Re: [PATCH v1] perf evlist: Force adding default events only to core PMUs

[James Clark]

On 29/05/2024 18:33, Ian Rogers wrote:
> On Wed, May 29, 2024 at 7:50 AM James Clark <james.clark@xxxxxxx> wrote:
>>
>> On 28/05/2024 20:51, Ian Rogers wrote:
>>> On Tue, May 28, 2024 at 12:44 PM Arnaldo Carvalho de Melo
>>> <acme@xxxxxxxxxx> wrote:
>>>>
>>>> On Mon, May 27, 2024 at 10:36:45PM -0700, Ian Rogers wrote:
>>>>> On Mon, May 27, 2024 at 3:58 AM Leo Yan <leo.yan@xxxxxxxxx> wrote:
>>>>>> On Sat, May 25, 2024 at 02:14:26PM -0700, Linus Torvalds wrote:
>>>>>>> On Sat, 25 May 2024 at 09:43, Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>>>>
>>>>>>>> This makes 'perf record' work for me again.
>>>>
>>>>>>> Oh, wait, no it doesn't.
>>>>
>>>>>>> It makes just the plain "perf record" without any arguments work,
>>>>>>> which was what I was testing because I was lazy.
>>>>
>>>>>>> So now
>>>>
>>>>>>>     $ perf record sleep 1
>>>>
>>>>>>> works fine. But
>>>>
>>>>>>>     $ perf record -e cycles:pp sleep 1
>>>>
>>>>>>> is still completely broken (with or without ":p" and ":pp").
>>>>
>>>>>> Seems to me that this patch fails to check if a PMU is a core-attached
>>>>>> PMU that can support common hardware events. Therefore, we should
>>>>>> consider adding the following check.
>>>>
>>>>>> +++ b/tools/perf/util/parse-events.c
>>>>>> @@ -1594,6 +1594,9 @@ int parse_events_multi_pmu_add(struct parse_events_state *parse_state,
>>>>>>         while ((pmu = perf_pmus__scan(pmu)) != NULL) {
>>>>>>                 bool auto_merge_stats;
>>>>>>
>>>>>> +               if (hw_config != PERF_COUNT_HW_MAX && !pmu->is_core)
>>>>>> +                       continue;
>>>>>> +
>>>>>>                 if (parse_events__filter_pmu(parse_state, pmu))
>>>>>>                         continue;
>>>>
>>>>>> To be clear, I only compiled this change but I have no chance to test
>>>>>> it. @Ian, could you confirm this?
>>>>
>>>>> Hi Leo,
>>>>
>>>>> so the code is working as intended. I believe it also agrees with what
>>>>> Arnaldo thinks.
>>>>
>>>>> If you do:
>>>>
>>>>> $ perf stat -e cycles ...
>>>>
>>>>> and you have
>>>>
>>>>> /sys/devices/pmu1/events/cycles
>>>>> /sys/devices/pmu2/events/cycles
>>>>
>>>>> The output of perf stat should contain counts for pmu1 and pmu2. Were
>>>>> the event 'data_read' or 'inst_retired.any' we wouldn't be having the
>>>>
>>>> Sure, what is being asked is to count events and if those two events in
>>>> those two PMUs can count, then do what the user asked.
>>>>
>>>> For 'perf record' we're asking for sampling, if the event has the name
>>>> specified and can't be sampled, skip it, warn the user and even so
>>>> only if verbose mode is asked, something like:
>>>>
>>>>   root@x1:~# perf record -e cycles -a sleep 1
>>>>   [ perf record: Woken up 1 times to write data ]
>>>>   [ perf record: Captured and wrote 1.998 MB perf.data (4472 samples) ]
>>>>   root@x1:~# perf evlist
>>>>   cpu_atom/cycles/
>>>>   cpu_core/cycles/
>>>>   dummy:u
>>>>   root@x1:~#
>>>>
>>>> Cool, there are two 'cycles' events, one in a PMU named 'cpu_atom',
>>>> another in a 'cpu_core' one, both can be sampled, my workload may
>>>> run/use resources on then, I'm interested, sample both.
>>>>
>>>> But if we had some other PMU, to use a name Jiri uses in tests/fake
>>>> PMUs, the 'krava' PMU and it has a 'cycles' event, so 'krava/cycles/'
>>>> and for some reason it doesn't support sampling, skip it, then the
>>>> result should be the same as above.
>>>>
>>>> If the user finds it strange after looking at sysfs that 'krava/cycles/'
>>>> isn't being sampled, the usual workflow is to ask perf for more
>>>> verbosity, using -v (or multiple 'v' letters to get increasing levels of
>>>> verbosity), in which case the user would see:
>>>>
>>>>   root@x1:~# perf record -v -e cycles -a sleep 1
>>>>   WARNING: skipping 'krava/cycles/' event, it doesn't support sampling.
>>>>   [ perf record: Woken up 1 times to write data ]
>>>>   [ perf record: Captured and wrote 1.998 MB perf.data (4472 samples) ]
>>>>   root@x1:~# perf evlist
>>
>> This makes sense to me. I like keeping the old apparent behavior unless
>> -v is used and it will feel like the tool "just works".
>>
>> In the context of the commit summary "perf parse-events: Prefer
>> sysfs/JSON hardware events over legacy":
>>
>> I don't follow why that should be "Prefer, even if it's an event that
>> can't be opened, sysfs/JSON...".
>>
>> Surely it should be "Prefer sysfs/JSON, unless it can't be opened, then
>> use legacy". If all events can be opened, sure go and open them all. If
>> only core events can be opened, do that too. If only uncore events can
>> be opened... etc.
> 

