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

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Message-ID: <429dd2fd-3ceb-6780-f970-4ea9b115388c@canonical.com>
Date:   Thu, 10 Feb 2022 15:13:15 +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 15:10, Sebastian Andrzej Siewior wrote:
> On 2022-02-10 15:05:24 [+0100], Krzysztof Kozlowski wrote:
>> 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?
> 
> It would be good to describe what the purpose of the change in terms of
> the actual problem and the aimed solution.

The purpose was explain - fix a failing test with PREEMPT_RT. I am not
planning to rework entire test, it is merely a fix.

> 
>> The implementation detail is that do_softirq() is in ifndef.
> 
> So let me ask again.  We have
>    FUNC1="schedule"
>    FUNC2="do_softirq"
> 
> What is the purpose of this? Do you need FUNC2 when ksoftirqd is run or
> when softirqs are served? Not sure how scheduler_tick fits in all this.

I guess this is more a question to the author of the test. Unless you
are now questioning the entire purpose of this test?

Best regards,
Krzysztof