[PATCH V2] libperf evlist: Fix per-thread mmaps for multi-threaded targets
From: Adrian Hunter
Date: Mon Sep 05 2022 - 07:42:26 EST
The offending commit removed mmap_per_thread(), which did not consider
the different set-output rules for per-thread mmaps i.e. in the per-thread
case set-output is used for file descriptors of the same thread not the
same cpu.
This was not immediately noticed because it only happens with
multi-threaded targets and we do not have a test for that yet.
Reinstate mmap_per_thread() expanding it to cover also system-wide per-cpu
events i.e. to continue to allow the mixing of per-thread and per-cpu
mmaps.
Debug messages (with -vv) show the file descriptors that are opened with
sys_perf_event_open. New debug messages are added (needs -vvv) that show
also which file descriptors are mmapped and which are redirected with
set-output.
In the per-cpu case (cpu != -1) file descriptors for the same CPU are
set-output to the first file descriptor for that CPU.
In the per-thread case (cpu == -1) file descriptors for the same thread are
set-output to the first file descriptor for that thread.
Example (process 17489 has 2 threads):
Before (but with new debug prints):
$ perf record --no-bpf-event -vvv --per-thread -p 17489
<SNIP>
sys_perf_event_open: pid 17489 cpu -1 group_fd -1 flags 0x8 = 5
sys_perf_event_open: pid 17490 cpu -1 group_fd -1 flags 0x8 = 6
<SNIP>
libperf: idx 0: mmapping fd 5
libperf: idx 0: set output fd 6 -> 5
failed to mmap with 22 (Invalid argument)
After:
$ perf record --no-bpf-event -vvv --per-thread -p 17489
<SNIP>
sys_perf_event_open: pid 17489 cpu -1 group_fd -1 flags 0x8 = 5
sys_perf_event_open: pid 17490 cpu -1 group_fd -1 flags 0x8 = 6
<SNIP>
libperf: mmap_per_thread: nr cpu values (may include -1) 1 nr threads 2
libperf: idx 0: mmapping fd 5
libperf: idx 1: mmapping fd 6
<SNIP>
[ perf record: Woken up 2 times to write data ]
[ perf record: Captured and wrote 0.018 MB perf.data (15 samples) ]
Per-cpu example (process 20341 has 2 threads, same as above):
$ perf record --no-bpf-event -vvv -p 20341
<SNIP>
sys_perf_event_open: pid 20341 cpu 0 group_fd -1 flags 0x8 = 5
sys_perf_event_open: pid 20342 cpu 0 group_fd -1 flags 0x8 = 6
sys_perf_event_open: pid 20341 cpu 1 group_fd -1 flags 0x8 = 7
sys_perf_event_open: pid 20342 cpu 1 group_fd -1 flags 0x8 = 8
sys_perf_event_open: pid 20341 cpu 2 group_fd -1 flags 0x8 = 9
sys_perf_event_open: pid 20342 cpu 2 group_fd -1 flags 0x8 = 10
sys_perf_event_open: pid 20341 cpu 3 group_fd -1 flags 0x8 = 11
sys_perf_event_open: pid 20342 cpu 3 group_fd -1 flags 0x8 = 12
sys_perf_event_open: pid 20341 cpu 4 group_fd -1 flags 0x8 = 13
sys_perf_event_open: pid 20342 cpu 4 group_fd -1 flags 0x8 = 14
sys_perf_event_open: pid 20341 cpu 5 group_fd -1 flags 0x8 = 15
sys_perf_event_open: pid 20342 cpu 5 group_fd -1 flags 0x8 = 16
sys_perf_event_open: pid 20341 cpu 6 group_fd -1 flags 0x8 = 17
sys_perf_event_open: pid 20342 cpu 6 group_fd -1 flags 0x8 = 18
sys_perf_event_open: pid 20341 cpu 7 group_fd -1 flags 0x8 = 19
sys_perf_event_open: pid 20342 cpu 7 group_fd -1 flags 0x8 = 20
<SNIP>
libperf: mmap_per_cpu: nr cpu values 8 nr threads 2
libperf: idx 0: mmapping fd 5
libperf: idx 0: set output fd 6 -> 5
libperf: idx 1: mmapping fd 7
libperf: idx 1: set output fd 8 -> 7
libperf: idx 2: mmapping fd 9
libperf: idx 2: set output fd 10 -> 9
libperf: idx 3: mmapping fd 11
libperf: idx 3: set output fd 12 -> 11
libperf: idx 4: mmapping fd 13
libperf: idx 4: set output fd 14 -> 13
libperf: idx 5: mmapping fd 15
libperf: idx 5: set output fd 16 -> 15
libperf: idx 6: mmapping fd 17
libperf: idx 6: set output fd 18 -> 17
libperf: idx 7: mmapping fd 19
libperf: idx 7: set output fd 20 -> 19
<SNIP>
[ perf record: Woken up 7 times to write data ]
[ perf record: Captured and wrote 0.020 MB perf.data (17 samples) ]
Fixes: ae4f8ae16a07 ("libperf evlist: Allow mixing per-thread and per-cpu mmaps")
Signed-off-by: Adrian Hunter <adrian.hunter@xxxxxxxxx>
---
Changes in V2:
Expand commit message.
