sched: fix inconsistency when redistribute per-cpu tg->cfs_rq shares

Impact: make load-balancing more consistent

In the update_shares() path leading to tg_shares_up(), the calculation of
per-cpu cfs_rq shares is rather erratic even under moderate task wake up
rate.  The problem is that the per-cpu tg->cfs_rq load weight used in the
sd_rq_weight aggregation and actual redistribution of the cfs_rq->shares
are collected at different time.  Under moderate system load, we've seen
quite a bit of variation on the cfs_rq->shares and ultimately wildly
affects sched_entity's load weight.

This patch caches the result of initial per-cpu load weight when doing the
sum calculation, and then pass it down to update_group_shares_cpu() for
redistributing per-cpu cfs_rq shares.  This allows consistent total cfs_rq
shares across all CPUs. It also simplifies the rounding and zero load
weight check.

Signed-off-by: Ken Chen <kenchen@google.com>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 15 insertions, 26 deletions

diff --git a/kernel/sched.c b/kernel/sched.c
index a4c156d9a4a5..93bfb086e60f 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -1453,27 +1453,13 @@ static void
 update_group_shares_cpu(struct task_group *tg, int cpu,
                         unsigned long sd_shares, unsigned long sd_rq_weight)
 {
-        int boost = 0;
         unsigned long shares;
         unsigned long rq_weight;
 
         if (!tg->se[cpu])
                 return;
 
-        rq_weight = tg->cfs_rq[cpu]->load.weight;
-
-        /*
-         * If there are currently no tasks on the cpu pretend there is one of
-         * average load so that when a new task gets to run here it will not
-         * get delayed by group starvation.
-         */
-        if (!rq_weight) {
-                boost = 1;
-                rq_weight = NICE_0_LOAD;
-        }
-
-        if (unlikely(rq_weight > sd_rq_weight))
-                rq_weight = sd_rq_weight;
+        rq_weight = tg->cfs_rq[cpu]->rq_weight;
 
         /*
          *           \Sum shares * rq_weight
@@ -1481,7 +1467,7 @@ update_group_shares_cpu(struct task_group *tg, int cpu,
          *               \Sum rq_weight
          *
          */
-        shares = (sd_shares * rq_weight) / (sd_rq_weight + 1);
+        shares = (sd_shares * rq_weight) / sd_rq_weight;
         shares = clamp_t(unsigned long, shares, MIN_SHARES, MAX_SHARES);
 
         if (abs(shares - tg->se[cpu]->load.weight) >
@@ -1490,11 +1476,7 @@ update_group_shares_cpu(struct task_group *tg, int cpu,
                 unsigned long flags;
 
                 spin_lock_irqsave(&rq->lock, flags);
-                /*
-                 * record the actual number of shares, not the boosted amount.
-                 */
-                tg->cfs_rq[cpu]->shares = boost ? 0 : shares;
-                tg->cfs_rq[cpu]->rq_weight = rq_weight;
+                tg->cfs_rq[cpu]->shares = shares;
 
                 __set_se_shares(tg->se[cpu], shares);
                 spin_unlock_irqrestore(&rq->lock, flags);
@@ -1508,13 +1490,23 @@ update_group_shares_cpu(struct task_group *tg, int cpu,
  */
 static int tg_shares_up(struct task_group *tg, void *data)
 {
-        unsigned long rq_weight = 0;
+        unsigned long weight, rq_weight = 0;
         unsigned long shares = 0;
         struct sched_domain *sd = data;
         int i;
 
         for_each_cpu_mask(i, sd->span) {
-                rq_weight += tg->cfs_rq[i]->load.weight;
+                /*
+                 * If there are currently no tasks on the cpu pretend there
+                 * is one of average load so that when a new task gets to
+                 * run here it will not get delayed by group starvation.
+                 */
+                weight = tg->cfs_rq[i]->load.weight;
+                if (!weight)
+                        weight = NICE_0_LOAD;
+
+                tg->cfs_rq[i]->rq_weight = weight;
+                rq_weight += weight;
                 shares += tg->cfs_rq[i]->shares;
         }
 
@@ -1524,9 +1516,6 @@ static int tg_shares_up(struct task_group *tg, void *data)
         if (!sd->parent || !(sd->parent->flags & SD_LOAD_BALANCE))
                 shares = tg->shares;
 
-        if (!rq_weight)
-                rq_weight = cpus_weight(sd->span) * NICE_0_LOAD;
-
         for_each_cpu_mask(i, sd->span)
                 update_group_shares_cpu(tg, i, shares, rq_weight);