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

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Date:   Thu, 10 Feb 2022 15:48:41 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Krzysztof Kozlowski <krzysztof.kozlowski@...onical.com>
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 2022-02-10 15:13:15 [+0100], Krzysztof Kozlowski wrote:
> On 10/02/2022 15:10, Sebastian Andrzej Siewior wrote:
> 
> The purpose was explain - fix a failing test with PREEMPT_RT. I am not
> planning to rework entire test, it is merely a fix.

What I got confused by is the fact that you do
s/do_softirq/scheduler_tick/ without any explanation why that is correct.

After looking into the test it appears that two random functions are
enough to be specified because the actual purpose is it to figure out if
the function is recorded and not the actual functionality behind the
function.

> >> 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?

I questioned the purpose of FUNC2 in this context so I don't have to
look into the actual test. But I did, see above ;)

> Best regards,
> Krzysztof

Sebastian