_t size);
 int perf_pmu__pathname_scnprintf(char *buf, size_t size,
 				 const char *pmu_name, const char *filename);
+int perf_pmu__event_source_devices_fd(void);
+int perf_pmu__pathname_fd(int dirfd, const char *pmu_name, const char *filename, int flags);
 FILE *perf_pmu__open_file(struct perf_pmu *pmu, const char *name);
 
 void perf_pmu__destroy(void);