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SubjectRe: RCU+KVM: making CPU guest mode a quiescent state.
On Wed, Apr 27, 2011 at 10:47:04AM +0300, Avi Kivity wrote:
> 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.
Definitely. This will allow other CPUs to complete rcu barrier much
earlier.

>
> 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?
>
One of them works on rcu_sched_data another on rcu_preempt_data as far
as I see.

--
Gleb.


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