sched, cgroup: Reduce rq->lock hold times for large cgroup hierarchies

Peter Portante reported that for large cgroup hierarchies (and or on
large CPU counts) we get immense lock contention on rq->lock and stuff
stops working properly.

His workload was a ton of processes, each in their own cgroup,
everybody idling except for a sporadic wakeup once every so often.

It was found that:

  schedule()
    idle_balance()
      load_balance()
        local_irq_save()
        double_rq_lock()
        update_h_load()
          walk_tg_tree(tg_load_down)
            tg_load_down()

Results in an entire cgroup hierarchy walk under rq->lock for every
new-idle balance and since new-idle balance isn't throttled this
results in a lot of work while holding the rq->lock.

This patch does two things, it removes the work from under rq->lock
based on the good principle of race and pray which is widely employed
in the load-balancer as a whole. And secondly it throttles the
update_h_load() calculation to max once per jiffy.

I considered excluding update_h_load() for new-idle balance
all-together, but purely relying on regular balance passes to update
this data might not work out under some rare circumstances where the
new-idle busiest isn't the regular busiest for a while (unlikely, but
a nightmare to debug if someone hits it and suffers).

Cc: pjt@google.com
Cc: Larry Woodman <lwoodman@redhat.com>
Cc: Mike Galbraith <efault@gmx.de>
Reported-by: Peter Portante <pportant@redhat.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/n/tip-aaarrzfpnaam7pqrekofu8a6@git.kernel.org
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

2 files changed, 14 insertions, 3 deletions

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index d0cc03b3e70..c219bf8d704 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -3387,6 +3387,14 @@ static int tg_load_down(struct task_group *tg, void *data)
 
 static void update_h_load(long cpu)
 {
+        struct rq *rq = cpu_rq(cpu);
+        unsigned long now = jiffies;
+
+        if (rq->h_load_throttle == now)
+                return;
+
+        rq->h_load_throttle = now;
+
         rcu_read_lock();
         walk_tg_tree(tg_load_down, tg_nop, (void *)cpu);
         rcu_read_unlock();
@@ -4293,11 +4301,10 @@ redo:
                 env.src_rq    = busiest;
                 env.loop_max  = min(sysctl_sched_nr_migrate, busiest->nr_running);
 
+                update_h_load(env.src_cpu);
 more_balance:
                 local_irq_save(flags);
                 double_rq_lock(this_rq, busiest);
-                if (!env.loop)
-                        update_h_load(env.src_cpu);
 
                 /*
                  * cur_ld_moved - load moved in current iteration
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index c35a1a7dd4d..531411b1044 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -374,7 +374,11 @@ struct rq {
 #ifdef CONFIG_FAIR_GROUP_SCHED
         /* list of leaf cfs_rq on this cpu: */
         struct list_head leaf_cfs_rq_list;
-#endif
+#ifdef CONFIG_SMP
+        unsigned long h_load_throttle;
+#endif /* CONFIG_SMP */
+#endif /* CONFIG_FAIR_GROUP_SCHED */
+
 #ifdef CONFIG_RT_GROUP_SCHED
         struct list_head leaf_rt_rq_list;
 #endif