Messages in this thread | | | Date | Wed, 27 Apr 2011 10:47:04 +0300 | From | Avi Kivity <> | Subject | Re: RCU+KVM: making CPU guest mode a quiescent state. |
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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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