sched: avoid large irq-latencies in smp-balancing

SMP balancing is done with IRQs disabled and can iterate the full rq.
When rqs are large this can cause large irq-latencies. Limit the nr of
iterations on each run.

This fixes a scheduling latency regression reported by the -rt folks.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
Tested-by: Gregory Haskins <ghaskins@novell.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

3 files changed, 19 insertions, 5 deletions

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 93fd30d6dac4..2cc789fef711 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1466,6 +1466,7 @@ extern unsigned int sysctl_sched_batch_wakeup_granularity;
 extern unsigned int sysctl_sched_child_runs_first;
 extern unsigned int sysctl_sched_features;
 extern unsigned int sysctl_sched_migration_cost;
+extern unsigned int sysctl_sched_nr_migrate;
 
 int sched_nr_latency_handler(struct ctl_table *table, int write,
                 struct file *file, void __user *buffer, size_t *length,
diff --git a/kernel/sched.c b/kernel/sched.c
index 2a107e4ad5ed..e195a4229418 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -472,6 +472,12 @@ const_debug unsigned int sysctl_sched_features =
 #define sched_feat(x) (sysctl_sched_features & SCHED_FEAT_##x)
 
 /*
+ * Number of tasks to iterate in a single balance run.
+ * Limited because this is done with IRQs disabled.
+ */
+const_debug unsigned int sysctl_sched_nr_migrate = 32;
+
+/*
  * For kernel-internal use: high-speed (but slightly incorrect) per-cpu
  * clock constructed from sched_clock():
  */
@@ -2235,7 +2241,7 @@ balance_tasks(struct rq *this_rq, int this_cpu, struct rq *busiest,
               enum cpu_idle_type idle, int *all_pinned,
               int *this_best_prio, struct rq_iterator *iterator)
 {
-        int pulled = 0, pinned = 0, skip_for_load;
+        int loops = 0, pulled = 0, pinned = 0, skip_for_load;
         struct task_struct *p;
         long rem_load_move = max_load_move;
 
@@ -2249,10 +2255,10 @@ balance_tasks(struct rq *this_rq, int this_cpu, struct rq *busiest,
          */
         p = iterator->start(iterator->arg);
 next:
-        if (!p)
+        if (!p || loops++ > sysctl_sched_nr_migrate)
                 goto out;
         /*
-         * To help distribute high priority tasks accross CPUs we don't
+         * To help distribute high priority tasks across CPUs we don't
          * skip a task if it will be the highest priority task (i.e. smallest
          * prio value) on its new queue regardless of its load weight
          */
@@ -2269,8 +2275,7 @@ next:
         rem_load_move -= p->se.load.weight;
 
         /*
-         * We only want to steal up to the prescribed number of tasks
-         * and the prescribed amount of weighted load.
+         * We only want to steal up to the prescribed amount of weighted load.
          */
         if (rem_load_move > 0) {
                 if (p->prio < *this_best_prio)
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index adddf682d4cb..3a1744fed2b6 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -301,6 +301,14 @@ static struct ctl_table kern_table[] = {
                 .mode           = 0644,
                 .proc_handler   = &proc_dointvec,
         },
+        {
+                .ctl_name       = CTL_UNNUMBERED,
+                .procname       = "sched_nr_migrate",
+                .data           = &sysctl_sched_nr_migrate,
+                .maxlen         = sizeof(unsigned int),
+                .mode           = 644,
+                .proc_handler   = &proc_dointvec,
+        },
 #endif
         {
                 .ctl_name       = CTL_UNNUMBERED,