Re: [RFC PATCH v1 0/2] Avoid rcu_core() if CPU just left guest vcpu

[Leonardo Bras]

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