linux-kernel - Re: [patch -rt 1/2] KVM: use simple waitqueue for vcpu->wq

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Date:	Tue, 20 Jan 2015 13:16:13 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Paul Mackerras <paulus@...ba.org>
Cc:	Marcelo Tosatti <mtosatti@...hat.com>,
	linux-kernel@...r.kernel.org, linux-rt-users@...r.kernel.org,
	Luiz Capitulino <lcapitulino@...hat.com>,
	Rik van Riel <riel@...hat.com>,
	Steven Rostedt <srostedt@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>, kvm@...r.kernel.org,
	Paolo Bonzini <pbonzini@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [patch -rt 1/2] KVM: use simple waitqueue for vcpu->wq

On Tue, 20 Jan 2015 16:46:53 +1100
Paul Mackerras <paulus@...ba.org> wrote:

> On Mon, Jan 19, 2015 at 12:41:00PM -0200, Marcelo Tosatti wrote:
> > On Fri, Jan 16, 2015 at 11:48:46AM -0500, Steven Rostedt wrote:
> > > >  static void kvmppc_vcore_blocked(struct kvmppc_vcore *vc)
> > > >  {
> > > > -	DEFINE_WAIT(wait);
> > > > +	DEFINE_SWAITER(wait);
> > > >  
> > > > -	prepare_to_wait(&vc->wq, &wait, TASK_INTERRUPTIBLE);
> > > > +	swait_prepare(&vc->wq, &wait, TASK_INTERRUPTIBLE);
> > > >  	vc->vcore_state = VCORE_SLEEPING;
> > > >  	spin_unlock(&vc->lock);
> > > >  	schedule();
> > > > -	finish_wait(&vc->wq, &wait);
> > > > +	swait_finish(&vc->wq, &wait);
> > > >  	spin_lock(&vc->lock);
> > > >  	vc->vcore_state = VCORE_INACTIVE;
> > > >  }
> > > > @@ -1613,7 +1613,7 @@
> > > >  			kvmppc_create_dtl_entry(vcpu, vc);
> > > >  			kvmppc_start_thread(vcpu);
> > > >  		} else if (vc->vcore_state == VCORE_SLEEPING) {
> > > > -			wake_up(&vc->wq);
> > > > +			swait_wake(&vc->wq);
> > > 
> > > I notice everywhere you have a swait_wake_interruptible() but here. Is
> > > there a reason why?
> > > 
> > > IIRC, Peter wants to make swait wakeup usage homogenous. That is, you
> > > either sleep in an interruptible state, or you don't. You can't mix and
> > > match it.
> > 
> > IIUC there is only one waiter on this waitqueue at any given time.
> > 
> > Paul is that correct?
> 
> Yes, that's right.  It's only the task that has taken the
> responsibility for running the virtual core that would be waiting on
> that wait queue.

Thanks Paul, but it still makes me nervious.

I'm actually wondering if we should just nuke the _interruptible()
version of swait. As it should only be all interruptible or all not
interruptible, that the swait_wake() should just do the wake up
regardless. In which case, swait_wake() is good enough. No need to have
different versions where people may think do something special.

Peter?

-- Steve
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