Re: [PATCH v1] perf lock contention: skip traceiter functions

Namhyung Kim
2024-03-11T18:17:14Z

Hello Anne,

On Sat, Mar 9, 2024 at 5:27 AM Anne Macedo <retpolanne@posteo.net> wrote:
>
> The perf lock contention program currently shows the caller of the locks
> as __traceiter_contention_begin+0x??. This caller can be ignored, as it is
> from the traceiter itself. Instead, it should show the real callers for
> the locks.
>
> When fiddling with the --stack-skip parameter, the actual callers for
> the locks start to show up. However, just ignore the
> __traceiter_contention_begin and the __traceiter_contention_end symbols
> so the actual callers will show up.
>
> Before this patch is applied:
>
> sudo perf lock con -a -b -- sleep 3
> contended total wait max wait avg wait type caller
>
> 8 2.33 s 2.28 s 291.18 ms rwlock:W __traceiter_contention_begin+0x44
> 4 2.33 s 2.28 s 582.35 ms rwlock:W __traceiter_contention_begin+0x44
> 7 140.30 ms 46.77 ms 20.04 ms rwlock:W __traceiter_contention_begin+0x44
> 2 63.35 ms 33.76 ms 31.68 ms mutex trace_contention_begin+0x84
> 2 46.74 ms 46.73 ms 23.37 ms rwlock:W __traceiter_contention_begin+0x44
> 1 13.54 us 13.54 us 13.54 us mutex trace_contention_begin+0x84
> 1 3.67 us 3.67 us 3.67 us rwsem:R __traceiter_contention_begin+0x44
>
> Before this patch is applied - using --stack-skip 5
>
> sudo perf lock con --stack-skip 5 -a -b -- sleep 3
> contended total wait max wait avg wait type caller
>
> 2 2.24 s 2.24 s 1.12 s rwlock:W do_epoll_wait+0x5a0
> 4 1.65 s 824.21 ms 412.08 ms rwlock:W do_exit+0x338
> 2 824.35 ms 824.29 ms 412.17 ms spinlock get_signal+0x108
> 2 824.14 ms 824.14 ms 412.07 ms rwlock:W release_task+0x68
> 1 25.22 ms 25.22 ms 25.22 ms mutex cgroup_kn_lock_live+0x58
> 1 24.71 us 24.71 us 24.71 us spinlock do_exit+0x44
> 1 22.04 us 22.04 us 22.04 us rwsem:R lock_mm_and_find_vma+0xb0
>
> After this patch is applied:
>
> sudo ./perf lock con -a -b -- sleep 3
> contended total wait max wait avg wait type caller
>
> 4 4.13 s 2.07 s 1.03 s rwlock:W release_task+0x68
> 2 2.07 s 2.07 s 1.03 s rwlock:R mm_update_next_owner+0x50
> 2 2.07 s 2.07 s 1.03 s rwlock:W do_exit+0x338
> 1 41.56 ms 41.56 ms 41.56 ms mutex cgroup_kn_lock_live+0x58
> 2 36.12 us 18.83 us 18.06 us rwlock:W do_exit+0x338
>
> changes since v0:
>
> - skip trace_contention functions
> - use sym->end instead of __traceiter_contention_end for text_end
>
> Signed-off-by: Anne Macedo <retpolanne@posteo.net>
> ---
> tools/perf/util/machine.c | 17 +++++++++++++++++
> tools/perf/util/machine.h | 2 +-
> 2 files changed, 18 insertions(+), 1 deletion(-)
>
> diff --git a/tools/perf/util/machine.c b/tools/perf/util/machine.c
> index 527517db3182..db443947ccd1 100644
> --- a/tools/perf/util/machine.c
> +++ b/tools/perf/util/machine.c
> @@ -3266,6 +3266,14 @@ bool machine__is_lock_function(struct machine *machine, u64 addr)
>
> sym = machine__find_kernel_symbol_by_name(machine, "__lock_text_end", &kmap);
> machine->lock.text_end = map__unmap_ip(kmap, sym->start);
> +
> + sym = machine__find_kernel_symbol_by_name(machine, "__traceiter_contention_begin", &kmap);

Unlike __sched_text_{start,end} and __lock_text_{start,end} I guess
this traceiter thing is optional so it might not be there. You need to
handle if it's NULL.

I think it depends on the kernel version and configuration. I remember
I saw a different symbol on old kernels. But it'd be hard to handle all
the cases. Let's have a single trace text section in the struct machine
and use __traceiter_contention_begin only. If it's not found you can
fallback to trace_contention_begin.

Thanks,
Namhyung

> + machine->traceiter.text_start = map__unmap_ip(kmap, sym->start);
> + machine->traceiter.text_end = map__unmap_ip(kmap, sym->end);
> +
> + sym = machine__find_kernel_symbol_by_name(machine, "trace_contention_begin", &kmap);
> + machine->trace.text_start = map__unmap_ip(kmap, sym->start);
> + machine->trace.text_end = map__unmap_ip(kmap, sym->end);
> }
>
> /* failed to get kernel symbols */
> @@ -3280,5 +3288,14 @@ bool machine__is_lock_function(struct machine *machine, u64 addr)
> if (machine->lock.text_start <= addr && addr < machine->lock.text_end)
> return true;
>
> + /* traceiter functions currently don't have their own section
> + * but we consider them lock functions
> + */
> + if (machine->traceiter.text_start <= addr && addr < machine->traceiter.text_end)
> + return true;
> +
> + if (machine->trace.text_start <= addr && addr < machine->trace.text_end)
> + return true;
> +
> return false;
> }
> diff --git a/tools/perf/util/machine.h b/tools/perf/util/machine.h
> index e28c787616fe..4312f6db6de0 100644
> --- a/tools/perf/util/machine.h
> +++ b/tools/perf/util/machine.h
> @@ -49,7 +49,7 @@ struct machine {
> struct {
> u64 text_start;
> u64 text_end;
> - } sched, lock;
> + } sched, lock, traceiter, trace;
> pid_t *current_tid;
> size_t current_tid_sz;
> union { /* Tool specific area */
> --
> 2.39.2
>