Re: [PATCH v3] perf lock contention: skip traceiter functions

[Namhyung Kim]

On Fri, Mar 15, 2024 at 4:19 AM Anne Macedo <retpolanne@xxxxxxxxxx> 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 v2:
>
> - add dummy value to machine->traceiter and machine->trace to make
> necessary checks, fixing possible null pointer access
>
> changes since v1:
>
> - consider trace_contention and __traceiter_contention functions as
> optional (i.e. check if sym is null to avoid segfault)
>
> changes since v0:
>
> - skip trace_contention functions
> - use sym->end instead of __traceiter_contention_end for text_end
>
> Signed-off-by: Anne Macedo <retpolanne@xxxxxxxxxx>
> ---
>  tools/perf/util/machine.c | 29 +++++++++++++++++++++++++++++
>  tools/perf/util/machine.h |  2 +-
>  2 files changed, 30 insertions(+), 1 deletion(-)
>
> diff --git a/tools/perf/util/machine.c b/tools/perf/util/machine.c
> index 527517db3182..373a3b852f72 100644
> --- a/tools/perf/util/machine.c
> +++ b/tools/perf/util/machine.c
> @@ -3266,6 +3266,22 @@ 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);
> +               if (sym) {
> +                       machine->traceiter.text_start = map__unmap_ip(kmap, sym->start);
> +                       machine->traceiter.text_end = map__unmap_ip(kmap, sym->end);
> +               } else {
> +                       machine->traceiter.text_start = 1;
> +               }

I think you can remove the else part as it's initialized to 0
and the addresses cannot be 0.

> +
> +               sym = machine__find_kernel_symbol_by_name(machine, "trace_contention_begin", &kmap);
> +               if (sym) {
> +                       machine->trace.text_start = map__unmap_ip(kmap, sym->start);
> +                       machine->trace.text_end = map__unmap_ip(kmap, sym->end);
> +               } else {
> +                       machine->trace.text_start = 1;
> +               }

Ditto.


>         }
>
>         /* failed to get kernel symbols */
> @@ -3280,5 +3296,18 @@ 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 != 1) {

Then you can just check if it's non-zero here.


> +               if (machine->traceiter.text_start <= addr && addr < machine->traceiter.text_end)
> +                       return true;
> +       }
> +
> +       if (machine->trace.text_start != 1) {

And here too.

Thanks,
Namhyung


> +               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
>