Re: [Patch v2 3/5] perf x86/topdown: Don't move topdown metrics events when sorting events

Mi, Dapeng
2024-07-11T04:48:12Z

On 7/10/2024 11:07 PM, Ian Rogers wrote:
> On Wed, Jul 10, 2024 at 2:40 AM Mi, Dapeng <dapeng1.mi@linux.intel.com> wrote:
>>
>> On 7/10/2024 6:37 AM, Ian Rogers wrote:
>>> On Mon, Jul 8, 2024 at 9:18 PM Mi, Dapeng <dapeng1.mi@linux.intel.com> wrote:
>>>> On 7/8/2024 11:08 PM, Ian Rogers wrote:
>>>>> On Mon, Jul 8, 2024 at 12:40 AM Dapeng Mi <dapeng1.mi@linux.intel.com> wrote:
>>>>>> when running below perf command, we say error is reported.
>>>>>>
>>>>>> perf record -e "{slots,instructions,topdown-retiring}:S" -vv -C0 sleep 1
>>>>>>
>>>>>> ------------------------------------------------------------
>>>>>> perf_event_attr:
>>>>>> type 4 (cpu)
>>>>>> size 168
>>>>>> config 0x400 (slots)
>>>>>> sample_type IP|TID|TIME|READ|CPU|PERIOD|IDENTIFIER
>>>>>> read_format ID|GROUP|LOST
>>>>>> disabled 1
>>>>>> sample_id_all 1
>>>>>> exclude_guest 1
>>>>>> ------------------------------------------------------------
>>>>>> sys_perf_event_open: pid -1 cpu 0 group_fd -1 flags 0x8 = 5
>>>>>> ------------------------------------------------------------
>>>>>> perf_event_attr:
>>>>>> type 4 (cpu)
>>>>>> size 168
>>>>>> config 0x8000 (topdown-retiring)
>>>>>> { sample_period, sample_freq } 4000
>>>>>> sample_type IP|TID|TIME|READ|CPU|PERIOD|IDENTIFIER
>>>>>> read_format ID|GROUP|LOST
>>>>>> freq 1
>>>>>> sample_id_all 1
>>>>>> exclude_guest 1
>>>>>> ------------------------------------------------------------
>>>>>> sys_perf_event_open: pid -1 cpu 0 group_fd 5 flags 0x8
>>>>>> sys_perf_event_open failed, error -22
>>>>>>
>>>>>> Error:
>>>>>> The sys_perf_event_open() syscall returned with 22 (Invalid argument) for event (topdown-retiring).
>>>>>>
>>>>>> The reason of error is that the events are regrouped and
>>>>>> topdown-retiring event is moved to closely after the slots event and
>>>>>> topdown-retiring event needs to do the sampling, but Intel PMU driver
>>>>>> doesn't support to sample topdown metrics events.
>>>>>>
>>>>>> For topdown metrics events, it just requires to be in a group which has
>>>>>> slots event as leader. It doesn't require topdown metrics event must be
>>>>>> closely after slots event. Thus it's a overkill to move topdown metrics
>>>>>> event closely after slots event in events regrouping and furtherly cause
>>>>>> the above issue.
>>>>>>
>>>>>> Thus delete the code that moving topdown metrics events to fix the
>>>>>> issue.
>>>>> I think this is wrong. The topdown events may not be in a group, such
>>>>> cases can come from metrics due to grouping constraints, and so they
>>>>> must be sorted together so that they may be gathered into a group to
>>>>> avoid the perf event opens failing for ungrouped topdown events. I'm
>>>>> not understanding what these patches are trying to do, if you want to
>>>>> prioritize the event for leader sampling why not modify it to compare
>>>> Per my understanding, this change doesn't break anything. The events
>>>> regrouping can be divided into below several cases.
>>>>
>>>> a. all events in a group
>>>>
>>>> perf stat -e "{instructions,topdown-retiring,slots}" -C0 sleep 1
>>>> WARNING: events were regrouped to match PMUs
>>>>
>>>> Performance counter stats for 'CPU(s) 0':
>>>>
>>>> 15,066,240 slots
>>>> 1,899,760 instructions
>>>> 2,126,998 topdown-retiring
>>>>
>>>> 1.045783464 seconds time elapsed
>>>>
>>>> In this case, slots event would be adjusted as the leader event and all
>>>> events are still in same group.
>>>>
>>>> b. all events not in a group
>>>>
>>>> perf stat -e "instructions,topdown-retiring,slots" -C0 sleep 1
>>>> WARNING: events were regrouped to match PMUs
>>>>
>>>> Performance counter stats for 'CPU(s) 0':
>>>>
>>>> 2,045,561 instructions
>>>> 17,108,370 slots
>>>> 2,281,116 topdown-retiring
>>>>
>>>> 1.045639284 seconds time elapsed
>>>>
>>>> In this case, slots and topdown-retiring are placed into a group and slots
>>>> is the group leader. instructions event is outside the group.
>>>>
>>>> c. slots event in group but topdown metric events outside the group
>>>>
>>>> perf stat -e "{instructions,slots},topdown-retiring" -C0 sleep 1
>>>> WARNING: events were regrouped to match PMUs
>>>>
>>>> Performance counter stats for 'CPU(s) 0':
>>>>
>>>> 20,323,878 slots
>>>> 2,634,884 instructions
>>>> 3,028,656 topdown-retiring
>>>>
>>>> 1.045076380 seconds time elapsed
>>>>
>>>> In this case, topdown-retiring event is placed into previous group and
>>>> slots is adjusted to leader event.
>>>>
>>>> d. multiple event groups
>>>>
>>>> perf stat -e "{instructions,slots},{topdown-retiring}" -C0 sleep 1
>>>> WARNING: events were regrouped to match PMUs
>>>>
>>>> Performance counter stats for 'CPU(s) 0':
>>>>
>>>> 26,319,024 slots
>>>> 2,427,791 instructions
>>>> 2,683,508 topdown-retiring
>>>>
>>>> 1.045495830 seconds time elapsed
>>>>
>>>> In this case, the two groups are merged to one group and slots event is
>>>> adjusted as leader.
>>>>
>>>> The key point of this patch is that it's unnecessary to move topdown
>>>> metrics events closely after slots event. It's a overkill since Intel core
>>>> PMU driver doesn't require that. Intel PMU driver just requires topdown
>>>> metrics events are in a group where slots event is the group leader, and
>>>> worse the movement for topdown metrics events causes the issue in the
>>>> commit message mentioned.
>>>>
>>>> This patch doesn't block to regroup topdown metrics event. It just removes
>>>> the unnecessary movement for topdown metrics events.
>>> But you will get the same behavior because of the non-arch dependent
>>> force group index - I guess you don't care as the sample read only
>>> happens when you have a group.
>>>
>>> I'm thinking of cases like (which admittedly is broken):
>>> ```
>>> $ perf stat -e "{slots,instructions},cycles,topdown-fe-bound" -a sleep 0.1
>>> [sudo] password for irogers:
>>>
>>> Performance counter stats for 'system wide':
>>>
>>> 2,589,345,900 slots
>>> 852,492,838 instructions
>>> 583,525,372 cycles
>>> <not supported> topdown-fe-bound
>>>
>>> 0.103930790 seconds time elapsed
>>> ```
>> I run the upstream code (commit 73e931504f8e0d42978bfcda37b323dbbd1afc08)
>> without this patchset, I see same issue.
>>
>> perf stat -e "{slots,instructions},cycles,topdown-fe-bound" -a sleep 0.1
>>
>> Performance counter stats for 'system wide':
>>
>> 262,448,922 slots
>> 29,630,373 instructions
>> 43,891,902 cycles
>> <not supported> topdown-fe-bound
>>
>> 0.150369560 seconds time elapsed
>>
>> #perf -v
>> perf version 6.10.rc6.g73e931504f8e
>>
>> This issue is not caused by this patchset.
> I agree, but I think what is broken above was caused by the forced
> grouping change that I did for Andi. The point of your change here is
> to say that topdown events don't need to be moved while sorting, but
> what should be happening here is just that. topdown-fe-bound should be
> moved into the group with slots and instructions so it isn't "<not
> supported>". So yes the current code has issues, but that's not the
> same as saying we don't need to move topdown events, we do so that we
> may group them better.
>
> Thanks,
> Ian

