Hello Sean, Marcelo and Paul, Thank you for your comments on this thread! I will try to reply some of the questions below: (Sorry for the delay, I was OOO for a while.) On Mon, Apr 01, 2024 at 01:21:25PM -0700, Sean Christopherson wrote: > On Thu, Mar 28, 2024, Leonardo Bras wrote: > > I am dealing with a latency issue inside a KVM guest, which is caused by > > a sched_switch to rcuc[1]. > > > > During guest entry, kernel code will signal to RCU that current CPU was on > > a quiescent state, making sure no other CPU is waiting for this one. > > > > If a vcpu just stopped running (guest_exit), and a syncronize_rcu() was > > issued somewhere since guest entry, there is a chance a timer interrupt > > will happen in that CPU, which will cause rcu_sched_clock_irq() to run. > > > > rcu_sched_clock_irq() will check rcu_pending() which will return true, > > and cause invoke_rcu_core() to be called, which will (in current config) > > cause rcuc/N to be scheduled into the current cpu. > > > > On rcu_pending(), I noticed we can avoid returning true (and thus invoking > > rcu_core()) if the current cpu is nohz_full, and the cpu came from either > > idle or userspace, since both are considered quiescent states. > > > > Since this is also true to guest context, my idea to solve this latency > > issue by avoiding rcu_core() invocation if it was running a guest vcpu. > > > > On the other hand, I could not find a way of reliably saying the current > > cpu was running a guest vcpu, so patch #1 implements a per-cpu variable > > for keeping the time (jiffies) of the last guest exit. > > > > In patch #2 I compare current time to that time, and if less than a second > > has past, we just skip rcu_core() invocation, since there is a high chance > > it will just go back to the guest in a moment. > > What's the downside if there's a false positive? False positive being guest_exit without going back in this CPU, right? If so in WSC, supposing no qs happens and there is a pending request, RCU will take a whole second to run again, possibly making other CPUs wait this long for a synchronize_rcu. This value (1 second) could defined in .config or as a parameter if needed, but does not seem a big deal, > > > What I know it's weird with this patch: > > 1 - Not sure if this is the best way of finding out if the cpu was > > running a guest recently. > > > > 2 - This per-cpu variable needs to get set at each guest_exit(), so it's > > overhead, even though it's supposed to be in local cache. If that's > > an issue, I would suggest having this part compiled out on > > !CONFIG_NO_HZ_FULL, but further checking each cpu for being nohz_full > > enabled seems more expensive than just setting this out. > > A per-CPU write isn't problematic, but I suspect reading jiffies will be quite > imprecise, e.g. it'll be a full tick "behind" on many exits. That would not be a problem, as it would mean 1 tick less waiting in the false positive WSC, and the 1s amount is plenty. > > > 3 - It checks if the guest exit happened over than 1 second ago. This 1 > > second value was copied from rcu_nohz_full_cpu() which checks if the > > grace period started over than a second ago. If this value is bad, > > I have no issue changing it. > > IMO, checking if a CPU "recently" ran a KVM vCPU is a suboptimal heuristic regardless > of what magic time threshold is used. IIUC, what you want is a way to detect if > a CPU is likely to _run_ a KVM vCPU in the near future. That's correct! > KVM can provide that > information with much better precision, e.g. KVM knows when when it's in the core > vCPU run loop. That would not be enough. I need to present the application/problem to make a point: - There is multiple isolated physical CPU (nohz_full) on which we want to run KVM_RT vcpus, which will be running a real-time (low latency) task. - This task should not miss deadlines (RT), so we test the VM to make sure the maximum latency on a long run does not exceed the latency requirement - This vcpu will run on SCHED_FIFO, but has to run on lower priority than rcuc, so we can avoid stalling other cpus. - There may be some scenarios where the vcpu will go back to userspace (from KVM_RUN ioctl), and that does not mean it's good to interrupt the this to run other stuff (like rcuc). Now, I understand it will cover most of our issues if we have a context tracking around the vcpu_run loop, since we can use that to decide not to run rcuc on the cpu if the interruption hapenned inside the loop. But IIUC we can have a thread that "just got out of the loop" getting interrupted by the timer, and asked to run rcu_core which will be bad for latency. I understand that the chance may be statistically low, but happening once may be enough to crush the latency numbers. Now, I can't think on a place to put this context trackers in kvm code that would avoid the chance of rcuc running improperly, that's why the suggested timeout, even though its ugly. About the false-positive, IIUC we could reduce it if we reset the per-cpu last_guest_exit on kvm_put. > > > 4 - Even though I could detect no issue, I included linux/kvm_host.h into > > rcu/tree_plugin.h, which is the first time it's getting included > > outside of kvm or arch code, and can be weird. > > Heh, kvm_host.h isn't included outside of KVM because several architectures can > build KVM as a module, which means referencing global KVM varibles from the kernel > proper won't work. > > > An alternative would be to create a new header for providing data for > > non-kvm code. > > I doubt a new .h or .c file is needed just for this, there's gotta be a decent > landing spot for a one-off variable. You are probably right > E.g. I wouldn't be at all surprised if there > is additional usefulness in knowing if a CPU is in KVM's core run loop and thus > likely to do a VM-Enter in the near future, at which point you could probably make > a good argument for adding a flag in "struct context_tracking". Even without a > separate use case, there's a good argument for adding that info to context_tracking. For the tracking solution, makes sense :) Not sure if the 'timeout' alternative will be that useful outside rcu. Thanks! Leo
