linux-kernel - Re: RCU+KVM: making CPU guest mode a quiescent state.

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Message-ID: <4DB7C9F8.5070906@redhat.com>
Date:	Wed, 27 Apr 2011 10:47:04 +0300
From:	Avi Kivity <avi@...hat.com>
To:	paulmck@...ux.vnet.ibm.com
CC:	Gleb Natapov <gleb@...hat.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: RCU+KVM: making CPU guest mode a quiescent state.

On 04/26/2011 06:55 PM, Paul E. McKenney wrote:
> On Tue, Apr 26, 2011 at 03:38:24PM +0300, Gleb Natapov wrote:
> >  Hello Paul,
> >
> >  I have a question about RCU + KVM. KVM does not hold any references to RCU
> >  protected data when it switches CPU into a guest mode. In fact switching
> >  to a guest mode is very similar to exiting to userspase from RCU point
> >  of view. In addition CPU may stay in a guest mode for quite a long time
> >  (up to one time slice). It looks like it will be beneficial to treat guest
> >  mode as quiescent state, just like user-mode execution. How can this be
> >  done? I was trying to find how RCU knows about cpu entering user-mode,
> >  but it seems that it does this by checking CPU mode in a timer interrupt
> >  (update_process_times()->rcu_check_callbacks()). This will not work for
> >  guest mode detection since timer interrupt will kick CPU out of a guest
> >  mode and timer interrupt will always see CPU in kernel mode. Do we have
> >  a simple function to call to notify RCU that CPU passed quiescent state
> >  which we can call just before entering guest?
>
> Hello, Gleb,
>
> You could call rcu_note_context_switch(), passing it the current
> CPU.  Please note that preemption -must- be disabled when calling
> this.  You could call this just after exiting the guest as well
> as just before entering guest.
>

It's expected that after exiting, we'd spend a very short time in the 
kernel, and then either re-enter the guest, exit to userspace, or switch 
to another task.  So I think calling it just before entry should be 
sufficient.

Looking at the code, I see rcu_note_context_switch() calls 
rcu_sched_qs(), which does

     rdp->passed_quiesc_completed = rdp->gpnum - 1;
     barrier();
     rdp->passed_quiesc = 1;

and also calls rcu_preempt_note_context_switch(), which calls 
rcu_preempt_qs(), which does

     rdp->passed_quiesc_completed = rdp->gpnum - 1;
     barrier();
     rdp->passed_quiesc = 1;
     current->rcu_read_unlock_special &= ~RCU_READ_UNLOCK_NEED_QS;

the similarity is remarkable.  Is this intended?  Or did I get lost in a 
maze of #ifdefs?

-- 
error compiling committee.c: too many arguments to function

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