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

[Anne Macedo]

Anne Macedo <retpolanne@xxxxxxxxxx> writes:

> Namhyung Kim <namhyung@xxxxxxxxxx> writes:
>
>> Hello Anne,
>>
>> On Sat, Mar 9, 2024 at 5:27 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 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 | 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.
>>
>
> Let me test with CONFIG_TRACEPOINTS=n then.
>
Thanks for pointing out, with CONFIG_TRACEPOINTS=n perf lock contention
won't work, but I tried with a faux symbol and got a segfault. v2 will
handle that.
>
>> 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.

However, if we fallback to trace_contention_begin, we won't be able to
filter out both __traceiter_contention_begin and trace_contention_begin
at the same time.

>>
> ACK :)
>>
>> 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
>>>
Thanks, Anne