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

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Date: Sun, 12 May 2024 22:06:20 -0300
From: Marcelo Tosatti <mtosatti@...hat.com>
To: Leonardo Bras <leobras@...hat.com>
Cc: Frederic Weisbecker <frederic@...nel.org>,
	"Paul E. McKenney" <paulmck@...nel.org>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Sean Christopherson <seanjc@...gle.com>,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [RFC PATCH 1/1] kvm: Note an RCU quiescent state on guest exit

On Sun, May 12, 2024 at 06:44:23PM -0300, Marcelo Tosatti wrote:
> On Fri, May 10, 2024 at 11:05:56PM -0300, 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.
> 
> This does not properly fix the current problem, as RCU work might be
> scheduled after the VM exit, followed by a timer interrupt.
> 
> Correct?

Not that i am against the patch... 

But, regarding the problem at hand, it does not fix it reliably.