Commit 8bd93a2c authored by Paul E. McKenney's avatar Paul E. McKenney Committed by Ingo Molnar

rcu: Accelerate grace period if last non-dynticked 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().

However, if any CPU other than the current one is not in
dyntick-idle mode, fall back to simply checking (with fix to bug
noted by Lai Jiangshan).  Also, take advantage of last
grace-period forcing, the opportunity to do so noted by Steve
Rostedt.  And apply simplified #ifdef condition suggested by
Frederic Weisbecker.
Signed-off-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: laijs@cn.fujitsu.com
Cc: dipankar@in.ibm.com
Cc: mathieu.desnoyers@polymtl.ca
Cc: josh@joshtriplett.org
Cc: dvhltc@us.ibm.com
Cc: niv@us.ibm.com
Cc: peterz@infradead.org
Cc: rostedt@goodmis.org
Cc: Valdis.Kletnieks@vt.edu
Cc: dhowells@redhat.com
LKML-Reference: <1266887105-1528-15-git-send-email-paulmck@linux.vnet.ibm.com>
Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
parent 998f2ac3
......@@ -143,6 +143,8 @@ static inline unsigned int cpumask_any_but(const struct cpumask *mask,
#define for_each_cpu(cpu, mask) \
for ((cpu) = 0; (cpu) < 1; (cpu)++, (void)mask)
#define for_each_cpu_not(cpu, mask) \
for ((cpu) = 0; (cpu) < 1; (cpu)++, (void)mask)
#define for_each_cpu_and(cpu, mask, and) \
for ((cpu) = 0; (cpu) < 1; (cpu)++, (void)mask, (void)and)
#else
......@@ -202,6 +204,18 @@ int cpumask_any_but(const struct cpumask *mask, unsigned int cpu);
(cpu) = cpumask_next((cpu), (mask)), \
(cpu) < nr_cpu_ids;)
/**
* for_each_cpu_not - iterate over every cpu in a complemented mask
* @cpu: the (optionally unsigned) integer iterator
* @mask: the cpumask pointer
*
* After the loop, cpu is >= nr_cpu_ids.
*/
#define for_each_cpu_not(cpu, mask) \
for ((cpu) = -1; \
(cpu) = cpumask_next_zero((cpu), (mask)), \
(cpu) < nr_cpu_ids;)
/**
* for_each_cpu_and - iterate over every cpu in both masks
* @cpu: the (optionally unsigned) integer iterator
......
......@@ -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
......
......@@ -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 ||
......
......@@ -906,3 +906,72 @@ static void __init __rcu_init_preempt(void)
}
#endif /* #else #ifdef CONFIG_TREE_PREEMPT_RCU */
#if !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 /* #if !defined(CONFIG_RCU_FAST_NO_HZ) */
#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_not(thatcpu, nohz_cpu_mask)
if (thatcpu != cpu)
return rcu_needs_cpu_quick_check(cpu);
/* Try to push remaining RCU-sched and RCU-bh callbacks through. */
for (i = 0; i < RCU_NEEDS_CPU_FLUSHES && c; i++) {
c = 0;
if (per_cpu(rcu_sched_data, cpu).nxtlist) {
rcu_sched_qs(cpu);
force_quiescent_state(&rcu_sched_state, 0);
__rcu_process_callbacks(&rcu_sched_state,
&per_cpu(rcu_sched_data, cpu));
c = !!per_cpu(rcu_sched_data, cpu).nxtlist;
}
if (per_cpu(rcu_bh_data, cpu).nxtlist) {
rcu_bh_qs(cpu);
force_quiescent_state(&rcu_bh_state, 0);
__rcu_process_callbacks(&rcu_bh_state,
&per_cpu(rcu_bh_data, cpu));
c = !!per_cpu(rcu_bh_data, cpu).nxtlist;
}
}
/* If RCU callbacks are still pending, RCU still needs this CPU. */
return c;
}
#endif /* #else #if !defined(CONFIG_RCU_FAST_NO_HZ) */
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