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

[Sean Christopherson]

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?

> 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.

> 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.  KVM can provide that
information with much better precision, e.g. KVM knows when when it's in the core
vCPU run loop.

> 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.  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.