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    SubjectRe: [PATCH, RFC] v7 scalable classic RCU implementation
    On Sun, Oct 12, 2008 at 05:52:56PM +0200, Manfred Spraul wrote:
    > Paul E. McKenney wrote:
    >> +/*
    >> + * If the specified CPU is offline, tell the caller that it is in
    >> + * a quiescent state. Otherwise, whack it with a reschedule IPI.
    >> + * Grace periods can end up waiting on an offline CPU when that
    >> + * CPU is in the process of coming online -- it will be added to the
    >> + * rcu_node bitmasks before it actually makes it online. Because this
    >> + * race is quite rare, we check for it after detecting that the grace
    >> + * period has been delayed rather than checking each and every CPU
    >> + * each and every time we start a new grace period.
    >> + */
    >
    > What about using CPU_DYING and CPU_STARTING?
    >
    > Then this race wouldn't exist anymore.

    Because I don't want to tie RCU too tightly to the details of the
    online/offline implementation. It is too easy for someone to make a
    "simple" change and break things, especially given that the online/offline
    code still seems to be adjusting a bit.

    So I might well use CPU_DYING and CPU_STARTING, but I would still keep
    the check offlined CPUs in the force_quiescent_state() processing.

    >> +static void force_quiescent_state(struct rcu_state *rsp, int relaxed)
    >> +{
    >> + [snip]
    >> + case RCU_FORCE_QS:
    >> +
    >> + /* Check dyntick-idle state, send IPI to laggarts. */
    >> + if (rcu_process_dyntick(rsp,
    >> dyntick_recall_completed(rsp),
    >> + rcu_implicit_dynticks_qs))
    >> + goto unlock_ret;
    >> +
    >> + /* Leave state in case more forcing is required. */
    >> +
    >> + break;
    >
    > Hmm - your code must loop multiple times over the cpus.
    > I've use a different approach: More forcing is only required for a nohz cpu
    > when it was hit within a long-running interrupt.
    > Thus I've added a '->kick_poller' flag, rcu_irq_exit() reports back when
    > the long-running interrupt completes. Never more than one loop over the
    > outstanding cpus is required.

    Do you send a reschedule IPI to CPUs that are not in dyntick idle mode,
    but who have failed to pass through a quiescent state?

    In my case, more forcing is required only for a nohz CPU in a long-running
    interrupt (as with your approach), for sending the aforementioned IPI,
    and for checking for offlined CPUs as noted above.

    Thanx, Paul


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