On Fri, Jul 12, 2024 at 5:02 PM Leonardo Bras <leobras@xxxxxxxxxx> wrote: > > On Fri, Jul 12, 2024 at 05:57:10PM +0200, Paolo Bonzini wrote: > > On 7/11/24 01:18, Leonardo Bras wrote: > > > What are your thoughts on above results? > > > Anything you would suggest changing? > > > > Hello Paolo, thanks for the feedback! > > > Can you run the test with a conditional on "!tick_nohz_full_cpu(vcpu->cpu)"? > > > > If your hunch is correct that nohz-full CPUs already avoid invoke_rcu_core() > > you might get the best of both worlds. > > > > tick_nohz_full_cpu() is very fast when there is no nohz-full CPU, because > > then it shortcuts on context_tracking_enabled() (which is just a static > > key). > > But that would mean not noting an RCU quiescent state in guest_exit of > nohz_full cpus, right? > > The original issue we were dealing was having invoke_rcu_core() running on > nohz_full cpus, and messing up the latency of RT workloads inside the VM. > > While most of the invoke_rcu_core() get ignored by the nohz_full rule, > there are some scenarios in which it the vcpu thread may take more than 1s > between a guest_entry and the next one (VM busy), and those which did > not get ignored have caused latency peaks in our tests. > > The main idea of this patch is to note RCU quiescent states on guest_exit > at nohz_full cpus (and use rcu.patience) to avoid running invoke_rcu_core() > between a guest_exit and the next guest_entry if it takes less than > rcu.patience miliseconds between exit and entry, and thus avoiding the > latency increase. > > What I tried to prove above is that it also improves non-Isolated cores as > well, since rcu_core will not be running as often, saving cpu cycles that > can be used by the VM. > > > What are your thoughts on that? Hello Paolo, Sean, Thanks for the feedback so far! Do you have any thoughts or suggestions for this patch? Thanks! Leo > > Thanks! > Leo
