Re: [PATCH v1 0/4] Prefer sysfs/JSON events also when no PMU is provided

[James Clark <james.clark@linaro.org>]

On 08/11/2024 6:37 pm, Atish Kumar Patra wrote:
> On Fri, Nov 8, 2024 at 4:16 AM James Clark <james.clark@linaro.org> wrote:
>>
>>
>>
>> On 07/11/2024 18:51, Ian Rogers wrote:
>>> On Sat, Oct 26, 2024 at 5:18 AM Ian Rogers <irogers@google.com> wrote:
>>>>
>>>> At the RISC-V summit the topic of avoiding event data being in the
>>>> RISC-V PMU kernel driver came up. There is a preference for sysfs/JSON
>>>> events being the priority when no PMU is provided so that legacy
>>>> events maybe supported via json. Originally Mark Rutland also
>>>> expressed at LPC 2023 that doing this would resolve bugs on ARM Apple
>>>> M? processors, but James Clark more recently tested this and believes
>>>> the driver issues there may not have existed or have been resolved. In
>>>> any case, it is inconsistent that with a PMU event names avoid legacy
>>>> encodings, but when wildcarding PMUs (ie without a PMU with the event
>>>> name) the legacy encodings have priority.
>>>>
>>>> The patch doing this work was reverted in a v6.10 release candidate
>>>> as, even though the patch was posted for weeks and had been on
>>>> linux-next for weeks without issue, Linus was in the habit of using
>>>> explicit legacy events with unsupported precision options on his
>>>> Neoverse-N1. This machine has SLC PMU events for bus and CPU cycles
>>>> where ARM decided to call the events bus_cycles and cycles, the latter
>>>> being also a legacy event name. ARM haven't renamed the cycles event
>>>> to a more consistent cpu_cycles and avoided the problem. With these
>>>> changes the problematic event will now be skipped, a large warning
>>>> produced, and perf record will continue for the other PMU events. This
>>>> solution was proposed by Arnaldo.
>>>>
>>>> Two minor changes have been added to help with the error message and
>>>> to work around issues occurring with "perf stat metrics (shadow stat)
>>>> test".
>>>>
>>>> The patches have only been tested on my x86 non-hybrid laptop.
>>>
>>> Hi Atish and James,
>>>
>>> Could I get your tags for this series?
>>>
> 
> Hi Ian,
> Thanks for your patches. It definitely helps to have a clean slate
> implementation
> for the perf tool. However, I have some open questions about other use cases
> that we discussed during the RVI Summit.
> 
>>> The patches were originally motivated by wanting to make the behavior
>>> of events parsed like "cycles" match that of "cpu/cycles/", the PMU is
>>> wildcarded to "cpu" in the first case. This was divergent because of
>>> ARM we switched from preferring legacy (type = PERF_TYPE_HARDWARE,
>>> config = PERF_COUNT_HW_CPU_CYCLES) to sysfs/json (type=<core PMU's
>>> type>, config=<encoding from event>) when a PMU name was given. This
>>> aligns with RISC-V wanting to use json encodings to avoid complexity
>>> in the PMU driver.
>>>
>>
>> I couldn't find the thread, but I remember fairly recently it was
>> mentioned that RISC-V would be supporting the legacy events after all,
>> maybe it was a comment from Atish? I'm not sure if that changes the
>> requirements for this or not?
>>
>> I still can't really imagine how tooling would work if every tool has to
>> maintain the mappings of basic events like instructions and branches.
>> For example all the perf_event_open tests in ltp use the legacy events.
>>
> 
> No it has not changed. While this series helps to avoid clunky vendor
> specific encodings
> in the driver for perf tool, I am still unsure how we will manage
> other applications
> (directly passing legacy events through perf_event_open or
> perf_evlist__open) will work.
> 
> I have only anecdotal data about folks relying perf legacy events
> directly to profile
> their application. All of them use mostly cycle/instruction events though.
> Are there any users who use other legacy events directly without perf tool ?
> 
> If not, we may have only cycle/instruction mapping in the driver and
> rely on json for everything else.
> The other use case is virtualization. I have been playing with these
> patches to find a clean solution to
> enable all the use cases. If you have any other ideas, please let me know.
> 

Yeah I would expect it's mostly cycles and instructions. I searched a 
bit for PERF_COUNT_HW_BRANCH_MISSES and only found tool/developer type 
usages, which I suppose we could expect to have to handle the mappings 
like perf. Although it's not the easiest thing to search for and 
obviously that only includes open source.

Usages do exist though, there are people posting on stack overflow using 
it, and other various bug tracker listings. So you would expect those 
same users to have to use raw event codes and some switch statement to 
pick the right one for their hardware, or use Perf.