On Wed, Apr 10, 2013 at 07:22:36PM +0200, Ingo Molnar wrote: > > * Robin Holt <h...@sgi.com> wrote: > > > On Wed, Apr 10, 2013 at 06:59:34PM +0200, Ingo Molnar wrote: > > > > > > * Russ Anderson <r...@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. > > > > What, exactly, are you proposing with the NMI profiling? [...] > > I'm proposing to make 'reboot' overhead profilable, via a debug hack: > > echo 1 > /proc/sys/kernel/magic_dont_fully_reboot_flag > > perf record reboot > > perf is using NMIs to profile - and since much of cpu_down() is with irqs > disabled, NMI profiling would be needed to see inside the overhead. > > (Assuming the 240 msecs is CPU overhead, not waiting for some IRQ/IPI event.)
Let me give it a try. Robin -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
