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authorPeter Zijlstra <peterz@infradead.org>2016-06-09 15:07:50 +0200
committerIngo Molnar <mingo@kernel.org>2016-06-14 10:58:34 +0200
commitb7fa30c9cc48c4f55663420472505d3b4f6e1705 (patch)
tree38f0c717a7a3044c2c1d285f06bd3d136cf746c4 /kernel
parentdb06d759d6cf903aeda8c107fd3abd366dd80200 (diff)
downloadlinux-b7fa30c9cc48c4f55663420472505d3b4f6e1705.tar.gz
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>
Diffstat (limited to 'kernel')
-rw-r--r--kernel/sched/core.c3
-rw-r--r--kernel/sched/fair.c8
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;