Re: [PATCH, RFC] v7 scalable classic RCU implementation

[Manfred Spraul <manfred@colorfullife.com>]

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.
> +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.

--
    Manfred