On Sun, May 12, 2024 at 06:44:23PM -0300, Marcelo Tosatti wrote: > On Fri, May 10, 2024 at 11:05:56PM -0300, Leonardo Bras wrote: > > As of today, KVM notes a quiescent state only in guest entry, which is good > > as it avoids the guest being interrupted for current RCU operations. > > > > While the guest vcpu runs, it can be interrupted by a timer IRQ that will > > check for any RCU operations waiting for this CPU. In case there are any of > > such, it invokes rcu_core() in order to sched-out the current thread and > > note a quiescent state. > > > > This occasional schedule work will introduce tens of microsseconds of > > latency, which is really bad for vcpus running latency-sensitive > > applications, such as real-time workloads. > > > > So, note a quiescent state in guest exit, so the interrupted guests is able > > to deal with any pending RCU operations before being required to invoke > > rcu_core(), and thus avoid the overhead of related scheduler work. > > This does not properly fix the current problem, as RCU work might be > scheduled after the VM exit, followed by a timer interrupt. > > Correct? Not that i am against the patch... But, regarding the problem at hand, it does not fix it reliably.
