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?
I was hoping for a stupid udelay when I first started looking at this code, but found nothing obvious. The bulk of the time (after making the cpu bitmask change) is spent in __cpu_notify(), as explained above. > 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. My testing was similar. I hacked a kernel module to call disable_nonboot_cpus() and enable_nonboot_cpus() and used printks to narrow down the slow functions. That points at the cpu notifier call chain. It's not clear if any of the functions on the call chain take a long time, or just going sequentially through the list for all cpus just takes a long time. -- Russ Anderson, OS RAS/Partitioning Project Lead SGI - Silicon Graphics Inc r...@sgi.com -- 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/
