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
<update_blocked_averages+0xc0>
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.
Ludicrous code generation if you ask me; I'd have expected something
like (note, r15 holds 0):
mov %r15, %rax
xchg %rax, cfs_rq->removed_load_avg
mov sa->load_avg, %rcx
sub %rax, %rcx
cmovs %r15, %rcx
mov %rcx, sa->load_avg
Adding the serialization (to _both_ call sites) should fix this.
Fixes: 2b8c41daba32 ("sched/fair: Initiate a new task's util avg to a bounded
value")
Reported-by: Chris Wilson <ch...@chris-wilson.co.uk>
Signed-off-by: Peter Zijlstra (Intel) <pet...@infradead.org>
---
kernel/sched/core.c | 3 +--
kernel/sched/fair.c | 8 +++++++-
2 files changed, 8 insertions(+), 3 deletions(-)
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 385c947482e1..4aff10e3bd14 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 c6dd8bab010c..f379da14c7dd 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8468,8 +8468,9 @@ 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);
@@ -8484,6 +8485,8 @@ 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)
@@ -8497,7 +8500,10 @@ 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;
