linux-kernel - Re: [PATCH] ARM: call disable_nonboot_cpus() from machine

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Message-ID: <510848E2.50203@wwwdotorg.org>
Date:	Tue, 29 Jan 2013 15:10:42 -0700
From:	Stephen Warren <swarren@...dotorg.org>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
CC:	Stephen Warren <swarren@...dia.com>,
	Stephen Boyd <sboyd@...eaurora.org>,
	Will Deacon <will.deacon@....com>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	"kexec@...ts.infradead.org" <kexec@...ts.infradead.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ARM: call disable_nonboot_cpus() from machine_shutdown()

On 01/10/2013 11:28 PM, Eric W. Biederman wrote:
...
> I don't have any problem with generic code in the reboot path
> doing:
> if (cpu_online(0))
> 	set_cpus_allowed_ptr(current, cpumask_of(0));

It looks like that API just affects the scheduler, and not whether the
other CPUs are actually active/hot-plugged-in. At least for my use-case,
I need something that really disables the other CPUs so they aren't
executing code, hence my tendency to hot-un-plug them using
disable_nonboot_cpus(), rather than just shift task execution off them
using the code above.

I wonder if all architectures shouldn't always do the following in all
reboot/shutdown/kexec cases:

* set_cpus_allowed_ptr() to limit code execution to a single CPU.
* disable_nonboot_cpus() (or equivalent) if it's available to turn off
all the other CPUs.

The issue here would be that disable_nonboot_cpus() isn't always
available; I assume that's part of the reason that there are
arch-specific machine_xxx() hooks, so that architectures can
power-off/reset their CPUs even when hotplug isn't enabled? I wonder if
that can be refactored so that reboot/poweroff/kexec can share some
CPU-disable code with CPU hotplug?
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