Messages in this thread | | | From | Namhyung Kim <> | Subject | [PATCHSET 0/9] perf tools: Update pmu scan using openat() (v1) | Date | Fri, 31 Mar 2023 13:29:40 -0700 |
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Hello,
This patchset changes PMU info scanning on sysfs using openat() basically. I got reports of occasional contention on the opening files in sysfs. While the root cause was a separate issue, I discovered some inefficiencies in the perf code.
To scan PMUs, it roughly does something 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) in the kernel unnecessarily. We can use openat(2) to open the file in the given directory.
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); ... }
I added a benchmark for pmu-scan and it showed a slight speedup in the normal case too.
$ ./perf.old bench internals pmu-scan # Running 'internals/pmu-scan' benchmark: Computing performance of sysfs PMU event scan for 100 times Average PMU scanning took: 6670.970 usec (+- 13.022 usec)
$ ./perf.new bench internals pmu-scan # Running 'internals/pmu-scan' benchmark: Computing performance of sysfs PMU event scan for 100 times Average PMU scanning took: 6296.980 usec (+- 14.891 usec)
The 5~6% of improvement might be small but it may have bigger impact when the system is contended.
You can get the code from 'perf/pmu-scan-v1' branch in
git://git.kernel.org/pub/scm/linux/kernel/git/namhyung/linux-perf.git
Thanks, Namhyung
Namhyung Kim (9): perf list: Use relative path for tracepoint scan perf tools: Fix a asan issue in parse_events_multi_pmu_add() perf pmu: Add perf_pmu__destroy() function perf bench: Add pmu-scan benchmark perf pmu: Use relative path for sysfs scan perf pmu: Use relative path in perf_pmu__caps_parse() perf pmu: Use relative path in setup_pmu_alias_list() perf pmu: Add perf_pmu__{open,scan}_file_at() perf intel-pt: Use perf_pmu__scan_file_at() if possible
tools/perf/arch/x86/util/intel-pt.c | 52 ++++-- tools/perf/arch/x86/util/pmu.c | 13 +- tools/perf/bench/Build | 1 + tools/perf/bench/bench.h | 1 + tools/perf/bench/pmu-scan.c | 184 ++++++++++++++++++ tools/perf/builtin-bench.c | 1 + tools/perf/tests/pmu.c | 9 +- tools/perf/util/parse-events.c | 2 +- tools/perf/util/pmu.c | 278 ++++++++++++++++++++-------- tools/perf/util/pmu.h | 12 +- tools/perf/util/print-events.c | 26 ++- 11 files changed, 466 insertions(+), 113 deletions(-) create mode 100644 tools/perf/bench/pmu-scan.c
base-commit: 417c6adfb155f906f0441cc1034827f6e2b3c372 -- 2.40.0.348.gf938b09366-goog
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