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    SubjectRe: [PATCH RFC tip/core/rcu] accelerate grace period if last non-dynticked CPU
    On Mon, Jan 25, 2010 at 08:28:34PM +0800, Lai Jiangshan wrote:
    > 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.

    Indeed, for this code to be correct, I would need to sequence through
    the non-dyntick-idle CPUs, not the dyntick-idle ones.

    Good catch!

    I will likely come back with something similar to Steve Rostedt's
    suggestion. Probably better to sequence through all the CPUs rather
    than to allocate a cpumask and invert it. Or a 'for_each_cpu_not()'
    or some such. ;-)

    There does appear to be a cpumask_next_zero() that I should be able to
    use.

    Thanx, Paul

    > -----------
    > 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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