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sched: use a 2-d bitmap for searching lowest-pri CPU

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The current code use a linear algorithm which causes scaling issues
on larger SMP machines.  This patch replaces that algorithm with a
2-dimensional bitmap to reduce latencies in the wake-up path.

Signed-off-by: Gregory Haskins <ghaskins@novell.com>
Acked-by: Steven Rostedt <srostedt@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
This commit is contained in:
Gregory Haskins 2008-05-12 21:21:01 +02:00 committed by Ingo Molnar
parent f333fdc909
commit 6e0534f278
5 changed files with 239 additions and 77 deletions
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1
kernel/Makefile
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@ -69,6 +69,7 @@ obj-$(CONFIG_TASK_DELAY_ACCT) += delayacct.o
obj-$(CONFIG_TASKSTATS) += taskstats.o tsacct.o
obj-$(CONFIG_MARKERS) += marker.o
obj-$(CONFIG_LATENCYTOP) += latencytop.o
obj-$(CONFIG_SMP) += sched_cpupri.o
ifneq ($(CONFIG_SCHED_NO_NO_OMIT_FRAME_POINTER),y)
# According to Alan Modra <alan@linuxcare.com.au>, the -fno-omit-frame-pointer is

7
kernel/sched.c
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@ -74,6 +74,8 @@
#include <asm/tlb.h>
#include <asm/irq_regs.h>
#include "sched_cpupri.h"
/*
* Convert user-nice values [ -20 ... 0 ... 19 ]
* to static priority [ MAX_RT_PRIO..MAX_PRIO-1 ],
@ -450,6 +452,9 @@ struct root_domain {
*/
cpumask_t rto_mask;
atomic_t rto_count;
#ifdef CONFIG_SMP
struct cpupri cpupri;
#endif
};
/*
@ -6392,6 +6397,8 @@ static void init_rootdomain(struct root_domain *rd)
cpus_clear(rd->span);
cpus_clear(rd->online);
cpupri_init(&rd->cpupri);
}
static void init_defrootdomain(void)

174
kernel/sched_cpupri.c Normal file
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@ -0,0 +1,174 @@
/*
* kernel/sched_cpupri.c
*
* CPU priority management
*
* Copyright (C) 2007-2008 Novell
*
* Author: Gregory Haskins <ghaskins@novell.com>
*
* This code tracks the priority of each CPU so that global migration
* decisions are easy to calculate. Each CPU can be in a state as follows:
*
* (INVALID), IDLE, NORMAL, RT1, ... RT99
*
* going from the lowest priority to the highest. CPUs in the INVALID state
* are not eligible for routing. The system maintains this state with
* a 2 dimensional bitmap (the first for priority class, the second for cpus
* in that class). Therefore a typical application without affinity
* restrictions can find a suitable CPU with O(1) complexity (e.g. two bit
* searches). For tasks with affinity restrictions, the algorithm has a
* worst case complexity of O(min(102, nr_domcpus)), though the scenario that
* yields the worst case search is fairly contrived.
*
* This program is free software; you can redistribute it and/or
* modify it under the terms of the GNU General Public License
* as published by the Free Software Foundation; version 2
* of the License.
*/
#include "sched_cpupri.h"
/* Convert between a 140 based task->prio, and our 102 based cpupri */
static int convert_prio(int prio)
{
int cpupri;
if (prio == CPUPRI_INVALID)
cpupri = CPUPRI_INVALID;
else if (prio == MAX_PRIO)
cpupri = CPUPRI_IDLE;
else if (prio >= MAX_RT_PRIO)
cpupri = CPUPRI_NORMAL;
else
cpupri = MAX_RT_PRIO - prio + 1;
return cpupri;
}
#define for_each_cpupri_active(array, idx) \
for (idx = find_first_bit(array, CPUPRI_NR_PRIORITIES); \
idx < CPUPRI_NR_PRIORITIES; \
idx = find_next_bit(array, CPUPRI_NR_PRIORITIES, idx+1))
/**
* cpupri_find - find the best (lowest-pri) CPU in the system
* @cp: The cpupri context
* @p: The task
* @lowest_mask: A mask to fill in with selected CPUs
*
* Note: This function returns the recommended CPUs as calculated during the
* current invokation. By the time the call returns, the CPUs may have in
* fact changed priorities any number of times. While not ideal, it is not
* an issue of correctness since the normal rebalancer logic will correct
* any discrepancies created by racing against the uncertainty of the current
* priority configuration.
*
* Returns: (int)bool - CPUs were found
*/
int cpupri_find(struct cpupri *cp, struct task_struct *p,
cpumask_t *lowest_mask)
{
int idx = 0;
int task_pri = convert_prio(p->prio);
for_each_cpupri_active(cp->pri_active, idx) {
struct cpupri_vec *vec = &cp->pri_to_cpu[idx];
cpumask_t mask;
if (idx >= task_pri)
break;
cpus_and(mask, p->cpus_allowed, vec->mask);
if (cpus_empty(mask))
continue;
*lowest_mask = mask;
return 1;
}
return 0;
}
/**
* cpupri_set - update the cpu priority setting
* @cp: The cpupri context
* @cpu: The target cpu
* @pri: The priority (INVALID-RT99) to assign to this CPU
*
* Note: Assumes cpu_rq(cpu)->lock is locked
*
* Returns: (void)
*/
void cpupri_set(struct cpupri *cp, int cpu, int newpri)
{
int *currpri = &cp->cpu_to_pri[cpu];
int oldpri = *currpri;
unsigned long flags;
newpri = convert_prio(newpri);
BUG_ON(newpri >= CPUPRI_NR_PRIORITIES);
if (newpri == oldpri)
return;
/*
* If the cpu was currently mapped to a different value, we
* first need to unmap the old value
*/
if (likely(oldpri != CPUPRI_INVALID)) {
struct cpupri_vec *vec = &cp->pri_to_cpu[oldpri];
spin_lock_irqsave(&vec->lock, flags);
vec->count--;
if (!vec->count)
clear_bit(oldpri, cp->pri_active);
cpu_clear(cpu, vec->mask);
spin_unlock_irqrestore(&vec->lock, flags);
}
if (likely(newpri != CPUPRI_INVALID)) {
struct cpupri_vec *vec = &cp->pri_to_cpu[newpri];
spin_lock_irqsave(&vec->lock, flags);
cpu_set(cpu, vec->mask);
vec->count++;
if (vec->count == 1)
set_bit(newpri, cp->pri_active);
spin_unlock_irqrestore(&vec->lock, flags);
}
*currpri = newpri;
}
/**
* cpupri_init - initialize the cpupri structure
* @cp: The cpupri context
*
* Returns: (void)
*/
void cpupri_init(struct cpupri *cp)
{
int i;
memset(cp, 0, sizeof(*cp));
for (i = 0; i < CPUPRI_NR_PRIORITIES; i++) {
struct cpupri_vec *vec = &cp->pri_to_cpu[i];
spin_lock_init(&vec->lock);
vec->count = 0;
cpus_clear(vec->mask);
}
for_each_possible_cpu(i)
cp->cpu_to_pri[i] = CPUPRI_INVALID;
}

