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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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