Re: [PATCH RFC tip/core/rcu] accelerate grace period if last non-dynticked CPU

[Steven Rostedt <rostedt@goodmis.org>]

On Sun, 2010-01-24 at 19:48 -0800, Paul E. McKenney wrote:

> +/*
> + * Check to see if any future RCU-related work will need to be done
> + * by the current CPU, even if none need be done immediately, returning
> + * 1 if so.  This function is part of the RCU implementation; it is -not-
> + * an exported member of the RCU API.
> + *
> + * Because we are not supporting preemptible RCU, attempt to accelerate
> + * any current grace periods so that RCU no longer needs this CPU, but
> + * only if all other CPUs are already in dynticks-idle mode.  This will
> + * allow the CPU cores to be powered down immediately, as opposed to after
> + * waiting many milliseconds for grace periods to elapse.
> + */
> +int rcu_needs_cpu(int cpu)
> +{
> +	int c = 1;
> +	int i;
> +	int thatcpu;
> +
> +	/* Don't bother unless we are the last non-dyntick-idle CPU. */
> +	for_each_cpu(thatcpu, nohz_cpu_mask)
> +		if (thatcpu != cpu)
> +			return rcu_needs_cpu_quick_check(cpu);
> +
> +	/* Try to push remaining RCU-sched and RCU-bh callbacks through. */
> +	for (i = 0; i < RCU_NEEDS_CPU_FLUSHES && c; i++) {
> +		c = 0;
> +		if (per_cpu(rcu_sched_data, cpu).nxtlist) {
> +			c = 1;
> +			rcu_sched_qs(cpu);
> +			force_quiescent_state(&rcu_sched_state, 0);
> +			__rcu_process_callbacks(&rcu_sched_state,
> +						&per_cpu(rcu_sched_data, cpu));

> +		}
> +		if (per_cpu(rcu_bh_data, cpu).nxtlist) {
> +			c = 1;
> +			rcu_bh_qs(cpu);
> +			force_quiescent_state(&rcu_bh_state, 0);
> +			__rcu_process_callbacks(&rcu_bh_state,
> +						&per_cpu(rcu_bh_data, cpu));
> +		}
> +	}
> +
> +	/* If RCU callbacks are still pending, RCU still needs this CPU. */
> +	return c;

What happens if the last loop pushes out all callbacks? Then we would be
returning 1 when we could really be returning 0. Wouldn't a better
answer be:

	return per_cpu(rcu_sched_data, cpu).nxtlist ||
		per_cpu(rcu_bh_data, cpu).nxtlist;

-- Steve


> +}
> +
> +#endif