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

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Date:   Thu, 10 Feb 2022 15:10:44 +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 15:05:24 [+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 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.

> Best regards,
> Krzysztof

Sebastian