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

[Ingo Molnar <mingo@kernel.org>]

* Russ Anderson <rja@sgi.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.

Thanks,

	Ingo