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Subjectscheduler fix / cross node balancer change
Date
A new attempt to fix the 1 CPU per node problem of the current NUMA
scheduler. The previous version was a bit too aggressive with cross
node balancing.

This patch:
- links the frequency of cross-node balances to the number of failed
local balance attempts while simplifying the code,
- fixes the 1 CPU/node issue, i.e. eliminates local balance attempts
for the nodes which have only one CPU. Can happen on any NUMA
platform, is a major issue on x86_64.
- makes the cross-node balance frequency tunable by the parameter
NUMA_FACTOR_BONUS. Its default setting is such that the scheduler
behaves like before: cross node balance every 5 local node balances on
an idle CPU, every 2 local node balances on a busy CPU. This parameter
should be tuned for each platform depending on its NUMA factor.

Regards,
Erich




diff -urNp 2.6.0-test3/include/linux/topology.h 2.6.0-test3-1cpuf/include/linux/topology.h
--- 2.6.0-test3/include/linux/topology.h 2003-08-09 06:38:46.000000000 +0200
+++ 2.6.0-test3-1cpuf/include/linux/topology.h 2003-08-17 00:51:13.000000000 +0200
@@ -48,4 +48,13 @@ static inline int __next_node_with_cpus(
#define for_each_node_with_cpus(node) \
for (node = 0; node < numnodes; node = __next_node_with_cpus(node))

+#ifndef NUMA_FACTOR_BONUS
+/*
+ * High NUMA_FACTOR_BONUS means rare cross-node load balancing. The default
+ * value of 3 means idle_node_rebalance after 5 (failed) local balances,
+ * busy_node_rebalance after 2 failed local balances. Should be tuned for
+ * each platform in asm/topology.h.
+ */
+#define NUMA_FACTOR_BONUS 3
+#endif
#endif /* _LINUX_TOPOLOGY_H */
diff -urNp 2.6.0-test3/kernel/sched.c 2.6.0-test3-1cpuf/kernel/sched.c
--- 2.6.0-test3/kernel/sched.c 2003-08-09 06:39:35.000000000 +0200
+++ 2.6.0-test3-1cpuf/kernel/sched.c 2003-08-17 00:46:56.000000000 +0200
@@ -164,6 +164,7 @@ struct runqueue {
prio_array_t *active, *expired, arrays[2];
int prev_cpu_load[NR_CPUS];
#ifdef CONFIG_NUMA
+ unsigned int nr_lb_failed;
atomic_t *node_nr_running;
int prev_node_load[MAX_NUMNODES];
#endif
@@ -856,6 +857,42 @@ static int find_busiest_node(int this_no
return node;
}

+/*
+ * Decide whether the scheduler should balance locally (inside the same node)
+ * or globally depending on the number of failed local balance attempts.
+ * The number of failed local balance attempts depends on the number of cpus
+ * in the current node. In case it's just one, go immediately for global
+ * balancing. On a busy cpu the number of retries is smaller.
+ * NUMA_FACTOR_BONUS can be used to tune the frequency of global load
+ * balancing (in topology.h).
+ */
+static inline unsigned long cpus_to_balance(int this_cpu, runqueue_t *this_rq)
+{
+ int node, retries, this_node = cpu_to_node(this_cpu);
+
+ if (nr_cpus_node(this_node) == 1) {
+ retries = 0;
+ } else {
+ retries = 2 + NUMA_FACTOR_BONUS;
+ /* less retries for busy CPUs */
+ if (this_rq->curr != this_rq->idle)
+ retries >>= 1;
+ }
+ if (this_rq->nr_lb_failed >= retries) {
+ node = find_busiest_node(this_node);
+ this_rq->nr_lb_failed = 0;
+ if (node >= 0)
+ return (node_to_cpumask(node) | (1UL << this_cpu));
+ }
+ return node_to_cpumask(this_node);
+}
+
+#else /* !CONFIG_NUMA */
+
+static inline unsigned long cpus_to_balance(int this_cpu, runqueue_t *this_rq)
+{
+ return cpu_online_map;
+}
#endif /* CONFIG_NUMA */

