linux-kernel - [PATCH 2/2] libperf evlist: Fix polling of system-wide events

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Date:   Thu, 15 Sep 2022 15:26:12 +0300
From:   Adrian Hunter <adrian.hunter@...el.com>
To:     Arnaldo Carvalho de Melo <acme@...nel.org>
Cc:     Jiri Olsa <jolsa@...hat.com>, Namhyung Kim <namhyung@...nel.org>,
        Ian Rogers <irogers@...gle.com>, linux-kernel@...r.kernel.org,
        linux-perf-users@...r.kernel.org
Subject: [PATCH 2/2] libperf evlist: Fix polling of system-wide events

Originally, (refer commit f90d194a867a ("perf evlist: Do not poll events
that use the system_wide flag") there wasn't much reason to poll
system-wide events because:
 1. The mmaps get "merged" via set-output anyway (the per-cpu case)
 2. perf reads all mmaps when any event is woken
 3. system-wide mmaps do not fill up as fast as the mmaps for user
 selected events
But there was 1 reason not to poll which was that it prevented
correct termination due to POLLHUP on all user selected events.
That issue is now easily resolved by using fdarray_flag__nonfilterable.

With the advent of commit ae4f8ae16a07 ("libperf evlist: Allow mixing
per-thread and per-cpu mmaps"), system-wide mmaps can be used also in
the per-thread case where reason 1 does not apply.

Fix the omission of system-wide events from polling by using the
fdarray_flag__nonfilterable flag.

Example:

 Before:

    $ perf record --no-bpf-event -vvv -e intel_pt// --per-thread uname 2>err.txt
    Linux
    $ grep 'sys_perf_event_open.*=\|pollfd' err.txt
    sys_perf_event_open: pid 155076  cpu -1  group_fd -1  flags 0x8 = 5
    sys_perf_event_open: pid 155076  cpu -1  group_fd -1  flags 0x8 = 6
    sys_perf_event_open: pid -1  cpu 0  group_fd -1  flags 0x8 = 7
    sys_perf_event_open: pid -1  cpu 1  group_fd -1  flags 0x8 = 9
    sys_perf_event_open: pid -1  cpu 2  group_fd -1  flags 0x8 = 10
    sys_perf_event_open: pid -1  cpu 3  group_fd -1  flags 0x8 = 11
    sys_perf_event_open: pid -1  cpu 4  group_fd -1  flags 0x8 = 12
    sys_perf_event_open: pid -1  cpu 5  group_fd -1  flags 0x8 = 13
    sys_perf_event_open: pid -1  cpu 6  group_fd -1  flags 0x8 = 14
    sys_perf_event_open: pid -1  cpu 7  group_fd -1  flags 0x8 = 15
    thread_data[0x55fb43c29e80]: pollfd[0] <- event_fd=5
    thread_data[0x55fb43c29e80]: pollfd[1] <- event_fd=6
    thread_data[0x55fb43c29e80]: pollfd[2] <- non_perf_event fd=4

 After:

    $ perf record --no-bpf-event -vvv -e intel_pt// --per-thread uname 2>err.txt
    Linux
    $ grep 'sys_perf_event_open.*=\|pollfd' err.txt
    sys_perf_event_open: pid 156316  cpu -1  group_fd -1  flags 0x8 = 5
    sys_perf_event_open: pid 156316  cpu -1  group_fd -1  flags 0x8 = 6
    sys_perf_event_open: pid -1  cpu 0  group_fd -1  flags 0x8 = 7
    sys_perf_event_open: pid -1  cpu 1  group_fd -1  flags 0x8 = 9
    sys_perf_event_open: pid -1  cpu 2  group_fd -1  flags 0x8 = 10
    sys_perf_event_open: pid -1  cpu 3  group_fd -1  flags 0x8 = 11
    sys_perf_event_open: pid -1  cpu 4  group_fd -1  flags 0x8 = 12
    sys_perf_event_open: pid -1  cpu 5  group_fd -1  flags 0x8 = 13
    sys_perf_event_open: pid -1  cpu 6  group_fd -1  flags 0x8 = 14
    sys_perf_event_open: pid -1  cpu 7  group_fd -1  flags 0x8 = 15
    thread_data[0x55cc19e58e80]: pollfd[0] <- event_fd=5
    thread_data[0x55cc19e58e80]: pollfd[1] <- event_fd=6
    thread_data[0x55cc19e58e80]: pollfd[2] <- event_fd=7
    thread_data[0x55cc19e58e80]: pollfd[3] <- event_fd=9
    thread_data[0x55cc19e58e80]: pollfd[4] <- event_fd=10
    thread_data[0x55cc19e58e80]: pollfd[5] <- event_fd=11
    thread_data[0x55cc19e58e80]: pollfd[6] <- event_fd=12
    thread_data[0x55cc19e58e80]: pollfd[7] <- event_fd=13
    thread_data[0x55cc19e58e80]: pollfd[8] <- event_fd=14
    thread_data[0x55cc19e58e80]: pollfd[9] <- event_fd=15
    thread_data[0x55cc19e58e80]: pollfd[10] <- non_perf_event fd=4

Fixes: ae4f8ae16a07 ("libperf evlist: Allow mixing per-thread and per-cpu mmaps")
Cc: stable@...r.kernel.org
Signed-off-by: Adrian Hunter <adrian.hunter@...el.com>
---
 tools/lib/perf/evlist.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/tools/lib/perf/evlist.c b/tools/lib/perf/evlist.c
index 80cc810c5097..0e7347d1583d 100644
--- a/tools/lib/perf/evlist.c
+++ b/tools/lib/perf/evlist.c
@@ -441,6 +441,7 @@ mmap_per_evsel(struct perf_evlist *evlist, struct perf_evlist_mmap_ops *ops,
 
 	perf_evlist__for_each_entry(evlist, evsel) {
 		bool overwrite = evsel->attr.write_backward;
+		enum fdarray_flags flgs;
 		struct perf_mmap *map;
 		int *output, fd, cpu;
 
@@ -506,8 +507,8 @@ mmap_per_evsel(struct perf_evlist *evlist, struct perf_evlist_mmap_ops *ops,
 
 		revent = !overwrite ? POLLIN : 0;
 
-		if (!evsel->system_wide &&
-		    perf_evlist__add_pollfd(evlist, fd, map, revent, fdarray_flag__default) < 0) {
+		flgs = evsel->system_wide ? fdarray_flag__nonfilterable : fdarray_flag__default;
+		if (perf_evlist__add_pollfd(evlist, fd, map, revent, flgs) < 0) {
 			perf_mmap__put(map);
 			return -1;
 		}
-- 
2.25.1