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

[Lai Jiangshan <laijs@cn.fujitsu.com>]

Paul E. McKenney wrote:
> [Experimental RFC, not for inclusion.]
> 
> I recently received a complaint that RCU was refusing to let a system
> go into low-power state immediately, instead waiting a few ticks after
> the system had gone idle before letting go of the last CPU.  Of course,
> the reason for this was that there were a couple of RCU callbacks on
> the last CPU.
> 
> Currently, rcu_needs_cpu() simply checks whether the current CPU has
> an outstanding RCU callback, which means that the last CPU to go into
> dyntick-idle mode might wait a few ticks for the relevant grace periods
> to complete.  However, if all the other CPUs are in dyntick-idle mode,
> and if this CPU is in a quiescent state (which it is for RCU-bh and
> RCU-sched any time that we are considering going into dyntick-idle mode),
> then the grace period is instantly complete.
> 
> This patch therefore repeatedly invokes the RCU grace-period machinery
> in order to force any needed grace periods to complete quickly.  It does
> so a limited number of times in order to prevent starvation by an RCU
> callback function that might pass itself to call_rcu().
> 
> Thoughts?
> 
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> 
> diff --git a/init/Kconfig b/init/Kconfig
> index d95ca7c..42bf914 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -396,6 +396,22 @@ config RCU_FANOUT_EXACT
>  
>  	  Say N if unsure.
>  
> +config RCU_FAST_NO_HZ
> +	bool "Accelerate last non-dyntick-idle CPU's grace periods"
> +	depends on TREE_RCU && NO_HZ && SMP
> +	default n
> +	help
> +	  This option causes RCU to attempt to accelerate grace periods
> +	  in order to allow the final CPU to enter dynticks-idle state
> +	  more quickly.  On the other hand, this option increases the
> +	  overhead of the dynticks-idle checking, particularly on systems
> +	  with large numbers of CPUs.
> +
> +	  Say Y if energy efficiency is critically important, particularly
> +	  	if you have relatively few CPUs.
> +
> +	  Say N if you are unsure.
> +
>  config TREE_RCU_TRACE
>  	def_bool RCU_TRACE && ( TREE_RCU || TREE_PREEMPT_RCU )
>  	select DEBUG_FS
> diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> index 099a255..29d88c0 100644
> --- a/kernel/rcutree.c
> +++ b/kernel/rcutree.c
> @@ -1550,10 +1550,9 @@ static int rcu_pending(int cpu)
>  /*
>   * 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.
> + * 1 if so.
>   */
> -int rcu_needs_cpu(int cpu)
> +static int rcu_needs_cpu_quick_check(int cpu)
>  {
>  	/* RCU callbacks either ready or pending? */
>  	return per_cpu(rcu_sched_data, cpu).nxtlist ||
> diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
> index e77cdf3..d6170a9 100644
> --- a/kernel/rcutree_plugin.h
> +++ b/kernel/rcutree_plugin.h
> @@ -906,3 +906,72 @@ static void __init __rcu_init_preempt(void)
>  }
>  
>  #endif /* #else #ifdef CONFIG_TREE_PREEMPT_RCU */
> +
> +#if defined(CONFIG_TREE_PREEMPT_RCU) || !defined(CONFIG_RCU_FAST_NO_HZ)
> +
> +/*
> + * 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 have preemptible RCU, just check whether this CPU needs
> + * any flavor of RCU.  Do not chew up lots of CPU cycles with preemption
> + * disabled in a most-likely vain attempt to cause RCU not to need this CPU.
> + */
> +int rcu_needs_cpu(int cpu)
> +{
> +	return rcu_needs_cpu_quick_check(cpu);
> +}
> +
> +#else
> +
> +#define RCU_NEEDS_CPU_FLUSHES 5
> +
> +/*
> + * 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);

The comment and the code are not the same, I think.

-----------
I found this thing, Although I think it is a ugly thing.
Is it help?

See select_nohz_load_balancer().

/*
 * This routine will try to nominate the ilb (idle load balancing)
 * owner among the cpus whose ticks are stopped. ilb owner will do the idle
 * load balancing on behalf of all those cpus. If all the cpus in the system
 * go into this tickless mode, then there will be no ilb owner (as there is
 * no need for one) and all the cpus will sleep till the next wakeup event
 * arrives...
 *
 * For the ilb owner, tick is not stopped. And this tick will be used
 * for idle load balancing. ilb owner will still be part of
 * nohz.cpu_mask..
 *
 * While stopping the tick, this cpu will become the ilb owner if there
 * is no other owner. And will be the owner till that cpu becomes busy
 * or if all cpus in the system stop their ticks at which point
 * there is no need for ilb owner.
 *
 * When the ilb owner becomes busy, it nominates another owner, during the
 * next busy scheduler_tick()
 */