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    Subject[PATCH 51/63] sched: numa: Prevent parallel updates to group stats during placement
    Date
    Having multiple tasks in a group go through task_numa_placement
    simultaneously can lead to a task picking a wrong node to run on, because
    the group stats may be in the middle of an update. This patch avoids
    parallel updates by holding the numa_group lock during placement
    decisions.

    Signed-off-by: Mel Gorman <mgorman@suse.de>
    ---
    kernel/sched/fair.c | 35 +++++++++++++++++++++++------------
    1 file changed, 23 insertions(+), 12 deletions(-)

    diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
    index d5873e5..dc0c376 100644
    --- a/kernel/sched/fair.c
    +++ b/kernel/sched/fair.c
    @@ -1233,6 +1233,7 @@ static void task_numa_placement(struct task_struct *p)
    {
    int seq, nid, max_nid = -1, max_group_nid = -1;
    unsigned long max_faults = 0, max_group_faults = 0;
    + spinlock_t *group_lock = NULL;

    seq = ACCESS_ONCE(p->mm->numa_scan_seq);
    if (p->numa_scan_seq == seq)
    @@ -1241,6 +1242,12 @@ static void task_numa_placement(struct task_struct *p)
    p->numa_migrate_seq++;
    p->numa_scan_period_max = task_scan_max(p);

    + /* If the task is part of a group prevent parallel updates to group stats */
    + if (p->numa_group) {
    + group_lock = &p->numa_group->lock;
    + spin_lock(group_lock);
    + }
    +
    /* Find the node with the highest number of faults */
    for_each_online_node(nid) {
    unsigned long faults = 0, group_faults = 0;
    @@ -1279,20 +1286,24 @@ static void task_numa_placement(struct task_struct *p)
    }
    }

    - /*
    - * If the preferred task and group nids are different,
    - * iterate over the nodes again to find the best place.
    - */
    - if (p->numa_group && max_nid != max_group_nid) {
    - unsigned long weight, max_weight = 0;
    -
    - for_each_online_node(nid) {
    - weight = task_weight(p, nid) + group_weight(p, nid);
    - if (weight > max_weight) {
    - max_weight = weight;
    - max_nid = nid;
    + if (p->numa_group) {
    + /*
    + * If the preferred task and group nids are different,
    + * iterate over the nodes again to find the best place.
    + */
    + if (max_nid != max_group_nid) {
    + unsigned long weight, max_weight = 0;
    +
    + for_each_online_node(nid) {
    + weight = task_weight(p, nid) + group_weight(p, nid);
    + if (weight > max_weight) {
    + max_weight = weight;
    + max_nid = nid;
    + }
    }
    }
    +
    + spin_unlock(group_lock);
    }

    /* Preferred node as the node with the most faults */
    --
    1.8.4


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