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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