On Wed, May 15, 2013 at 10:31:42AM -0700, Paul E. McKenney wrote: > On Wed, May 15, 2013 at 11:02:34AM +0200, Peter Zijlstra wrote:
> > Earlier you said that improving EQS behaviour was expensive in that it > > would require taking (global) locks or somesuch. > > > > Would it not be possible to have the cpu performing a FQS finish this > > work; that way the first FQS would be a little slow, but after that no > > FQS would be needed anymore, right? Since we'd no longer require the > > other CPUs to end a grace period. > > It is not just the first FQS that would be slow, it would also be slow > the next time that this CPU transitioned from idle to non-idle, which > is when this work would need to be undone. Hurm, yes I suppose that is true. If you've saved more on FQS cost it might be worth it for the throughput people though. But somehow I imagined making a CPU part of the GP would be easier than taking it out. After all, taking it out is dangerous and careful work, one is not to accidentally execute a callback or otherwise end a GP before time. When entering the GP cycle there is no such concern, the CPU state is clean after all. > Furthermore, in this approach, RCU would still need to scan all the CPUs > to see if any did the first part of the transition to idle. And if we > have to scan either way, why not keep the idle-nonidle transitions cheap > and continue to rely on the scan? Here are the rationales I can think > of and what I am thinking in terms of doing instead: > > 1. The scan could become a scalability bottleneck. There is one > way to handle this today, and one possible future change. The way > to handle this today is to increas rcutree.jiffies_till_first_fqs, > for example, the SGI guys set it to 20 or thereabouts. If this > becomes problematic, I could easily create multiple kthreads to > carry out the FQS scan in parallel for large systems. *groan* whoever thought all this SMP nonsense was worth it again? :-) > 2. Someone could demonstrate that RCU's grace periods were significantly > delaying boot. There are several ways of dealing with this: Surely there's also non-boot cases where most of the machine is 'idle' and we're running into FQS? Esp. now with that userspace NO_HZ stuff from Frederic. -- 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/
