Re: [PATCH V2] libperf evlist: Fix per-thread mmaps for multi-threaded targets

From: Namhyung Kim
Date: Tue Sep 06 2022 - 13:50:59 EST


On Mon, Sep 5, 2022 at 4:42 AM Adrian Hunter <adrian.hunter@xxxxxxxxx> wrote:
>
> 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.

Yeah, this is unfortunate. I feel like I need to spend some time on it.

>
> 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) ]

It'd be nice if the example had 2 events so that it could check the
set-output rule actually worked.

Thanks,
Namhyung

>
> 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>