linux-kernel - Re: [PATCH] Do not force shutdown/reboot to boot cpu.

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Date:	Wed, 10 Apr 2013 08:10:05 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Robin Holt <holt@....com>, "H. Peter Anvin" <hpa@...or.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Russ Anderson <rja@....com>, Shawn Guo <shawn.guo@...aro.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"the arch/x86 maintainers" <x86@...nel.org>
Subject: Re: [PATCH] Do not force shutdown/reboot to boot cpu.

On Wed, Apr 10, 2013 at 4:16 AM, Ingo Molnar <mingo@...nel.org> wrote:
>
> I think rebooting on the same CPU where we booted up is something worth having in
> general, as a firmware robustness feature. (assuming the CPU in question is still
> online)

Yeah, we've had issues with ACPI in the past, so I do think we should
always reboot using the BP. Even if it almost certainly works on 99+%
of all machines on any random CPU.

The optimal solution would be to just speed up the
disable_nonboot_cpus() code so much that it isn't an issue. That would
be good for suspending too, although I guess suspend isn't a big issue
if you have a thousand CPU's.

Has anybody checked whether we could do the cpu_down() on non-boot
CPU's in parallel? Right now we serialize the thing completely, with
one single

    for_each_online_cpu(cpu) {
        ...

loop that does a synchrinous _cpu_down() for each CPU. No wonder it
takes forever. We do __stop_machine() over and over and over again:
the whole thing is basically O(n**2) in CPU's.

                        Linus
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