36
kernel/sched_cpupri.h Normal file
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@ -0,0 +1,36 @@
#ifndef _LINUX_CPUPRI_H
#define _LINUX_CPUPRI_H
#include <linux/sched.h>
#define CPUPRI_NR_PRIORITIES 2+MAX_RT_PRIO
#define CPUPRI_NR_PRI_WORDS CPUPRI_NR_PRIORITIES/BITS_PER_LONG
#define CPUPRI_INVALID -1
#define CPUPRI_IDLE 0
#define CPUPRI_NORMAL 1
/* values 2-101 are RT priorities 0-99 */
struct cpupri_vec {
spinlock_t lock;
int count;
cpumask_t mask;
};
struct cpupri {
struct cpupri_vec pri_to_cpu[CPUPRI_NR_PRIORITIES];
long pri_active[CPUPRI_NR_PRI_WORDS];
int cpu_to_pri[NR_CPUS];
};
#ifdef CONFIG_SMP
int cpupri_find(struct cpupri *cp,
struct task_struct *p, cpumask_t *lowest_mask);
void cpupri_set(struct cpupri *cp, int cpu, int pri);
void cpupri_init(struct cpupri *cp);
#else
#define cpupri_set(cp, cpu, pri) do { } while (0)
#define cpupri_init() do { } while (0)
#endif
#endif /* _LINUX_CPUPRI_H */