#ifdef CONFIG_SMP
@@ -960,6 +997,12 @@ static inline runqueue_t *find_busiest_q
busiest = NULL;
}
out:
+#ifdef CONFIG_NUMA
+ if (!busiest)
+ this_rq->nr_lb_failed++;
+ else
+ this_rq->nr_lb_failed = 0;
+#endif
return busiest;
}

@@ -995,7 +1038,7 @@ static inline void pull_task(runqueue_t
* We call this with the current runqueue locked,
* irqs disabled.
*/
-static void load_balance(runqueue_t *this_rq, int idle, unsigned long cpumask)
+static void load_balance(runqueue_t *this_rq, int idle)
{
int imbalance, idx, this_cpu = smp_processor_id();
runqueue_t *busiest;
@@ -1003,7 +1046,8 @@ static void load_balance(runqueue_t *thi
struct list_head *head, *curr;
task_t *tmp;

- busiest = find_busiest_queue(this_rq, this_cpu, idle, &imbalance, cpumask);
+ busiest = find_busiest_queue(this_rq, this_cpu, idle, &imbalance,
+ cpus_to_balance(this_cpu, this_rq));
if (!busiest)
goto out;

@@ -1085,29 +1129,9 @@ out:
*/
#define IDLE_REBALANCE_TICK (HZ/1000 ?: 1)
#define BUSY_REBALANCE_TICK (HZ/5 ?: 1)
-#define IDLE_NODE_REBALANCE_TICK (IDLE_REBALANCE_TICK * 5)
-#define BUSY_NODE_REBALANCE_TICK (BUSY_REBALANCE_TICK * 2)
-
-#ifdef CONFIG_NUMA
-static void balance_node(runqueue_t *this_rq, int idle, int this_cpu)
-{
- int node = find_busiest_node(cpu_to_node(this_cpu));
- unsigned long cpumask, this_cpumask = 1UL << this_cpu;
-
- if (node >= 0) {
- cpumask = node_to_cpumask(node) | this_cpumask;
- spin_lock(&this_rq->lock);
- load_balance(this_rq, idle, cpumask);
- spin_unlock(&this_rq->lock);
- }
-}
-#endif

static void rebalance_tick(runqueue_t *this_rq, int idle)
{
-#ifdef CONFIG_NUMA
- int this_cpu = smp_processor_id();
-#endif
unsigned long j = jiffies;

/*
@@ -1119,24 +1143,16 @@ static void rebalance_tick(runqueue_t *t
* are not balanced.)
*/
if (idle) {
-#ifdef CONFIG_NUMA
- if (!(j % IDLE_NODE_REBALANCE_TICK))
- balance_node(this_rq, idle, this_cpu);
-#endif
if (!(j % IDLE_REBALANCE_TICK)) {
spin_lock(&this_rq->lock);
- load_balance(this_rq, idle, cpu_to_node_mask(this_cpu));
+ load_balance(this_rq, idle);
spin_unlock(&this_rq->lock);
}
return;
}
-#ifdef CONFIG_NUMA
- if (!(j % BUSY_NODE_REBALANCE_TICK))
- balance_node(this_rq, idle, this_cpu);
-#endif
if (!(j % BUSY_REBALANCE_TICK)) {
spin_lock(&this_rq->lock);
- load_balance(this_rq, idle, cpu_to_node_mask(this_cpu));
+ load_balance(this_rq, idle);
spin_unlock(&this_rq->lock);
}
}
@@ -1308,7 +1324,7 @@ need_resched:
pick_next_task:
if (unlikely(!rq->nr_running)) {
#ifdef CONFIG_SMP
- load_balance(rq, 1, cpu_to_node_mask(smp_processor_id()));
+ load_balance(rq, 1);
if (rq->nr_running)
goto pick_next_task;
#endif
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