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sched: optimize RT affinity

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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>
This commit is contained in:
Gregory Haskins 2008-01-25 21:08:11 +01:00 committed by Ingo Molnar
parent 318e0893ce
commit 6e1254d2c4
1 changed files with 88 additions and 12 deletions
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100
kernel/sched_rt.c
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@ -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 lowest_rq ? lowest_rq->cpu : -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;
}
}
/*
* 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 */