Re: [RFC PATCH 1/1] libperf: evlist: Fix --cpu argument on hybrid platform

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

On 18/10/2024 00:10, Namhyung Kim wrote:
> On Wed, Oct 16, 2024 at 08:01:21AM -0700, Ian Rogers wrote:
>> On Wed, Oct 16, 2024 at 1:29 AM James Clark <james.clark@linaro.org> wrote:
>>>
>>> On 15/10/2024 4:14 pm, Ian Rogers wrote:
>>>> On Tue, Oct 15, 2024 at 7:54 AM James Clark <james.clark@linaro.org> wrote:
>>>>>
>>>>> Since the linked fixes: commit, specifying a CPU on hybrid platforms
>>>>> results in an error because Perf tries to open an extended type event
>>>>> on "any" CPU which isn't valid. Extended type events can only be opened
>>>>> on CPUs that match the type.
>>>>>
>>>>> Before (working):
>>>>>
>>>>>     $ perf record --cpu 1 -- true
>>>>>     [ perf record: Woken up 1 times to write data ]
>>>>>     [ perf record: Captured and wrote 2.385 MB perf.data (7 samples) ]
>>>>>
>>>>> After (not working):
>>>>>
>>>>>     $ perf record -C 1 -- true
>>>>>     WARNING: A requested CPU in '1' is not supported by PMU 'cpu_atom' (CPUs 16-27) for event 'cycles:P'
>>>>>     Error:
>>>>>     The sys_perf_event_open() syscall returned with 22 (Invalid argument) for event (cpu_atom/cycles:P/).
>>>>>     /bin/dmesg | grep -i perf may provide additional information.
>>>>>
>>>>> (Ignore the warning message, that's expected and not particularly
>>>>> relevant to this issue).
>>>>>
>>>>> This is because perf_cpu_map__intersect() of the user specified CPU (1)
>>>>> and one of the PMU's CPUs (16-27) correctly results in an empty (NULL)
>>>>> CPU map. However for the purposes of opening an event, libperf converts
>>>>> empty CPU maps into an any CPU (-1) which the kernel rejects.
>>>>
>>>> Ugh. The cpumap API tries its best to confuse NULL == empty but empty
>>>> can give you dummy, dummy is also known as 'any' or -1, 'any' sounds a
>>>> lot like 'all' but sometimes 'all' is only online CPUs. I tried to
>>>> tidy up the naming a while ago, but there is still a mess.
>>>>
>>>
>>> I don't know if you think this is a good opportunity for me to have a go
>>> at finishing separating those? Or is it a dead end?
>>
>> So cpumap (and threadmap) underpin a lot of things, we also used to
>> routinely confuse CPU numbers with cpumap indices that are used to
>> densely index xyarrays with file descriptors, etc. My thought was that
>> we may end up doing a proper Rust libperf that can be under a more
>> permissive license like libbpf - currently libperf is a source of GPL
>> infection. The rewrite would be a good time to clear these things up.
>> I believe someone at RedHat has looked at doing a Rust libperf.
> 
> I really want to rewrite CPU/thread map related code but didn't have
> time to work on it. :(
> 
> It'd be great if we can rewrite it in Rust!  But current libperf API is
> pretty bad and it's not clearly separated from the tools code.  For
> example, accessing internals like evsel->core.xxx should be changed
> first.
> 
>>
>>>>> Fix it by deleting evsels with empty CPU maps in the specific case where
>>>>> user requested CPU maps are evaluated.
>>>>
>>>> If we delete evsels than the indices can be broken for certain things.
>>>> I'm guessing asan testing is clean but the large number of side data
>>>> structures that are indexed by things in another data structure makes
>>>> the whole code base brittle and I am nervous around this change.
>>>>
>>>>> Fixes: 251aa040244a ("perf parse-events: Wildcard most "numeric" events")
>>>>> Signed-off-by: James Clark <james.clark@linaro.org>
>>>>
>>>> Reviewed-by: Ian Rogers <irogers@google.com>
>>>>
>>>> Thanks,
>>>> Ian
>>>>
>>>
>>> Ok if we're not completely opposed to doing it this way I will dig a bit
>>> more and double check everything is working.
>>
>> I think it is okay to do it this way (hence the reviewed-by tag) as
>> propagate maps should happen before the xyarrays are set up, it'd be
>> nice if these things were checked at runtime, or by the compiler...
> 
> Right, evsel index is used some places probably we need to update it
> too.
> 
> Thanks,
> Namhyung
> 

Ok I'll check for that.

James