sched: Fix select_idle_sibling() logic in select_task_rq_fair()

Issues in the current select_idle_sibling() logic in select_task_rq_fair()
in the context of a task wake-up:

a) Once we select the idle sibling, we use that domain (spanning the cpu that
   the task is currently woken-up and the idle sibling that we found) in our
   wake_affine() decisions. This domain is completely different from the
   domain(we are supposed to use) that spans the cpu that the task currently
   woken-up and the cpu where the task previously ran.

b) We do select_idle_sibling() check only for the cpu that the task is
   currently woken-up on. If select_task_rq_fair() selects the previously run
   cpu for waking the task, doing a select_idle_sibling() check
   for that cpu also helps and we don't do this currently.

c) In the scenarios where the cpu that the task is woken-up is busy but
   with its HT siblings are idle, we are selecting the task be woken-up
   on the idle HT sibling instead of a core that it previously ran
   and currently completely idle. i.e., we are not taking decisions based on
   wake_affine() but directly selecting an idle sibling that can cause
   an imbalance at the SMT/MC level which will be later corrected by the
   periodic load balancer.

Fix this by first going through the load imbalance calculations using
wake_affine() and once we make a decision of woken-up cpu vs previously-ran cpu,
then choose a possible idle sibling for waking up the task on.

Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <1270079265.7835.8.camel@sbs-t61.sc.intel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 40 insertions, 42 deletions

diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
index 0a413c7e3ab8..cbd8b8a296d1 100644
--- a/kernel/sched_fair.c
+++ b/kernel/sched_fair.c
@@ -1375,29 +1375,48 @@ find_idlest_cpu(struct sched_group *group, struct task_struct *p, int this_cpu)
 /*
  * Try and locate an idle CPU in the sched_domain.
  */
-static int
-select_idle_sibling(struct task_struct *p, struct sched_domain *sd, int target)
+static int select_idle_sibling(struct task_struct *p, int target)
 {
         int cpu = smp_processor_id();
         int prev_cpu = task_cpu(p);
+        struct sched_domain *sd;
         int i;
 
         /*
-         * If this domain spans both cpu and prev_cpu (see the SD_WAKE_AFFINE
-         * test in select_task_rq_fair) and the prev_cpu is idle then that's
-         * always a better target than the current cpu.
+         * If the task is going to be woken-up on this cpu and if it is
+         * already idle, then it is the right target.
+         */
+        if (target == cpu && idle_cpu(cpu))
+                return cpu;
+
+        /*
+         * If the task is going to be woken-up on the cpu where it previously
+         * ran and if it is currently idle, then it the right target.
          */
-        if (target == cpu && !cpu_rq(prev_cpu)->cfs.nr_running)
+        if (target == prev_cpu && idle_cpu(prev_cpu))
                 return prev_cpu;
 
         /*
-         * Otherwise, iterate the domain and find an elegible idle cpu.
+         * Otherwise, iterate the domains and find an elegible idle cpu.
          */
-        for_each_cpu_and(i, sched_domain_span(sd), &p->cpus_allowed) {
-                if (!cpu_rq(i)->cfs.nr_running) {
-                        target = i;
+        for_each_domain(target, sd) {
+                if (!(sd->flags & SD_SHARE_PKG_RESOURCES))
                         break;
+
+                for_each_cpu_and(i, sched_domain_span(sd), &p->cpus_allowed) {
+                        if (idle_cpu(i)) {
+                                target = i;
+                                break;
+                        }
                 }
+
+                /*
+                 * Lets stop looking for an idle sibling when we reached
+                 * the domain that spans the current cpu and prev_cpu.
+                 */
+                if (cpumask_test_cpu(cpu, sched_domain_span(sd)) &&
+                    cpumask_test_cpu(prev_cpu, sched_domain_span(sd)))
+                        break;
         }
 
         return target;
@@ -1421,7 +1440,7 @@ select_task_rq_fair(struct rq *rq, struct task_struct *p, int sd_flag, int wake_
         int cpu = smp_processor_id();
         int prev_cpu = task_cpu(p);
         int new_cpu = cpu;
-        int want_affine = 0, cpu_idle = !current->pid;
+        int want_affine = 0;
         int want_sd = 1;
         int sync = wake_flags & WF_SYNC;
 
@@ -1460,36 +1479,13 @@ select_task_rq_fair(struct rq *rq, struct task_struct *p, int sd_flag, int wake_
                 }
 
                 /*
-                 * While iterating the domains looking for a spanning
-                 * WAKE_AFFINE domain, adjust the affine target to any idle cpu
-                 * in cache sharing domains along the way.
+                 * If both cpu and prev_cpu are part of this domain,
+                 * cpu is a valid SD_WAKE_AFFINE target.
                  */
-                if (want_affine) {
-                        int target = -1;
-
-                        /*
-                         * If both cpu and prev_cpu are part of this domain,
-                         * cpu is a valid SD_WAKE_AFFINE target.
-                         */
-                        if (cpumask_test_cpu(prev_cpu, sched_domain_span(tmp)))
-                                target = cpu;
-
-                        /*
-                         * If there's an idle sibling in this domain, make that
-                         * the wake_affine target instead of the current cpu.
-                         */
-                        if (!cpu_idle && tmp->flags & SD_SHARE_PKG_RESOURCES)
-                                target = select_idle_sibling(p, tmp, target);
-
-                        if (target >= 0) {
-                                if (tmp->flags & SD_WAKE_AFFINE) {
-                                        affine_sd = tmp;
-                                        want_affine = 0;
-                                        if (target != cpu)
-                                                cpu_idle = 1;
-                                }
-                                cpu = target;
-                        }
+                if (want_affine && (tmp->flags & SD_WAKE_AFFINE) &&
+                    cpumask_test_cpu(prev_cpu, sched_domain_span(tmp))) {
+                        affine_sd = tmp;
+                        want_affine = 0;
                 }
 
                 if (!want_sd && !want_affine)
@@ -1520,8 +1516,10 @@ select_task_rq_fair(struct rq *rq, struct task_struct *p, int sd_flag, int wake_
 #endif
 
         if (affine_sd) {
-                if (cpu_idle || cpu == prev_cpu || wake_affine(affine_sd, p, sync))
-                        return cpu;
+                if (cpu == prev_cpu || wake_affine(affine_sd, p, sync))
+                        return select_idle_sibling(p, cpu);
+                else
+                        return select_idle_sibling(p, prev_cpu);
         }
 
         while (sd) {