sched: Fix cgroup smp fairness

Commit ec4e0e2fe018992d980910db901637c814575914 ("fix
inconsistency when redistribute per-cpu tg->cfs_rq shares")
broke cgroup smp fairness.

In order to avoid starvation of newly placed tasks, we never
quite set the share of an empty cpu group-task to 0, but
instead we set it as if there's a single NICE-0 task present.

If however we actually set this in cfs_rq[cpu]->shares, that
means the total shares for that group will be slightly inflated
every time we balance, causing the observed unfairness.

Fix this by setting cfs_rq[cpu]->shares to 0 but actually
setting the effective weight of the related se to the inflated
number.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <1248696557.6987.1615.camel@twins>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 20 insertions, 8 deletions

diff --git a/kernel/sched.c b/kernel/sched.c
index ce1056e9b02a..26976cd8be0f 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -1523,13 +1523,18 @@ static void
 update_group_shares_cpu(struct task_group *tg, int cpu,
                         unsigned long sd_shares, unsigned long sd_rq_weight)
 {
-        unsigned long shares;
         unsigned long rq_weight;
+        unsigned long shares;
+        int boost = 0;
 
         if (!tg->se[cpu])
                 return;
 
         rq_weight = tg->cfs_rq[cpu]->rq_weight;
+        if (!rq_weight) {
+                boost = 1;
+                rq_weight = NICE_0_LOAD;
+        }
 
         /*
          *           \Sum shares * rq_weight
@@ -1546,8 +1551,7 @@ update_group_shares_cpu(struct task_group *tg, int cpu,
                 unsigned long flags;
 
                 spin_lock_irqsave(&rq->lock, flags);
-                tg->cfs_rq[cpu]->shares = shares;
-
+                tg->cfs_rq[cpu]->shares = boost ? 0 : shares;
                 __set_se_shares(tg->se[cpu], shares);
                 spin_unlock_irqrestore(&rq->lock, flags);
         }
@@ -1560,7 +1564,7 @@ update_group_shares_cpu(struct task_group *tg, int cpu,
  */
 static int tg_shares_up(struct task_group *tg, void *data)
 {
-        unsigned long weight, rq_weight = 0;
+        unsigned long weight, rq_weight = 0, eff_weight = 0;
         unsigned long shares = 0;
         struct sched_domain *sd = data;
         int i;
@@ -1572,11 +1576,13 @@ static int tg_shares_up(struct task_group *tg, void *data)
                  * run here it will not get delayed by group starvation.
                  */
                 weight = tg->cfs_rq[i]->load.weight;
+                tg->cfs_rq[i]->rq_weight = weight;
+                rq_weight += weight;
+
                 if (!weight)
                         weight = NICE_0_LOAD;
 
-                tg->cfs_rq[i]->rq_weight = weight;
-                rq_weight += weight;
+                eff_weight += weight;
                 shares += tg->cfs_rq[i]->shares;
         }
 
@@ -1586,8 +1592,14 @@ static int tg_shares_up(struct task_group *tg, void *data)
         if (!sd->parent || !(sd->parent->flags & SD_LOAD_BALANCE))
                 shares = tg->shares;
 
-        for_each_cpu(i, sched_domain_span(sd))
-                update_group_shares_cpu(tg, i, shares, rq_weight);
+        for_each_cpu(i, sched_domain_span(sd)) {
+                unsigned long sd_rq_weight = rq_weight;
+
+                if (!tg->cfs_rq[i]->rq_weight)
+                        sd_rq_weight = eff_weight;
+
+                update_group_shares_cpu(tg, i, shares, sd_rq_weight);
+        }
 
         return 0;
 }