Re: [PATCH] sched/fair: fix enqueue_task_fair warning

On Thu, 5 Mar 2020 at 20:07, Dietmar Eggemann dietmar.eggemann@arm.com wrote:

On 05/03/2020 18:29, Vincent Guittot wrote:

...

When a cfs rq is throttled, the latter and its child are removed from the leaf list but their nr_running is not changed which includes staying higher than 1. When a task is enqueued in this throttled branch, the cfs rqs must be added back in order to ensure correct ordering in the list but this can only happens if nr_running == 1. When cfs bandwidth is used, we call unconditionnaly list_add_leaf_cfs_rq() when enqueuing an entity to make sure that the complete branch will be added.

Reported-by: Christian Borntraeger borntraeger@de.ibm.com Tested-by: Christian Borntraeger borntraeger@de.ibm.com Cc: stable@vger.kernel.org #v5.1+ Signed-off-by: Vincent Guittot vincent.guittot@linaro.org

---

kernel/sched/fair.c | 11 +++++++++-- 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index fcc968669aea..bdc5bb72ab31 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -4117,6 +4117,7 @@ static inline void check_schedstat_required(void) #endif }

+static inline bool cfs_bandwidth_used(void);

/*

• MIGRATION

@@ -4195,10 +4196,16 @@ enqueue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags) __enqueue_entity(cfs_rq, se); se->on_rq = 1;

• if (cfs_rq->nr_running == 1) {
  

• /*
  

•  * When bandwidth control is enabled, cfs might have been removed because of
  

•  * a parent been throttled but cfs->nr_running > 1. Try to add it
  

•  * unconditionnally.
  

•  */
  

• if (cfs_rq->nr_running == 1 || cfs_bandwidth_used())
          list_add_leaf_cfs_rq(cfs_rq);
  

• if (cfs_rq->nr_running == 1)
          check_enqueue_throttle(cfs_rq);
  

• }
  

}

static void __clear_buddies_last(struct sched_entity *se)

I experimented with an rt-app based setup on Arm64 Juno (6 CPUs):

cgroupv1 hierarchy A/B/C, all CFS bw controlled (30,000/100,000)

I create A/B/C outside rt-app so I can have rt-app runs with an already existing taskgroup hierarchy. There is a 4 secs gap between consecutive rt-app runs.

The rt-app files contains 6 periodic CFS tasks (25,000/100,000) running in /A/B/C, /A/B, /A (3 rt-app task phases).

I get w/ the patch (and the debug patch applied to unthrottle_cfs_rq()):

root@juno:~# [ 409.236925] CPU1 path=/A/B on_list=1 nr_running=1 throttled=1 [ 409.242682] CPU1 path=/A on_list=0 nr_running=0 throttled=1 [ 409.248260] CPU1 path=/ on_list=1 nr_running=0 throttled=0 [ 409.253748] ------------[ cut here ]------------ [ 409.258365] rq->tmp_alone_branch != &rq->leaf_cfs_rq_list [ 409.258382] WARNING: CPU: 1 PID: 0 at kernel/sched/fair.c:380 unthrottle_cfs_rq+0x21c/0x2a8 ... [ 409.275196] CPU: 1 PID: 0 Comm: swapper/1 Not tainted 5.6.0-rc3-dirty #62 [ 409.281990] Hardware name: ARM Juno development board (r0) (DT) ... [ 409.384644] Call trace: [ 409.387089] unthrottle_cfs_rq+0x21c/0x2a8 [ 409.391188] distribute_cfs_runtime+0xf4/0x198 [ 409.395634] sched_cfs_period_timer+0x134/0x240 [ 409.400168] __hrtimer_run_queues+0x10c/0x3c0 [ 409.404527] hrtimer_interrupt+0xd4/0x250 [ 409.408539] tick_handle_oneshot_broadcast+0x17c/0x208 [ 409.413683] sp804_timer_interrupt+0x30/0x40

If I add the following snippet the issue goes away:

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index e9fd5379bb7e..5e03be046aba 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -4627,11 +4627,17 @@ void unthrottle_cfs_rq(struct cfs_rq *cfs_rq) break; }

•   assert_list_leaf_cfs_rq(rq);
  

•   if (!se)
            add_nr_running(rq, task_delta);
  
  

•   for_each_sched_entity(se) {
  

•           cfs_rq = cfs_rq_of(se);
  

•           list_add_leaf_cfs_rq(cfs_rq);
  

•   }
  

Yes make sense.

•   assert_list_leaf_cfs_rq(rq);
  

•   /* Determine whether we need to wake up potentially idle CPU: */
    if (rq->curr == rq->idle && rq->cfs.nr_running)
            resched_curr(rq);