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SubjectRe: [v7 0/8] Reduce cross CPU IPI interference
On Thu, Feb 09, 2012 at 03:41:45PM -0800, Paul E. McKenney wrote:
> On Thu, Feb 09, 2012 at 07:22:19PM +0100, Frederic Weisbecker wrote:
> > On Thu, Feb 09, 2012 at 06:05:07PM +0200, Avi Kivity wrote:
> > > On 02/09/2012 05:22 PM, Frederic Weisbecker wrote:
> > > > > >
> > > > > > Looks like there are new rcu_user_enter() and rcu_user_exit() APIs which
> > > > > > we can use. Hopefully they subsume rcu_virt_note_context_switch() so we
> > > > > > only need one set of APIs.
> > > > >
> > > > > Now that you mention it, that is a good goal. However, it requires
> > > > > coordination with Frederic's code as well, so some investigation
> > > > > is required. Bad things happen if you tell RCU you are idle when you
> > > > > really are not and vice versa!
> > > > >
> > > > > Thanx, Paul
> > > > >
> > > >
> > > > Right. Avi I need to know more about what you need. rcu_virt_note_context_switch()
> > > > notes a quiescent state while rcu_user_enter() shuts down RCU (it's in fact the same
> > > > thing than rcu_idle_enter() minus the is_idle_cpu() checks).
> > >
> > > I don't know enough about RCU to say if it's okay or not (I typically
> > > peek at the quick quiz answers). However, switching to guest mode is
> > > very similar to exiting to user mode: we're guaranteed not to be in an
> > > rcu critical section, and to remain so until the guest exits back to
> > > us.
> >
> > Awesome!
> >
> > > What guarantees does rcu_user_enter() provide? With luck guest
> > > entry satisifies them all.
> >
> > So rcu_user_enter() puts the CPU into RCU idle mode, which means the CPU
> > won't need to be part of the global RCU grace period completion. This
> > prevents it to depend on the timer tick (although for now you keep it)
> > and to complete some RCU specific work during the tick.
> >
> > Paul, do you think that would be a win?
>
> As long as the code doesn't enter RCU read-side critical sections in
> the time between rcu_idle_enter() and rcu_idle_exit(), this should
> work fine.

This should work fine yeah but further the correctness, I wonder if this
is going to be a win.

We use rcu_idle_enter() in idle to avoid to keep the tick for RCU. But
what about falling into guest mode? I guess the tick is kept there
so is it going to be a win in throughput or something to use rcu_idle_enter()?


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