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

From: Sean Christopherson <seanjc@google.com>
To: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Leonardo Bras <leobras@redhat.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	 Frederic Weisbecker <frederic@kernel.org>,
	Neeraj Upadhyay <quic_neeraju@quicinc.com>,
	 Joel Fernandes <joel@joelfernandes.org>,
	Josh Triplett <josh@joshtriplett.org>,
	 Boqun Feng <boqun.feng@gmail.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	Lai Jiangshan <jiangshanlai@gmail.com>,
	 Zqiang <qiang.zhang1211@gmail.com>,
	Marcelo Tosatti <mtosatti@redhat.com>,
	kvm@vger.kernel.org,  linux-kernel@vger.kernel.org,
	rcu@vger.kernel.org
Subject: Re: [RFC PATCH v1 0/2] Avoid rcu_core() if CPU just left guest vcpu
Date: Tue, 7 May 2024 14:00:12 -0700	[thread overview]
Message-ID: <ZjqWXPFuoYWWcxP3@google.com> (raw)
In-Reply-To: <663a659d-3a6f-4bec-a84b-4dd5fd16c3c1@paulmck-laptop>

On Tue, May 07, 2024, Paul E. McKenney wrote:
> On Tue, May 07, 2024 at 10:55:54AM -0700, Sean Christopherson wrote:
> > On Fri, May 03, 2024, Paul E. McKenney wrote:
> > > On Fri, May 03, 2024 at 02:29:57PM -0700, Sean Christopherson wrote:
> > > > So if we're comfortable relying on the 1 second timeout to guard against a
> > > > misbehaving userspace, IMO we might as well fully rely on that guardrail.  I.e.
> > > > add a generic PF_xxx flag (or whatever flag location is most appropriate) to let
> > > > userspace communicate to the kernel that it's a real-time task that spends the
> > > > overwhelming majority of its time in userspace or guest context, i.e. should be
> > > > given extra leniency with respect to rcuc if the task happens to be interrupted
> > > > while it's in kernel context.
> > > 
> > > But if the task is executing in host kernel context for quite some time,
> > > then the host kernel's RCU really does need to take evasive action.
> > 
> > Agreed, but what I'm saying is that RCU already has the mechanism to do so in the
> > form of the 1 second timeout.
> 
> Plus RCU will force-enable that CPU's scheduler-clock tick after about
> ten milliseconds of that CPU not being in a quiescent state, with
> the time varying depending on the value of HZ and the number of CPUs.
> After about ten seconds (halfway to the RCU CPU stall warning), it will
> resched_cpu() that CPU every few milliseconds.
> 
> > And while KVM does not guarantee that it will immediately resume the guest after
> > servicing the IRQ, neither does the existing userspace logic.  E.g. I don't see
> > anything that would prevent the kernel from preempting the interrupt task.
> 
> Similarly, the hypervisor could preempt a guest OS's RCU read-side
> critical section or its preempt_disable() code.
> 
> Or am I missing your point?

I think you're missing my point?  I'm talking specifically about host RCU, what
is or isn't happening in the guest is completely out of scope.

My overarching point is that the existing @user check in rcu_pending() is optimistic,
in the sense that the CPU is _likely_ to quickly enter a quiescent state if @user
is true, but it's not 100% guaranteed.  And because it's not guaranteed, RCU has
the aforementioned guardrails.

And I'm arguing that, since the @user check isn't bombproof, there's no reason to
try to harden against every possible edge case in an equivalent @guest check,
because it's unnecessary for kernel safety, thanks to the guardrails.