White-space fixups.
tools/lib/perf/evlist.c | 50 +++++++++++++++++++++++++++++++++++++++++
1 file changed, 50 insertions(+)
diff --git a/tools/lib/perf/evlist.c b/tools/lib/perf/evlist.c
index e6c98a6e3908..6b1bafe267a4 100644
--- a/tools/lib/perf/evlist.c
+++ b/tools/lib/perf/evlist.c
@@ -486,6 +486,7 @@ mmap_per_evsel(struct perf_evlist *evlist, struct perf_evlist_mmap_ops *ops,
if (ops->idx)
ops->idx(evlist, evsel, mp, idx);
+ pr_debug("idx %d: mmapping fd %d\n", idx, *output);
if (ops->mmap(map, mp, *output, evlist_cpu) < 0)
return -1;
@@ -494,6 +495,7 @@ mmap_per_evsel(struct perf_evlist *evlist, struct perf_evlist_mmap_ops *ops,
if (!idx)
perf_evlist__set_mmap_first(evlist, map, overwrite);
} else {
+ pr_debug("idx %d: set output fd %d -> %d\n", idx, fd, *output);
if (ioctl(fd, PERF_EVENT_IOC_SET_OUTPUT, *output) != 0)
return -1;
@@ -519,6 +521,48 @@ mmap_per_evsel(struct perf_evlist *evlist, struct perf_evlist_mmap_ops *ops,
return 0;
}
+static int
+mmap_per_thread(struct perf_evlist *evlist, struct perf_evlist_mmap_ops *ops,
+ struct perf_mmap_param *mp)
+{
+ int nr_threads = perf_thread_map__nr(evlist->threads);
+ int nr_cpus = perf_cpu_map__nr(evlist->all_cpus);
+ int cpu, thread, idx = 0;
+ int nr_mmaps = 0;
+
+ pr_debug("%s: nr cpu values (may include -1) %d nr threads %d\n",
+ __func__, nr_cpus, nr_threads);
+
+ /* per-thread mmaps */
+ for (thread = 0; thread < nr_threads; thread++, idx++) {
+ int output = -1;
+ int output_overwrite = -1;
+
+ if (mmap_per_evsel(evlist, ops, idx, mp, 0, thread, &output,
+ &output_overwrite, &nr_mmaps))
+ goto out_unmap;
+ }
+
+ /* system-wide mmaps i.e. per-cpu */
+ for (cpu = 1; cpu < nr_cpus; cpu++, idx++) {
+ int output = -1;
+ int output_overwrite = -1;
+
+ if (mmap_per_evsel(evlist, ops, idx, mp, cpu, 0, &output,
+ &output_overwrite, &nr_mmaps))
+ goto out_unmap;
+ }
+
+ if (nr_mmaps != evlist->nr_mmaps)
+ pr_err("Miscounted nr_mmaps %d vs %d\n", nr_mmaps, evlist->nr_mmaps);
+
+ return 0;
+
+out_unmap:
+ perf_evlist__munmap(evlist);
+ return -1;
+}
+
static int
mmap_per_cpu(struct perf_evlist *evlist, struct perf_evlist_mmap_ops *ops,
struct perf_mmap_param *mp)
@@ -528,6 +572,8 @@ mmap_per_cpu(struct perf_evlist *evlist, struct perf_evlist_mmap_ops *ops,
int nr_mmaps = 0;
int cpu, thread;
+ pr_debug("%s: nr cpu values %d nr threads %d\n", __func__, nr_cpus, nr_threads);
+
for (cpu = 0; cpu < nr_cpus; cpu++) {
int output = -1;
int output_overwrite = -1;
@@ -569,6 +615,7 @@ int perf_evlist__mmap_ops(struct perf_evlist *evlist,
struct perf_evlist_mmap_ops *ops,
struct perf_mmap_param *mp)
{
+ const struct perf_cpu_map *cpus = evlist->all_cpus;
struct perf_evsel *evsel;
if (!ops || !ops->get || !ops->mmap)
@@ -588,6 +635,9 @@ int perf_evlist__mmap_ops(struct perf_evlist *evlist,
if (evlist->pollfd.entries == NULL && perf_evlist__alloc_pollfd(evlist) < 0)
return -ENOMEM;
+ if (perf_cpu_map__empty(cpus))
+ return mmap_per_thread(evlist, ops, mp);
+
return mmap_per_cpu(evlist, ops, mp);
}
--
2.25.1