linux-kernel - Re: [RFC][PATCH 4/4] acpi: Take over RCU-idle for C3-BM idle

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Date:   Fri, 25 Sep 2020 17:24:03 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Guenter Roeck <linux@...ck-us.net>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Borislav Petkov <bp@...en8.de>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Tony Luck <tony.luck@...el.com>, Len Brown <lenb@...nel.org>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Naresh Kamboju <naresh.kamboju@...aro.org>
Subject: Re: [RFC][PATCH 4/4] acpi: Take over RCU-idle for C3-BM idle

On Fri, Sep 25, 2020 at 5:20 PM Guenter Roeck <linux@...ck-us.net> wrote:
>
> On Tue, Sep 15, 2020 at 12:32:01PM +0200, Peter Zijlstra wrote:
> > The C3 BusMaster idle code takes lock in a number of places, some deep
> > inside the ACPI code. Instead of wrapping it all in RCU_NONIDLE, have
> > the driver take over RCU-idle duty and avoid flipping RCU state back
> > and forth a lot.
> >
> > ( by marking 'C3 && bm_check' as RCU_IDLE, we _must_ call enter_bm() for
> >   that combination, otherwise we'll loose RCU-idle, this requires
> >   shuffling some code around )
> >
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
>
> ia64:defconfig:
>
> ERROR: modpost: "rcu_idle_enter" [drivers/acpi/processor.ko] undefined!
> ERROR: modpost: "rcu_idle_exit" [drivers/acpi/processor.ko] undefined!
>
> I realize that this has already been reported more than a week ago, with
> no visible reaction. Another problem introduced in the same file, resulting
> in
>
> drivers/acpi/processor_idle.c: In function 'lapic_timer_needs_broadcast':
> drivers/acpi/processor_idle.c:179:1: warning:
>         no return statement in function returning non-void
>
> may cause ia64 boot problems since a non-zero return value will trigger
> a function call. AFAICS that is not supposed to happen on ia64.

There are fixes for the above in my tree, they will go to Linus shortly.

Thanks!