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    Subject[PATCH 5.4 104/232] sched/fair: Fix enqueue_task_fair warning
    Date
    From: Vincent Guittot <vincent.guittot@linaro.org>

    commit fe61468b2cbc2b7ce5f8d3bf32ae5001d4c434e9 upstream.

    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.

    Similarly unthrottle_cfs_rq() can stop adding cfs in the list when a parent
    is throttled. Iterate the remaining entity to ensure that the complete
    branch will be added in the list.

    Reported-by: Christian Borntraeger <borntraeger@de.ibm.com>
    Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
    Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
    Tested-by: Christian Borntraeger <borntraeger@de.ibm.com>
    Tested-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
    Cc: stable@vger.kernel.org
    Cc: stable@vger.kernel.org #v5.1+
    Link: https://lkml.kernel.org/r/20200306135257.25044-1-vincent.guittot@linaro.org
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    kernel/sched/fair.c | 26 ++++++++++++++++++++++----
    1 file changed, 22 insertions(+), 4 deletions(-)

    --- a/kernel/sched/fair.c
    +++ b/kernel/sched/fair.c
    @@ -3927,6 +3927,7 @@ static inline void check_schedstat_requi
    #endif
    }

    +static inline bool cfs_bandwidth_used(void);

    /*
    * MIGRATION
    @@ -4005,10 +4006,16 @@ enqueue_entity(struct cfs_rq *cfs_rq, st
    __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)
    @@ -4589,11 +4596,22 @@ void unthrottle_cfs_rq(struct cfs_rq *cf
    break;
    }

    - assert_list_leaf_cfs_rq(rq);
    -
    if (!se)
    add_nr_running(rq, task_delta);

    + /*
    + * The cfs_rq_throttled() breaks in the above iteration can result in
    + * incomplete leaf list maintenance, resulting in triggering the
    + * assertion below.
    + */
    + for_each_sched_entity(se) {
    + cfs_rq = cfs_rq_of(se);
    +
    + list_add_leaf_cfs_rq(cfs_rq);
    + }
    +
    + 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);

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