linux-kernel - Re: [PATCH v2] selftests/ftrace: Do not trace do_softirq because of PREEMPT

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Message-ID: <546f147d-1682-fb92-42e5-7247dc19dcf1@canonical.com>
Date:   Thu, 10 Feb 2022 15:05:24 +0100
From:   Krzysztof Kozlowski <krzysztof.kozlowski@...onical.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>, Shuah Khan <shuah@...nel.org>,
        linux-kselftest@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-rt-users@...r.kernel.org, joseph.salisbury@...onical.com,
        Shuah Khan <skhan@...uxfoundation.org>
Subject: Re: [PATCH v2] selftests/ftrace: Do not trace do_softirq because of
 PREEMPT_RT

On 10/02/2022 14:47, Sebastian Andrzej Siewior wrote:
> On 2022-02-10 09:33:56 [+0100], Krzysztof Kozlowski wrote:
>> The PREEMPT_RT patchset does not use soft IRQs thus trying to filter for
>> do_softirq fails for such kernel:
> 
> PREEMPT_RT does use soft IRQs.

Correct. It does not use do_softirq() code, but follows different path
with ksoftirqd.
Shall I rephrase it towards something like this? Or maybe you have some
more accurate description?

The implementation detail is that do_softirq() is in ifndef.

> 
>>   echo do_softirq
>>   ftracetest: 81: echo: echo: I/O error
>>
>> Choose some other visible function for the test.
>>
> …
> 
>> --- a/tools/testing/selftests/ftrace/test.d/ftrace/func_set_ftrace_file.tc
>> +++ b/tools/testing/selftests/ftrace/test.d/ftrace/func_set_ftrace_file.tc
>> @@ -19,7 +19,7 @@ fail() { # mesg
>>  
>>  FILTER=set_ftrace_filter
>>  FUNC1="schedule"
>> -FUNC2="do_softirq"
>> +FUNC2="scheduler_tick"
> 
> What is the purpose of this?
> 
>>  ALL_FUNCS="#### all functions enabled ####"
>>  
> 
> Sebastian


Best regards,
Krzysztof