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

From: Robin Holt
Date: Wed Apr 10 2013 - 13:14:27 EST

On Wed, Apr 10, 2013 at 06:59:34PM +0200, Ingo Molnar wrote:
> * Russ Anderson <rja@xxxxxxx> 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?

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