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

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Date: Fri, 10 May 2024 23:08:42 -0300
From: Leonardo Bras <leobras@...hat.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Leonardo Bras <leobras@...hat.com>,
	"Paul E. McKenney" <paulmck@...nel.org>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Frederic Weisbecker <frederic@...nel.org>,
	Neeraj Upadhyay <quic_neeraju@...cinc.com>,
	Joel Fernandes <joel@...lfernandes.org>,
	Josh Triplett <josh@...htriplett.org>,
	Boqun Feng <boqun.feng@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Lai Jiangshan <jiangshanlai@...il.com>,
	Zqiang <qiang.zhang1211@...il.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	rcu@...r.kernel.org
Subject: Re: [RFC PATCH v1 0/2] Avoid rcu_core() if CPU just left guest vcpu

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