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

[Leonardo Bras <leobras@redhat.com>]

On Tue, May 14, 2024 at 03:54:16PM -0700, Paul E. McKenney wrote:
> On Mon, May 13, 2024 at 06:47:13PM -0300, Leonardo Bras Soares Passos wrote:
> > 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).
> 
> Yes, it is cheap, especially if interrupts are already disabled.
> (As in the scheduler asks RCU to do the same amount of work on its
> context-switch fastpath.)

Thanks!

> 
> > 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?
> 
> Try both on the workload of interest with appropriate tracing and
> see what happens?  The hardware's opinion overrides mine.  ;-)

That's a great approach!

But in this case I think noting a quiescent state in guest exit is 
necessary to avoid a scenario in which a VM takes longer than RCU 
patience, and it ends up running rcuc in a nohz_full cpu, even if guest 
exit was quite brief. 

IIUC Sean's question is more on the tone of "Why KVM does not note a 
quiescent state in guest exit already, if it does in guest entry", and I 
just came with a few arguments to try finding a possible rationale, since 
I could find no discussion on that topic in the lore for the original 
commit.

Since noting a quiescent state in guest exit is cheap enough, avoids rcuc 
schedules when grace period starts during guest execution, and enables a 
much more rational usage of RCU patience, it's a safe to assume it's a 
better way of dealing with RCU compared to current implementation.

Sean, what do you think?

Thanks!
Leo

> 
> 							Thanx, Paul
>