[PATCH 8/8] RT: Use a 2-d bitmap for searching lowest-pri CPU
From: Gregory Haskins
Date: Mon Nov 05 2007 - 19:52:30 EST
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@xxxxxxxxxx>
---
kernel/Makefile | 1
kernel/sched.c | 4 +
kernel/sched_cpupri.c | 186 +++++++++++++++++++++++++++++++++++++++++++++++++
kernel/sched_cpupri.h | 10 +++
kernel/sched_rt.c | 52 ++------------
5 files changed, 210 insertions(+), 43 deletions(-)
diff --git a/kernel/Makefile b/kernel/Makefile
index e4e2acf..a822706 100644
--- a/kernel/Makefile
+++ b/kernel/Makefile
@@ -66,6 +66,7 @@ obj-$(CONFIG_RELAY) += relay.o
obj-$(CONFIG_SYSCTL) += utsname_sysctl.o
obj-$(CONFIG_TASK_DELAY_ACCT) += delayacct.o
obj-$(CONFIG_TASKSTATS) += taskstats.o tsacct.o
+obj-$(CONFIG_SMP) += sched_cpupri.o
ifneq ($(CONFIG_SCHED_NO_NO_OMIT_FRAME_POINTER),y)
# According to Alan Modra <alan@xxxxxxxxxxxxxxxx>, the -fno-omit-frame-pointer is
diff --git a/kernel/sched.c b/kernel/sched.c
index 0eced8c..6f24aa0 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -70,6 +70,8 @@
#include <asm/tlb.h>
+#include "sched_cpupri.h"
+
/*
* Scheduler clock - returns current time in nanosec units.
* This is default implementation.
@@ -6842,6 +6844,8 @@ void __init sched_init(void)
fair_sched_class.next = &idle_sched_class;
idle_sched_class.next = NULL;
+ cpupri_init();
+
for_each_possible_cpu(i) {
struct rt_prio_array *array;
struct rq *rq;
diff --git a/kernel/sched_cpupri.c b/kernel/sched_cpupri.c
new file mode 100644
index 0000000..e6280b1
--- /dev/null
+++ b/kernel/sched_cpupri.c
@@ -0,0 +1,186 @@
+/*
+ * kernel/sched_cpupri.c
+ *
+ * CPU priority management
+ *
+ * Copyright (C) 2007 Novell
+ *
+ * Author: Gregory Haskins <ghaskins@xxxxxxxxxx>
+ *
+ * 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_CPUS)), 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"
+
+#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 pri_vec
+{
+ raw_spinlock_t lock;
+ cpumask_t mask;
+};
+
+struct cpu_priority {
+ struct pri_vec pri_to_cpu[CPUPRI_NR_PRIORITIES];
+ long pri_active[CPUPRI_NR_PRI_WORDS];
+ int cpu_to_pri[NR_CPUS];
+};
+
+static __cacheline_aligned_in_smp struct cpu_priority cpu_priority;
+
+/* Convert between a 140 based task->prio, and our 102 based cpupri */
+static int convert_prio(int prio)
+{
+ int cpupri;
+
+ 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
+ * @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 task_struct *p, cpumask_t *lowest_mask)
+{
+ int idx = 0;
+ struct cpu_priority *cp = &cpu_priority;
+ int task_pri = convert_prio(p->prio);
+
+ for_each_cpupri_active(cp->pri_active, idx) {
+ struct pri_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
+ * @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(int cpu, int newpri)
+{
+ struct cpu_priority *cp = &cpu_priority;
+ int *currpri = &cp->cpu_to_pri[cpu];
+ int oldpri = *currpri;
+ unsigned long flags;
+
+ newpri = convert_prio(newpri);
+
+ 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 pri_vec *vec = &cp->pri_to_cpu[oldpri];
+
+ spin_lock_irqsave(&vec->lock, flags);
+
+ cpu_clear(cpu, vec->mask);
+ if (cpus_empty(vec->mask))
+ clear_bit(oldpri, cp->pri_active);
+
+ spin_unlock_irqrestore(&vec->lock, flags);
+ }
+
+ if (likely(newpri != CPUPRI_INVALID)) {
+ struct pri_vec *vec = &cp->pri_to_cpu[newpri];
+
+ spin_lock_irqsave(&vec->lock, flags);
+
+ cpu_set(cpu, vec->mask);
+ set_bit(newpri, cp->pri_active);
+
+ spin_unlock_irqrestore(&vec->lock, flags);
+ }
+
+ *currpri = newpri;
+}
+
+/**
+ * cpupri_init - initialize the cpupri subsystem
+ *
+ * This must be called during the scheduler initialization before the
+ * other methods may be used.
