sched: reintroduce cache-hot affinity

reintroduce a simplified version of cache-hot/cold scheduling
affinity. This improves performance with certain SMP workloads,
such as sysbench.

Signed-off-by: Ingo Molnar <mingo@elte.hu>

3 files changed, 37 insertions, 0 deletions

diff --git a/kernel/sched.c b/kernel/sched.c
index 791dd08c692f..089d8b12ab76 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -2119,6 +2119,17 @@ static void pull_task(struct rq *src_rq, struct task_struct *p,
 }
 
 /*
+ * Is this task likely cache-hot:
+ */
+static inline int
+task_hot(struct task_struct *p, unsigned long long now, struct sched_domain *sd)
+{
+        s64 delta = now - p->se.exec_start;
+
+        return delta < (long long)sysctl_sched_migration_cost;
+}
+
+/*
  * can_migrate_task - may task p from runqueue rq be migrated to this_cpu?
  */
 static
@@ -2139,6 +2150,22 @@ int can_migrate_task(struct task_struct *p, struct rq *rq, int this_cpu,
         if (task_running(rq, p))
                 return 0;
 
+        /*
+         * Aggressive migration if:
+         * 1) task is cache cold, or
+         * 2) too many balance attempts have failed.
+         */
+
+        if (sd->nr_balance_failed > sd->cache_nice_tries) {
+#ifdef CONFIG_SCHEDSTATS
+                if (task_hot(p, rq->clock, sd))
+                        schedstat_inc(sd, lb_hot_gained[idle]);
+#endif
+                return 1;
+        }
+
+        if (task_hot(p, rq->clock, sd))
+                return 0;
         return 1;
 }
 
diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
index cea1fa32b3f4..a17b785d7000 100644
--- a/kernel/sched_fair.c
+++ b/kernel/sched_fair.c
@@ -74,6 +74,8 @@ const_debug unsigned int sysctl_sched_batch_wakeup_granularity = 10000000UL;
  */
 const_debug unsigned int sysctl_sched_wakeup_granularity = 10000000UL;
 
+const_debug unsigned int sysctl_sched_migration_cost = 500000UL;
+
 /**************************************************************
  * CFS operations on generic schedulable entities:
  */
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index 230ca4eb57fe..ec14aa8ac51f 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -277,6 +277,14 @@ static ctl_table kern_table[] = {
                 .mode           = 0644,
                 .proc_handler   = &proc_dointvec,
         },
+        {
+                .ctl_name       = CTL_UNNUMBERED,
+                .procname       = "sched_migration_cost",
+                .data           = &sysctl_sched_migration_cost,
+                .maxlen         = sizeof(unsigned int),
+                .mode           = 0644,
+                .proc_handler   = &proc_dointvec,
+        },
 #endif
         {
                 .ctl_name       = CTL_UNNUMBERED,