98
kernel/sched_rt.c
View File

@ -391,8 +391,11 @@ void inc_rt_tasks(struct sched_rt_entity *rt_se, struct rt_rq *rt_rq)
WARN_ON(!rt_prio(rt_se_prio(rt_se)));
rt_rq->rt_nr_running++;
#if defined CONFIG_SMP || defined CONFIG_RT_GROUP_SCHED
if (rt_se_prio(rt_se) < rt_rq->highest_prio)
if (rt_se_prio(rt_se) < rt_rq->highest_prio) {
struct rq *rq = rq_of_rt_rq(rt_rq);
rt_rq->highest_prio = rt_se_prio(rt_se);
cpupri_set(&rq->rd->cpupri, rq->cpu, rt_se_prio(rt_se));
}
#endif
#ifdef CONFIG_SMP
if (rt_se->nr_cpus_allowed > 1) {
@ -416,6 +419,10 @@ void inc_rt_tasks(struct sched_rt_entity *rt_se, struct rt_rq *rt_rq)
static inline
void dec_rt_tasks(struct sched_rt_entity *rt_se, struct rt_rq *rt_rq)
{
#ifdef CONFIG_SMP
int highest_prio = rt_rq->highest_prio;
#endif
WARN_ON(!rt_prio(rt_se_prio(rt_se)));
WARN_ON(!rt_rq->rt_nr_running);
rt_rq->rt_nr_running--;
@ -439,6 +446,11 @@ void dec_rt_tasks(struct sched_rt_entity *rt_se, struct rt_rq *rt_rq)
rq->rt.rt_nr_migratory--;
}
if (rt_rq->highest_prio != highest_prio) {
struct rq *rq = rq_of_rt_rq(rt_rq);
cpupri_set(&rq->rd->cpupri, rq->cpu, rt_rq->highest_prio);
}
update_rt_migration(rq_of_rt_rq(rt_rq));
#endif /* CONFIG_SMP */
#ifdef CONFIG_RT_GROUP_SCHED
@ -763,73 +775,6 @@ static struct task_struct *pick_next_highest_task_rt(struct rq *rq, int cpu)
static DEFINE_PER_CPU(cpumask_t, local_cpu_mask);
static int find_lowest_cpus(struct task_struct *task, cpumask_t *lowest_mask)
{
int lowest_prio = -1;
int lowest_cpu = -1;
int count = 0;
int cpu;
cpus_and(*lowest_mask, task_rq(task)->rd->online, task->cpus_allowed);
/*
* Scan each rq for the lowest prio.
*/
for_each_cpu_mask(cpu, *lowest_mask) {
struct rq *rq = cpu_rq(cpu);
/* We look for lowest RT prio or non-rt CPU */
if (rq->rt.highest_prio >= MAX_RT_PRIO) {
/*
* if we already found a low RT queue
* and now we found this non-rt queue
* clear the mask and set our bit.
* Otherwise just return the queue as is
* and the count==1 will cause the algorithm
* to use the first bit found.
*/
if (lowest_cpu != -1) {
cpus_clear(*lowest_mask);
cpu_set(rq->cpu, *lowest_mask);
}
return 1;
}
/* no locking for now */
if ((rq->rt.highest_prio > task->prio)
&& (rq->rt.highest_prio >= lowest_prio)) {
if (rq->rt.highest_prio > lowest_prio) {
/* new low - clear old data */
lowest_prio = rq->rt.highest_prio;
lowest_cpu = cpu;
count = 0;
}
count++;
} else
cpu_clear(cpu, *lowest_mask);
}
/*
* Clear out all the set bits that represent
* runqueues that were of higher prio than
* the lowest_prio.
*/
if (lowest_cpu > 0) {
/*
* Perhaps we could add another cpumask op to
* zero out bits. Like cpu_zero_bits(cpumask, nrbits);
* Then that could be optimized to use memset and such.
*/
for_each_cpu_mask(cpu, *lowest_mask) {
if (cpu >= lowest_cpu)
break;
cpu_clear(cpu, *lowest_mask);
}
}
return count;
}
static inline int pick_optimal_cpu(int this_cpu, cpumask_t *mask)
{
int first;
@ -851,18 +796,13 @@ static int find_lowest_rq(struct task_struct *task)
cpumask_t *lowest_mask = &__get_cpu_var(local_cpu_mask);
int this_cpu = smp_processor_id();
int cpu = task_cpu(task);
int count = find_lowest_cpus(task, lowest_mask);
if (!count)
if (task->rt.nr_cpus_allowed == 1)
return -1; /* No other targets possible */
if (!cpupri_find(&task_rq(task)->rd->cpupri, task, lowest_mask))
return -1; /* No targets found */
/*
* There is no sense in performing an optimal search if only one
* target is found.
*/
if (count == 1)
return first_cpu(*lowest_mask);
/*
* At this point we have built a mask of cpus representing the
* lowest priority tasks in the system. Now we want to elect
@ -1218,6 +1158,8 @@ static void join_domain_rt(struct rq *rq)
{
if (rq->rt.overloaded)
rt_set_overload(rq);
cpupri_set(&rq->rd->cpupri, rq->cpu, rq->rt.highest_prio);
}
/* Assumes rq->lock is held */
@ -1225,6 +1167,8 @@ static void leave_domain_rt(struct rq *rq)
{
if (rq->rt.overloaded)
rt_clear_overload(rq);
cpupri_set(&rq->rd->cpupri, rq->cpu, CPUPRI_INVALID);
}
/*