On Thu, Feb 05, 2015 at 03:23:47PM -0500, riel@xxxxxxxxxx wrote: > When running a KVM guest on a system with NOHZ_FULL enabled I just need to clarify the motivation first, does the above situation really happen? Ok some distros enable NOHZ_FULL to let the user stop the tick in userspace. So most of the time, CONFIG_NOHZ_FULL=y but nohz full is runtime disabled (we need to pass a nohz_full= boot parameter to enable it). And when it is runtime disabled, there should be no rcu nocb CPU. (Although not setting CPUs in nocb mode when nohz full is runtime disabled is perhaps a recent change.) So for the problem to arise, one need to enable nohz_full and run KVM guest. And I never heard about such workloads. That said it's potentially interesting to turn off the tick on the host when the guest runs. >, and the > KVM guest running with idle=poll mode, we still get wakeups of the > rcuos/N threads. So we need nohz_full on the host and idle=poll mode on the guest. Is it likely to happen? (sorry, again I'm just trying to make sure we agree on why we do this change). > > This problem has already been solved for user space by telling the > RCU subsystem that the CPU is in an extended quiescent state while > running user space code. > > This patch series extends that code a little bit to make it usable > to track KVM guest space, too. > > I tested the code by booting a KVM guest with idle=poll, on a system > with NOHZ_FULL enabled on most CPUs, and a VCPU thread bound to a > CPU. In a 10 second interval, rcuos/N threads on other CPUs got woken > up several times, while the rcuos thread on the CPU running the bound > and alwasy running VCPU thread never got woken up once. So what you're describing is to set RCU in extended quiescent state, right? This doesn't include stopping the tick while running in guest mode? Those are indeed two different thing, although stopping the tick most often requires to set RCU in extended quiescent state. > > Thanks to Christian Borntraeger and Paul McKenney for reviewing the > first version of this patch series, and helping optimize patch 4/5. > -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
