Re: [PATCH RFC tip/core/rcu 27/28] rcu: Allow dyntick-idle mode forCPUs with callbacks

[Josh Triplett]

On Wed, Nov 02, 2011 at 01:30:48PM -0700, 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.
> + * Allow the CPU to enter dyntick-idle mode if either: (1) There are no
> + * callbacks on this CPU, (2) this CPU has not yet attempted to enter
> + * dyntick-idle mode, and (3) this CPU is in the process of attempting to
> + * enter dyntick-idle mode.  Otherwise, if we have recently tried and failed

This sentence doesn't quite work; "if either...and..." should become
"either...or" or "if...and".

> + * to enter dyntick-idle mode, we refuse to try to enter it.  After all,
> + * it is better to incur scheduling-clock interrupts than to spin
> + * continuously for the same time duration!
> + */
> +int rcu_needs_cpu(int cpu)
> +{
> +	/* If no callbacks, RCU doesn't need the CPU. */
> +	if (!rcu_cpu_has_callbacks(cpu))
> +		return 0;
> +	/* Otherwise, RCU needs the CPU only if it recently tried and failed. */
> +	return per_cpu(rcu_dyntick_holdoff, cpu) == jiffies;

Sigh, one more use of jiffies. :(

- Josh Triplett
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