I see your point. I think the key is to ensure the topdown metrics events
in a group which has slots as the leader. As for where the topdown metrics
event is in the group, it doesn't matter. I would see if there is a better
method to fix this issue. Thanks.

>
>>> As the slots event is grouped there's no force group index on it, we
>>> want to shuffle the topdown-fe-bound into the group so we want it to
>>> compare as less than cycles - ie we're comparing topdown events with
>>> non topdown events and trying to shuffle the topdown events first.
>> Current evlist__cmp() won't really swap the order of cycles and
>> topdown-fe-bound.
>>
>> if (lhs_sort_idx != rhs_sort_idx)
>> return lhs_sort_idx - rhs_sort_idx;
>>
>> When comparing cycles and topdown-fe-bound events, lhs_sort_idx is 2 and
>> rhs_sort_idx is 3, so the swap won't happen.
>>
>> So the event sequence after sorting is still "slots, instructions ,cycles,
>> topdown-fe-bound". Both cycles and topdown-fe-bound events won't be placed
>> into the group.
>>
>>
>>> Thanks,
>>> Ian
>>>
>>>
>>>
>>>>> first?
>>>>>
>>>>> Thanks,
>>>>> Ian
>>>>>
>>>>>> Signed-off-by: Dapeng Mi <dapeng1.mi@linux.intel.com>
>>>>>> ---
>>>>>> tools/perf/arch/x86/util/evlist.c | 5 -----
>>>>>> 1 file changed, 5 deletions(-)
>>>>>>
>>>>>> diff --git a/tools/perf/arch/x86/util/evlist.c b/tools/perf/arch/x86/util/evlist.c
>>>>>> index 332e8907f43e..6046981d61cf 100644
>>>>>> --- a/tools/perf/arch/x86/util/evlist.c
>>>>>> +++ b/tools/perf/arch/x86/util/evlist.c
>>>>>> @@ -82,11 +82,6 @@ int arch_evlist__cmp(const struct evsel *lhs, const struct evsel *rhs)
>>>>>> return -1;
>>>>>> if (arch_is_topdown_slots(rhs))
>>>>>> return 1;
>>>>>> - /* Followed by topdown events. */
>>>>>> - if (arch_is_topdown_metrics(lhs) && !arch_is_topdown_metrics(rhs))
>>>>>> - return -1;
>>>>>> - if (!arch_is_topdown_metrics(lhs) && arch_is_topdown_metrics(rhs))
>>>>>> - return 1;
>>>>>> }
>>>>>>
>>>>>> /* Default ordering by insertion index. */
>>>>>> --
>>>>>> 2.40.1
>>>>>>