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

From: Ingo Molnar
Date: Wed Apr 10 2013 - 12:59:42 EST

* 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.


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