+ *
+ * Returns: (void)
+ */
+void cpupri_init(void)
+{
+ struct cpu_priority *cp = &cpu_priority;
+ int i;
+
+ memset(cp, 0, sizeof(*cp));
+
+ for (i = 0; i < CPUPRI_NR_PRIORITIES; i++)
+ spin_lock_init(&cp->pri_to_cpu[i].lock);
+
+ for_each_possible_cpu(i)
+ cp->cpu_to_pri[i] = CPUPRI_INVALID;
+}
+
+
diff --git a/kernel/sched_cpupri.h b/kernel/sched_cpupri.h
new file mode 100644
index 0000000..8cdd15d
--- /dev/null
+++ b/kernel/sched_cpupri.h
@@ -0,0 +1,10 @@
+#ifndef _LINUX_CPUPRI_H
+#define _LINUX_CPUPRI_H
+
+#include <linux/sched.h>
+
+int cpupri_find(struct task_struct *p, cpumask_t *lowest_mask);
+void cpupri_set(int cpu, int pri);
+void cpupri_init(void);
+
+#endif /* _LINUX_CPUPRI_H */
diff --git a/kernel/sched_rt.c b/kernel/sched_rt.c
index 698f4d9..71ae9e6 100644
--- a/kernel/sched_rt.c
+++ b/kernel/sched_rt.c
@@ -72,8 +72,10 @@ static inline void inc_rt_tasks(struct task_struct *p, struct rq *rq)
WARN_ON(!rt_task(p));
rq->rt.rt_nr_running++;
#ifdef CONFIG_SMP
- if (p->prio < rq->rt.highest_prio)
+ if (p->prio < rq->rt.highest_prio) {
rq->rt.highest_prio = p->prio;
+ cpupri_set(rq->cpu, p->prio);
+ }
if (p->nr_cpus_allowed > 1)
rq->rt.rt_nr_migratory++;
@@ -96,6 +98,7 @@ static inline void dec_rt_tasks(struct task_struct *p, struct rq *rq)
array = &rq->rt.active;
rq->rt.highest_prio =
sched_find_first_bit(array->bitmap);
+ cpupri_set(rq->cpu, rq->rt.highest_prio);
} /* otherwise leave rq->highest prio alone */
} else
rq->rt.highest_prio = MAX_RT_PRIO;
@@ -333,46 +336,6 @@ static struct task_struct *pick_next_highest_task_rt(struct rq *rq,
return next;
}
-static int find_lowest_cpus(struct task_struct *task, cpumask_t *lowest_mask)
-{
- int cpu;
- cpumask_t valid_mask;
- int lowest_prio = -1;
- int ret = 0;
-
- cpus_clear(*lowest_mask);
- cpus_and(valid_mask, cpu_online_map, task->cpus_allowed);
-
- /*
- * Scan each rq for the lowest prio.
- */
- for_each_cpu_mask(cpu, valid_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 (ret)
- 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;
- cpus_clear(*lowest_mask);
- }
- cpu_set(rq->cpu, *lowest_mask);
- ret = 1;
- }
- }
-
- return ret;
-}
-
static inline int pick_optimal_cpu(int this_cpu, cpumask_t *mask)
{
int first;
@@ -394,9 +357,12 @@ static int find_lowest_rq(struct task_struct *task)
cpumask_t lowest_mask;
int this_cpu = smp_processor_id();
int cpu = task_cpu(task);
+
+ if (task->nr_cpus_allowed == 1)
+ return -1; /* No other targets possible */
- if (!find_lowest_cpus(task, &lowest_mask))
- return -1;
+ if (!cpupri_find(task, &lowest_mask))
+ return -1; /* No better targets found */
/*
* At this point we have built a mask of cpus representing the
-
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