sched/fair: Fix post_init_entity_util_avg() serialization

Chris Wilson reported a divide by 0 at:

 post_init_entity_util_avg():

 >    725	if (cfs_rq->avg.util_avg != 0) {
 >    726		sa->util_avg  = cfs_rq->avg.util_avg * se->load.weight;
 > -> 727		sa->util_avg /= (cfs_rq->avg.load_avg + 1);
 >    728
 >    729		if (sa->util_avg > cap)
 >    730			sa->util_avg = cap;
 >    731	} else {

Which given the lack of serialization, and the code generated from
update_cfs_rq_load_avg() is entirely possible:

	if (atomic_long_read(&cfs_rq->removed_load_avg)) {
		s64 r = atomic_long_xchg(&cfs_rq->removed_load_avg, 0);
		sa->load_avg = max_t(long, sa->load_avg - r, 0);
		sa->load_sum = max_t(s64, sa->load_sum - r * LOAD_AVG_MAX, 0);
		removed_load = 1;
	}

turns into:

  ffffffff81087064:       49 8b 85 98 00 00 00    mov    0x98(%r13),%rax
  ffffffff8108706b:       48 85 c0                test   %rax,%rax
  ffffffff8108706e:       74 40                   je     ffffffff810870b0
  ffffffff81087070:       4c 89 f8                mov    %r15,%rax
  ffffffff81087073:       49 87 85 98 00 00 00    xchg   %rax,0x98(%r13)
  ffffffff8108707a:       49 29 45 70             sub    %rax,0x70(%r13)
  ffffffff8108707e:       4c 89 f9                mov    %r15,%rcx
  ffffffff81087081:       bb 01 00 00 00          mov    $0x1,%ebx
  ffffffff81087086:       49 83 7d 70 00          cmpq   $0x0,0x70(%r13)
  ffffffff8108708b:       49 0f 49 4d 70          cmovns 0x70(%r13),%rcx

Which you'll note ends up with 'sa->load_avg - r' in memory at
ffffffff8108707a.

By calling post_init_entity_util_avg() under rq->lock we're sure to be
fully serialized against PELT updates and cannot observe intermediate
state like this.

Reported-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Yuyang Du <yuyang.du@intel.com>
Cc: bsegall@google.com
Cc: morten.rasmussen@arm.com
Cc: pjt@google.com
Cc: steve.muckle@linaro.org
Fixes: 2b8c41daba32 ("sched/fair: Initiate a new task's util avg to a bounded value")
Link: http://lkml.kernel.org/r/20160609130750.GQ30909@twins.programming.kicks-ass.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>

2 files changed, 8 insertions, 3 deletions

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 017d5394f5dc..13d0896aff87 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2535,10 +2535,9 @@ void wake_up_new_task(struct task_struct *p)
          */
         set_task_cpu(p, select_task_rq(p, task_cpu(p), SD_BALANCE_FORK, 0));
 #endif
-        /* Post initialize new task's util average when its cfs_rq is set */
+        rq = __task_rq_lock(p, &rf);
         post_init_entity_util_avg(&p->se);
 
-        rq = __task_rq_lock(p, &rf);
         activate_task(rq, p, 0);
         p->on_rq = TASK_ON_RQ_QUEUED;
         trace_sched_wakeup_new(p);
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 218f8e83db73..4e33ad12bb68 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8496,8 +8496,9 @@ void free_fair_sched_group(struct task_group *tg)
 
 int alloc_fair_sched_group(struct task_group *tg, struct task_group *parent)
 {
-        struct cfs_rq *cfs_rq;
         struct sched_entity *se;
+        struct cfs_rq *cfs_rq;
+        struct rq *rq;
         int i;
 
         tg->cfs_rq = kzalloc(sizeof(cfs_rq) * nr_cpu_ids, GFP_KERNEL);
@@ -8512,6 +8513,8 @@ int alloc_fair_sched_group(struct task_group *tg, struct task_group *parent)
         init_cfs_bandwidth(tg_cfs_bandwidth(tg));
 
         for_each_possible_cpu(i) {
+                rq = cpu_rq(i);
+
                 cfs_rq = kzalloc_node(sizeof(struct cfs_rq),
                                       GFP_KERNEL, cpu_to_node(i));
                 if (!cfs_rq)
@@ -8525,7 +8528,10 @@ int alloc_fair_sched_group(struct task_group *tg, struct task_group *parent)
                 init_cfs_rq(cfs_rq);
                 init_tg_cfs_entry(tg, cfs_rq, se, i, parent->se[i]);
                 init_entity_runnable_average(se);
+
+                raw_spin_lock_irq(&rq->lock);
                 post_init_entity_util_avg(se);
+                raw_spin_unlock_irq(&rq->lock);
         }
 
         return 1;