On Wed, 2014-07-02 at 10:08 -0700, Paul E. McKenney wrote: > On Wed, Jul 02, 2014 at 06:04:12PM +0200, Peter Zijlstra wrote: > > On Wed, Jul 02, 2014 at 08:39:15AM -0700, Paul E. McKenney wrote: > > > On Wed, Jul 02, 2014 at 02:34:12PM +0200, Peter Zijlstra wrote: > > > > On Fri, Jun 27, 2014 at 07:20:38AM -0700, Paul E. McKenney wrote: > > > > > An 80-CPU system with a context-switch-heavy workload can require so > > > > > many NOCB kthread wakeups that the RCU grace-period kthreads spend > > > > > several > > > > > tens of percent of a CPU just awakening things. This clearly will not > > > > > scale well: If you add enough CPUs, the RCU grace-period kthreads > > > > > would > > > > > get behind, increasing grace-period latency. > > > > > > > > > > To avoid this problem, this commit divides the NOCB kthreads into > > > > > leaders > > > > > and followers, where the grace-period kthreads awaken the leaders > > > > > each of > > > > > whom in turn awakens its followers. By default, the number of groups > > > > > of > > > > > kthreads is the square root of the number of CPUs, but this default > > > > > may > > > > > be overridden using the rcutree.rcu_nocb_leader_stride boot parameter. > > > > > This reduces the number of wakeups done per grace period by the RCU > > > > > grace-period kthread by the square root of the number of CPUs, but of > > > > > course by shifting those wakeups to the leaders. In addition, because > > > > > the leaders do grace periods on behalf of their respective followers, > > > > > the number of wakeups of the followers decreases by up to a factor of > > > > > two. > > > > > Instead of being awakened once when new callbacks arrive and again > > > > > at the end of the grace period, the followers are awakened only at > > > > > the end of the grace period. > > > > > > > > > > For a numerical example, in a 4096-CPU system, the grace-period > > > > > kthread > > > > > would awaken 64 leaders, each of which would awaken its 63 followers > > > > > at the end of the grace period. This compares favorably with the 79 > > > > > wakeups for the grace-period kthread on an 80-CPU system. > > > > > > > > Urgh, how about we kill the entire nocb nonsense and try again? This is > > > > getting quite rediculous. > > > > > > Sure thing, Peter. > > > > So you don't think this has gotten a little out of hand? The NOCB stuff > > has lead to these masses of rcu threads and now you're adding extra > > cache misses to the perfectly sane and normal code paths just to deal > > with so many threads. > > Indeed it appears to have gotten a bit out of hand. But let's please > attack the real problem rather than the immediate irritant. > > And in this case, the real problem is that users are getting callback > offloading even when there is no reason for it. > > > And all to support a feature that nearly nobody uses. And you were > > talking about making nocb the default rcu... > > As were others, not that long ago. Today is the first hint that I got > that you feel otherwise. But it does look like the softirq approach to > callback processing needs to stick around for awhile longer. Nice to > hear that softirq is now "sane and normal" again, I guess. ;-) > > Please see my patch in reply to Rik's email. The idea is to neither > rip callback offloading from the kernel nor to keep callback offloading > as the default, but instead do callback offloading only for those CPUs > specifically marked as NO_HZ_FULL CPUs, or when specifically requested > at build time or at boot time. In other words, only do it when it is > needed.
Exactly! Like dynamically, when the user isolates CPUs via the cpuset interface, none of it making much sense without that particular property of a set of CPUs, and cpuset being the manager of CPU set properties. NO_HZ_FULL is a property of a set of CPUs. isolcpus is supposed to go away as being a redundant interface to manage a single property of a set of CPUs, but it's perfectly fine for NO_HZ_FULL to add an interface to manage a single property of a set of CPUs. What am I missing? -Mike -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
