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

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Message-ID: <c1987667-c4e5-4c73-8de1-2acacc967c40@linaro.org>
Date: Fri, 18 Oct 2024 11:18:54 +0100
From: James Clark <james.clark@...aro.org>
To: Namhyung Kim <namhyung@...nel.org>, Ian Rogers <irogers@...gle.com>
Cc: linux-perf-users@...r.kernel.org, acme@...nel.org,
 Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>,
 Mark Rutland <mark.rutland@....com>,
 Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
 Jiri Olsa <jolsa@...nel.org>, Adrian Hunter <adrian.hunter@...el.com>,
 "Liang, Kan" <kan.liang@...ux.intel.com>, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 1/1] libperf: evlist: Fix --cpu argument on hybrid
 platform



On 18/10/2024 00:02, Namhyung Kim wrote:
> On Tue, Oct 15, 2024 at 03:54:15PM +0100, James Clark 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.
>>
>> Fix it by deleting evsels with empty CPU maps in the specific case where
>> user requested CPU maps are evaluated.
> 
> I think there's a discussion about making default events skippable and
> hide them in the output unless all of them fail.
> 
> Thanks,
> Namhyung
> 

In this case it's still broken if you do -e cycles, not just for default 
events. So I'm not sure if that would help.

James