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

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Date:	Wed, 10 Apr 2013 12:14:20 -0500
From:	Robin Holt <holt@....com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Russ Anderson <rja@....com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	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>,
	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 06:59:34PM +0200, Ingo Molnar wrote:
> 
> * Russ Anderson <rja@....com> wrote:
> 
> > Yes, I have a test patch that replaces for_each_online_cpu(cpu) with a cpu 
> > bitmask in disable_nonboot_cpus().  The lower level routines already take a 
> > bitmask.  It allows __stop_machine() to be called just once.  That change 
> > reduces shutdown time on a 1024 cpu machine from 16 minutes 4 minutes.  
> > Significant improvement, but not good enough.
> > 
> > The next significant bottleneck is __cpu_notify().  Tried creating worker 
> > threads to parallelize the shutdown, but the problem is __cpu_notify() is not 
> > thread safe.  Putting a lock around it caused all the worker threads to fight 
> > over the lock.
> 
> 4 minutes bootup is 240 seconds, with 1024 CPUs that's about 240 msecs per CPU.
> 
> That sounds a lot, given that unlike bootup there's not much real work to be done 
> during shutdown - we don't initialize anything, etc.
> 
> Maybe much of those 240 msecs are spent in some stupid udelay loop or so, which 
> could be made parallel?
> 
> Would it be possible to create a 'reboot but stop at the end and reactivate all 
> CPUs again' reboot flag, so that it can all be NMI-profiled, to see where the true 
> bottleneck is? A naked disable_nonboot_cpus() call in essence.

What, exactly, are you proposing with the NMI profiling?  Currently,
if I NMI the system, I get dump_stack() output for all cpus.  Without
introducing a lock to serialize those, they stacks are really just a
jumbled mess.  With a lock, things are fairly slow.

Are you proposing something other than looking at stack dumps?

Thanks,
Robin
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