linux-kernel - Re: [PATCH v2 3/5] powerpc: Use preemption model accessors

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Date:   Mon, 22 Nov 2021 16:44:36 +0000
From:   Valentin Schneider <valentin.schneider@....com>
To:     Christophe Leroy <christophe.leroy@...roup.eu>,
        linux-kernel@...r.kernel.org, kasan-dev@...glegroups.com,
        linuxppc-dev@...ts.ozlabs.org, linux-kbuild@...r.kernel.org
Cc:     Marco Elver <elver@...gle.com>,
        Michal Marek <michal.lkml@...kovi.net>,
        Peter Zijlstra <peterz@...radead.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Mike Galbraith <efault@....de>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Paul Mackerras <paulus@...ba.org>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Dmitry Vyukov <dvyukov@...gle.com>
Subject: Re: [PATCH v2 3/5] powerpc: Use preemption model accessors

On 16/11/21 14:41, Christophe Leroy wrote:
> Le 10/11/2021 à 21:24, Valentin Schneider a écrit :
>> Per PREEMPT_DYNAMIC, checking CONFIG_PREEMPT doesn't tell you the actual
>> preemption model of the live kernel. Use the newly-introduced accessors
>> instead.
>
> Is that change worth it for now ? As far as I can see powerpc doesn't
> have DYNAMIC PREEMPT, a lot of work needs to be done before being able
> to use it:
> - Implement GENERIC_ENTRY
> - Implement STATIC_CALLS (already done on PPC32, to be done on PPC64)
>

You're right, I ditched this patch for v3 - AFAICT the change wasn't even
valid as the preempt_schedule_irq() call needs to be replaced with
irqentry_exit_cond_resched() (IOW this needs to make use of the generic
entry code).

>>
>> sched_init() -> preempt_dynamic_init() happens way before IRQs are set up,
>> so this should be fine.
>
> It looks like you are mixing up interrupts and IRQs (also known as
> "external interrupts").
>
> ISI (Instruction Storage Interrupt) and DSI (Data Storage Interrupt) for
> instance are also interrupts. They happen everytime there is a page
> fault so may happen pretty early.
>
> Traps generated by WARN_ON() are also interrupts that may happen at any
> time.
>

Michael pointed this out and indeed triggering a WARN_ON() there is not
super smart. Thanks for teaching me a bit of what I'm putting my grubby
hands in :)