Messages in this thread |  | | | Date | Mon, 25 Jan 2010 20:28:34 +0800 | | From | Lai Jiangshan <> | | Subject | Re: [PATCH RFC tip/core/rcu] accelerate grace period if last non-dynticked CPU |
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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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