sched: optimize RT affinity

The current code base assumes a relatively flat CPU/core topology and will
route RT tasks to any CPU fairly equally.  In the real world, there are
various toplogies and affinities that govern where a task is best suited to
run with the smallest amount of overhead.  NUMA and multi-core CPUs are
prime examples of topologies that can impact cache performance.

Fortunately, linux is already structured to represent these topologies via
the sched_domains interface.  So we change our RT router to consult a
combination of topology and affinity policy to best place tasks during
migration.

Signed-off-by: Gregory Haskins <ghaskins@novell.com>
Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 88 insertions, 12 deletions

diff --git a/kernel/sched_rt.c b/kernel/sched_rt.c
index ac7d06786454..a9d7d4408160 100644
--- a/kernel/sched_rt.c
+++ b/kernel/sched_rt.c
@@ -281,35 +281,111 @@ static struct task_struct *pick_next_highest_task_rt(struct rq *rq,
 }
 
 static DEFINE_PER_CPU(cpumask_t, local_cpu_mask);
+static DEFINE_PER_CPU(cpumask_t, valid_cpu_mask);
 
-static int find_lowest_rq(struct task_struct *task)
+static int find_lowest_cpus(struct task_struct *task, cpumask_t *lowest_mask)
 {
-        int cpu;
-        cpumask_t *cpu_mask = &__get_cpu_var(local_cpu_mask);
-        struct rq *lowest_rq = NULL;
+        int       cpu;
+        cpumask_t *valid_mask = &__get_cpu_var(valid_cpu_mask);
+        int       lowest_prio = -1;
+        int       ret         = 0;
 
-        cpus_and(*cpu_mask, cpu_online_map, task->cpus_allowed);
+        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, *cpu_mask) {
+        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) {
-                        lowest_rq = rq;
-                        break;
+                        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 &&
-                    (!lowest_rq || rq->rt.highest_prio > lowest_rq->rt.highest_prio)) {
-                        lowest_rq = rq;
+                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;
+
+        /* "this_cpu" is cheaper to preempt than a remote processor */
+        if ((this_cpu != -1) && cpu_isset(this_cpu, *mask))
+                return this_cpu;
+
+        first = first_cpu(*mask);
+        if (first != NR_CPUS)
+                return first;
+
+        return -1;
+}
+
+static int find_lowest_rq(struct task_struct *task)
+{
+        struct sched_domain *sd;
+        cpumask_t *lowest_mask = &__get_cpu_var(local_cpu_mask);
+        int this_cpu = smp_processor_id();
+        int cpu      = task_cpu(task);
+
+        if (!find_lowest_cpus(task, lowest_mask))
+                return -1;
+
+        /*
+         * At this point we have built a mask of cpus representing the
+         * lowest priority tasks in the system.  Now we want to elect
+         * the best one based on our affinity and topology.
+         *
+         * We prioritize the last cpu that the task executed on since
+         * it is most likely cache-hot in that location.
+         */
+        if (cpu_isset(cpu, *lowest_mask))
+                return cpu;
+
+        /*
+         * Otherwise, we consult the sched_domains span maps to figure
+         * out which cpu is logically closest to our hot cache data.
+         */
+        if (this_cpu == cpu)
+                this_cpu = -1; /* Skip this_cpu opt if the same */
+
+        for_each_domain(cpu, sd) {
+                if (sd->flags & SD_WAKE_AFFINE) {
+                        cpumask_t domain_mask;
+                        int       best_cpu;
+
+                        cpus_and(domain_mask, sd->span, *lowest_mask);
+
+                        best_cpu = pick_optimal_cpu(this_cpu,
+                                                    &domain_mask);
+                        if (best_cpu != -1)
+                                return best_cpu;
                 }
         }
 
-        return lowest_rq ? lowest_rq->cpu : -1;
+        /*
+         * And finally, if there were no matches within the domains
+         * just give the caller *something* to work with from the compatible
+         * locations.
+         */
+        return pick_optimal_cpu(this_cpu, lowest_mask);
 }
 
 /* Will lock the rq it finds */