linux-kernel - Re: [PATCH v3 tip/core/rcu 40/40] rcu: Make non-preemptive schedule be Tasks RCU quiescent state

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Message-ID: <20170929103424.o4yje6sv4s3c7hmq@hirez.programming.kicks-ass.net>
Date:   Fri, 29 Sep 2017 12:34:24 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Boqun Feng <boqun.feng@...il.com>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        "Levin, Alexander (Sasha Levin)" <alexander.levin@...izon.com>,
        Sasha Levin <levinsasha928@...il.com>,
        "linux-kernel@...r.kernel.org List" <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        "jiangshanlai@...il.com" <jiangshanlai@...il.com>,
        "dipankar@...ibm.com" <dipankar@...ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Josh Triplett <josh@...htriplett.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "dhowells@...hat.com" <dhowells@...hat.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Fr??d??ric Weisbecker <fweisbec@...il.com>,
        Oleg Nesterov <oleg@...hat.com>,
        "bobby.prani@...il.com" <bobby.prani@...il.com>,
        Radim Kr??m???? <rkrcmar@...hat.com>, kvm@...r.kernel.org
Subject: Re: [PATCH v3 tip/core/rcu 40/40] rcu: Make non-preemptive schedule
 be Tasks RCU quiescent state

On Fri, Sep 29, 2017 at 12:01:24PM +0200, Paolo Bonzini wrote:
> > Does this mean whenever we get a page fault in a RCU read-side critical
> > section, we may hit this?
> > 
> > Could we simply avoid to schedule() in kvm_async_pf_task_wait() if the
> > fault process is in a RCU read-side critical section as follow?
> > 
> > diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
> > index aa60a08b65b1..291ea13b23d2 100644
> > --- a/arch/x86/kernel/kvm.c
> > +++ b/arch/x86/kernel/kvm.c
> > @@ -140,7 +140,7 @@ void kvm_async_pf_task_wait(u32 token)
> >  
> >  	n.token = token;
> >  	n.cpu = smp_processor_id();
> > -	n.halted = is_idle_task(current) || preempt_count() > 1;
> > +	n.halted = is_idle_task(current) || preempt_count() > 1 || rcu_preempt_depth();
> >  	init_swait_queue_head(&n.wq);
> >  	hlist_add_head(&n.link, &b->list);
> >  	raw_spin_unlock(&b->lock);
> > 
> > (Add KVM folks and list Cced)
> 
> Yes, that would work.  Mind to send it as a proper patch?

I'm confused, why would we do an ASYNC PF at all here? Thing is, a
printk() shouldn't trigger a major fault _ever_. At worst it triggers
something like a vmalloc minor fault. And I'm thinking we should not do
the whole ASYNC machinery for minor faults.