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 08:20:00 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     rjw@...ysocki.net, bp@...en8.de, x86@...nel.org,
        tony.luck@...el.com, lenb@...nel.org, daniel.lezcano@...aro.org,
        linux-kernel@...r.kernel.org, linux-acpi@...r.kernel.org,
        linux-pm@...r.kernel.org, ulf.hansson@...aro.org,
        paulmck@...nel.org, tglx@...utronix.de, naresh.kamboju@...aro.org
Subject: Re: [RFC][PATCH 4/4] acpi: Take over RCU-idle for C3-BM idle

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.

This makes me wonder - if no one cares about buiding (much less running)
ia64 images with the upstream kernel, is it possibly time to remove it ?

Guenter