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

From: Leonardo Bras <leobras@redhat.com>
To: Sean Christopherson <seanjc@google.com>
Cc: Leonardo Bras <leobras@redhat.com>,
	"Paul E. McKenney" <paulmck@kernel.org>,
	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: Fri, 10 May 2024 23:08:42 -0300	[thread overview]
Message-ID: <Zj7TKg9tHTwgWOIQ@LeoBras> (raw)
In-Reply-To: <ZjuFuZHKUy7n6-sG@google.com>

On Wed, May 08, 2024 at 07:01:29AM -0700, Sean Christopherson wrote:
> On Wed, May 08, 2024, Leonardo Bras wrote:
> > Something just hit me, and maybe I need to propose something more generic.
> 
> Yes.  This is what I was trying to get across with my complaints about keying off
> of the last VM-Exit time.  It's effectively a broad stroke "this task will likely
> be quiescent soon" and so the core concept/functionality belongs in common code,
> not KVM.
> 

Hello Sean,

Paul implemented the RCU patience cmdline option, that will help avoiding 
rcuc waking up if the grace period is younger than X miliseconds, which 
means the last quiescent state needs to be at least X miliseconds old.

With that I just have to add a quiescent state in guest_exit(), and we will 
be able to get the same effect of last_guest_exit patch. 

I sent this RFC patch doing that:
https://lore.kernel.org/all/20240511020557.1198200-1-leobras@redhat.com/

Please take a look.

Thanks!
Leo