Re: [RFC PATCH 1/1] kvm: Note an RCU quiescent state on guest exit

[Leonardo Bras Soares Passos <leobras@redhat.com>]

On Mon, May 13, 2024 at 4:40 PM Sean Christopherson <seanjc@google.com> wrote:
>
> On Fri, May 10, 2024, 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.
>
> Are there any downsides to this?  E.g. extra latency or anything?  KVM will note
> a context switch on the next VM-Enter, so even if there is extra latency or
> something, KVM will eventually take the hit in the common case no matter what.
> But I know some setups are sensitive to handling select VM-Exits as soon as possible.
>
> I ask mainly because it seems like a no brainer to me to have both VM-Entry and
> VM-Exit note the context switch, which begs the question of why KVM isn't already
> doing that.  I assume it was just oversight when commit 126a6a542446 ("kvm,rcu,nohz:
> use RCU extended quiescent state when running KVM guest") handled the VM-Entry
> case?

I don't know, by the lore I see it happening in guest entry since the
first time it was introduced at
https://lore.kernel.org/all/1423167832-17609-5-git-send-email-riel@redhat.com/

Noting a quiescent state is cheap, but it may cost a few accesses to
possibly non-local cachelines. (Not an expert in this, Paul please let
me know if I got it wrong).

I don't have a historic context on why it was just implemented on
guest_entry, but it would make sense when we don't worry about latency
to take the entry-only approach:
- It saves the overhead of calling rcu_virt_note_context_switch()
twice per guest entry in the loop
- KVM will probably run guest entry soon after guest exit (in loop),
so there is no need to run it twice
- Eventually running rcu_core() may be cheaper than noting quiescent
state every guest entry/exit cycle

Upsides of the new strategy:
- Noting a quiescent state in guest exit avoids calling rcu_core() if
there was a grace period request while guest was running, and timer
interrupt hits the cpu.
- If the loop re-enter quickly there is a high chance that guest
entry's rcu_virt_note_context_switch() will be fast (local cacheline)
as there is low probability of a grace period request happening
between exit & re-entry.
- It allows us to use the rcu patience strategy to avoid rcu_core()
running if any grace period request happens between guest exit and
guest re-entry, which is very important for low latency workloads
running on guests as it reduces maximum latency in long runs.

What do you think?

Thanks!
Leo