linux-kernel - Re: [PATCH] perf/tests: Fix test case 95 on s390 and use same event

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Date:   Thu, 28 Jul 2022 13:28:51 +0100
From:   German Gomez <german.gomez@....com>
To:     Thomas Richter <tmricht@...ux.ibm.com>,
        linux-kernel@...r.kernel.org, linux-perf-users@...r.kernel.org,
        acme@...nel.org, james.clark@....com
Cc:     svens@...ux.ibm.com, gor@...ux.ibm.com, sumanthk@...ux.ibm.com,
        hca@...ux.ibm.com, Namhyung Kim <namhyung@...nel.org>,
        Jiri Olsa <jolsa@...nel.org>
Subject: Re: [PATCH] perf/tests: Fix test case 95 on s390 and use same event


On 28/07/2022 13:20, Thomas Richter wrote:
> On 7/28/22 11:25, German Gomez wrote:
>> Thanks for the fix, Thomas
>>
>> On 27/07/2022 15:14, Thomas Richter wrote:
>>> On linux-next tree perf test 95 was added recently.
>>> s390 does not support branch sampling at all and the test case fails
>>> despite for checking branch support before hand.
>>> The check for support of branching
>>> uses the software event named dummy, as seen in the line:
>>>
>>>  perf record -b -o- -e dummy -B true > /dev/null 2>&1 || exit 2
>> Just curious, do you know why the command succeeds in this platform (and
>> potentially others)?
>>
>> I got the idea of using "dummy" from [1] but only tested on arm64 and
>> x86. I thought the platforms would reject it if -b was not implemented
>> regardless of the event. Did I misunderstand the use of dummy?
>>
>> Thanks,
>> German
>>
>> [1] https://lore.kernel.org/all/20220617073840.GA45710@leoy-ThinkPad-X240s/
>>
> Well, dummy is a predefined event of type software, you see that when you add
> some -v options as in 
> # perf record -e dummy -vvv
> DEBUGINFOD_URLS=
> Using CPUID IBM,3931,704,A01,3.7,002f
> nr_cblocks: 0
> affinity: SYS
> mmap flush: 1
> comp level: 0
> ------------------------------------------------------------
> perf_event_attr:
>   type                             1      <----------- type 1 is software
>   size                             128
>   config                           0x9
>   { sample_period, sample_freq }   4000
>
>
> So this event is never sent to an hardware PMU (which would have type 0) and thus
> mostly succeeds where as the perf record command without event uses default
> event cycles. And that one is sent to hardware PMU on s390...

Thanks for explaining! My bad for missing this.

Acked-by: German Gomez <german.gomez@....com>

>