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SubjectRe: [PATCH RFC tip/core/rcu] accelerate grace period if last non-dynticked CPU
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()
*/

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