Re: [PATCH tip/core/rcu] Do not keep timekeeping CPU tick running for non-nohz_full= CPUs

[Josh Triplett <josh@joshtriplett.org>]

On Sat, Jul 19, 2014 at 09:53:50AM -0700, Paul E. McKenney wrote:
> If a non-nohz_full= CPU is non-idle, it will have a scheduling-clock
> interrupt, and therefore doesn't need the timekeeping CPU to keep
> its scheduling-clock interrupt going.  This commit therefore ignores
> the idle state of non-nohz_full CPUs when determining whether or not
> the timekeeping CPU can safely turn off its scheduling-clock interrupt.
> 
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

Reviewed-by: Josh Triplett <josh@joshtriplett.org>

> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index ddad959a9132..eaa32e4c228d 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -2789,8 +2789,13 @@ static void rcu_sysidle_exit(struct rcu_dynticks *rdtp, int irq)
>  	 * system-idle state.  This means that the timekeeping CPU must
>  	 * invoke rcu_sysidle_force_exit() directly if it does anything
>  	 * more than take a scheduling-clock interrupt.
> +	 *
> +	 * In addition if we are not a nohz_full= CPU, then when we are
> +	 * non-idle we have our own tick, so we don't need the timekeeping
> +	 * CPU to keep a tick on our behalf.  We assume that the timekeeping
> +	 * CPU is also a nohz_full= CPU.
>  	 */
> -	if (smp_processor_id() == tick_do_timer_cpu)
> +	if (!tick_nohz_full_cpu(smp_processor_id()))
>  		return;
>  
>  	/* Update system-idle state: We are clearly no longer fully idle! */
> @@ -2810,11 +2815,11 @@ static void rcu_sysidle_check_cpu(struct rcu_data *rdp, bool *isidle,
>  
>  	/*
>  	 * If some other CPU has already reported non-idle, if this is
> -	 * not the flavor of RCU that tracks sysidle state, or if this
> -	 * is an offline or the timekeeping CPU, nothing to do.
> +	 * not the flavor of RCU that tracks sysidle state, or if this is
> +	 * an offline or !nohz_full= or the timekeeping CPU, nothing to do.
>  	 */
>  	if (!*isidle || rdp->rsp != rcu_sysidle_state ||
> -	    cpu_is_offline(rdp->cpu) || rdp->cpu == tick_do_timer_cpu)
> +	    cpu_is_offline(rdp->cpu) || !tick_nohz_full_cpu(rdp->cpu))
>  		return;
>  	if (rcu_gp_in_progress(rdp->rsp))
>  		WARN_ON_ONCE(smp_processor_id() != tick_do_timer_cpu);
>