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    SubjectRe: [PATCH 05/32] nohz: Move rcu dynticks idle mode handling to idle enter/exit APIs
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    On Mon, 2011-08-29 at 19:59 +0200, Frederic Weisbecker wrote:
    > On Mon, Aug 29, 2011 at 07:49:15PM +0200, Peter Zijlstra wrote:
    > > On Mon, 2011-08-29 at 19:11 +0200, Frederic Weisbecker wrote:
    > > > On Mon, Aug 29, 2011 at 04:25:22PM +0200, Peter Zijlstra wrote:
    > > > > On Mon, 2011-08-15 at 17:52 +0200, Frederic Weisbecker wrote:
    > > > > > To prepare for nohz / idle logic split, pull out the rcu dynticks
    > > > > > idle mode switching to strict idle entry/exit areas.
    > > > > >
    > > > > > So we make the dyntick mode possible without always involving rcu
    > > > > > extended quiescent state.
    > > > >
    > > > > Why is this a good thing? I would be thinking that if we're a userspace
    > > > > bound task and we disable the tick rcu would be finished on this cpu and
    > > > > thus the extended quiescent state is just what we want?
    > > >
    > > > But we can stop the tick from the kernel, not just userspace.
    > >
    > > Humm!? I'm confused, I thought the idea was to only stop the tick when
    > > we're 'stuck' in a user bound task. Now I get that we have to stop the
    > > tick from kernel space (as in the interrupt will clearly run in kernel
    > > space), but assuming the normal return from interrupt path doesn't use
    > > rcu, and using rcu (as per a later patch) re-enables the tick again, it
    > > doesn't matter, right?
    >
    > Yeah. Either the interrupt returns to userspace and then we call
    > rcu_enter_nohz() or we return to kernel space and then a further
    > use of rcu will restart the tick.
    >
    > Now this is not any use of rcu. Uses of rcu read side critical section
    > don't need the tick.

    But but but, then how is it going to stop a grace period from happening?
    The grace period state is per-cpu and the whole state machine is tick
    driven.

    Now some of the new RCU things go kick cpus with IPIs to push grace
    periods along, but I would expect you don't want that to happen either,
    the whole purpose here is to leave a cpu alone, unperturbed.

    That means it has to be in an extended grace period when we stop the
    tick.

    > But we need it as long as there is an RCU callback
    > enqueued on some CPU.

    Well, no, only if there's one enqueued on this cpu because then we can't
    enter the extended grace period.

    > > Also, RCU needs the tick to drive the state machine, so how can you stop
    > > the tick and not also stop the RCU state machine?
    >
    > This is why we have rcu_needs_cpu() and rcu_pending() checks before
    > stopping the tick.
    >
    > rcu_needs_cpu() checks we have no local callback enqueued, in which
    > case the local CPU is responsible of the RCU state machine.
    >
    > rcu_pending() is there to know if another CPU started a grace period
    > so we need the tick to complete it.

    Hence the extended grace period, so we don't need to complete grace
    periods.


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