Messages in this thread | | | Date | Tue, 26 Jan 2010 21:17:54 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH RFC tip/core/rcu] accelerate grace period if last non-dynticked CPU |
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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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