[...]

> So great, ignoring the revert, that fixed everything? Well no. The
> tool in places was hard coding 'struct perf_event_attr' which is of
> course broken were things to be hybrid or BIG.little. So the fix for
> that was to not hard code things. We need a set of 'struct
> perf_event_attr', ah I know a way to get that let's just use our event
> parsing logic. So a 'struct perf_event_attr' hard coding type to
> PERF_TYPE_HARDWARE, the config to PERF_COUNT_HW_CPU_CYCLES and also
> setting the precision to maximum was changed into parsing the string
> "cycles:P". Sounds good, no? Well no. Somebody decided to create an
> ARM event called cycles (Intel's name to avoid conflicts is
> clockticks) and now that event was getting added to our set. Although
> the patch sat for weeks (months?) on linux-next nobody had discovered
> a fairly obvious breakage.
> 

We did see the test failure on our Ampere test machine 7 days ago, but
for some reason only on mainline (I was also on holiday at the same
time). I'm checking if that machine is running all the branches and will
make sure it does from now on.

We are running perf-tools-next on other machines and I try to look at
all the test failures. Just this one had a bit of an obscure combination
of needing the DSU PMU.

One thing we don't have in CI is any Apple M hardware. I can look into
it but I wouldn't have high hopes for anything soon.

[...]

> It isn't new behavior for perf to scan all PMUs, it always has, the
> new behavior is around legacy events. We want multiple PMU scanning
> for hybrid, we want all PMU scanning for uncore. The legacy changes
> happened because of the Apple M? PMU with me being complained at by
> folks at ARM who have now created this mess by their arm_dsu event
> name. Shouldn't it be a 1 liner fix to change "DSU_EVENT_ATTR(cycles,
> 0x11)," to "DSU_EVENT_ATTR(clockticks, 0x11),":
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/perf/arm_dsu_pmu.c#n177
> that's up to ARM but it would make sense to me.
> 

Not sure about that one, that would break anyone's scripts or tools that
are looking at DSU cycles. And it wouldn't fix the issue in the future
if there were other reasons the event doesn't open (like non sampling
core events, or someone's brand new uncore PMU that also has a cycles
event).

It seems like we're converging one something that works though in the
other threads, but I'm still digesting the problems a bit.

>> Because the user could always use the defaults (no argument) or -e
>> cycles and historically Perf correctly picked the one that could be
>> opened. Or if they want the DSU one they could specify it. That can all
>> still work _and_ we can support "prefer sysfs/JSON" as long as we don't
>> prefer it when opening the event doesn't work.
> 
> Hopefully this is all explained above.
> 
> Thanks,
> Ian
> 
>> Thanks
>> James
>>
>>> Thanks,
>>> Ian
>>>
>>>> - Arnaldo