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Date: Mon, 05 Nov 2007 16:49:03 -0700
From: Gregory Haskins <ghaskins@...ell.com>
To: rostedt@...dmis.org
Cc: mingo@...e.hu, dvhltc@...ibm.com, zijlstra@...hat.com,
linux-kernel@...r.kernel.org, linux-rt-users@...r.kernel.org,
ghaskins@...ell.com
Subject: [PATCH 6/8] RT: Optimize our cpu selection based on topology
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@...ell.com>
---
kernel/sched.c | 1 +
kernel/sched_rt.c | 99 +++++++++++++++++++++++++++++++++++++++++++++++------
2 files changed, 88 insertions(+), 12 deletions(-)
diff --git a/kernel/sched.c b/kernel/sched.c
index d16c686..8a27f09 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -26,6 +26,7 @@
* 2007-07-01 Group scheduling enhancements by Srivatsa Vaddagiri
* 2007-10-22 RT overload balancing by Steven Rostedt
* (with thanks to Gregory Haskins)
+ * 2007-11-05 RT migration/wakeup tuning by Gregory Haskins
*/
#include <linux/mm.h>
diff --git a/kernel/sched_rt.c b/kernel/sched_rt.c
index 43d4ea6..6ba5921 100644
--- a/kernel/sched_rt.c
+++ b/kernel/sched_rt.c
@@ -331,34 +331,109 @@ static struct task_struct *pick_next_highest_task_rt(struct rq *rq,
return next;
}
-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;
- struct rq *lowest_rq = NULL;
+ int cpu;
+ cpumask_t valid_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;
+ 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 */
-
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