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SubjectRe: [PATCH 46/50] sched: numa: Prevent parallel updates to group stats during placement
On Tue, Sep 10, 2013 at 10:32:26AM +0100, Mel Gorman wrote:
> 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 3a92c58..4653f71 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -1231,6 +1231,7 @@ 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)
> @@ -1239,6 +1240,12 @@ 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;
> @@ -1277,20 +1284,24 @@ static void task_numa_placement(struct task_struct *p)
> }
> }
>
> + 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 */

If you're going to hold locks you can also do away with all that
atomic_long_*() nonsense :-)


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