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

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Date:   Thu, 10 Feb 2022 09:10:55 +0100
From:   Krzysztof Kozlowski <krzysztof.kozlowski@...onical.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     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
Subject: Re: [PATCH] selftests/ftrace: Do not trace do_softirq because of
 PREEMPT_RT

On 10/02/2022 06:00, Steven Rostedt wrote:
> On Wed,  9 Feb 2022 17:24:30 +0100
> Krzysztof Kozlowski <krzysztof.kozlowski@...onical.com> wrote:
> 
>> The PREEMPT_RT patchset does not use soft IRQs thus trying to filter for
>> do_softirq fails for such kernel:
>>
>>   echo do_softirq
>>   ftracetest: 81: echo: echo: I/O error
>>
>> Choose some other externally visible function for the test.
>>
>> Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@...onical.com>
>>
>> ---
>>
>> I understand that the failure does not exist on mainline kernel (only
>> with PREEMPT_RT patchset) but the change does not harm it.
>>
>> If it is not suitable alone, please consider it for RT patchset.
>> ---
>>  .../selftests/ftrace/test.d/ftrace/func_set_ftrace_file.tc      | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git 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
>> index e96e279e0533..1d0c7601865f 100644
>> --- 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="_printk"
> 
> The problem with the above, is that it is not guaranteed to trigger (and
> probably will not), where as the do_softirq is. The filtering is suppose to
> trace something that actually happens.
> 
> How about using: scheduler_tick ?
> 

This works as well. I had impression that the test is only about
filtering interface and actual function does not have to hit/trigger.

I'll send v2 with scheduler_tick.


Best regards,
Krzysztof