sched: optimize group load balancer
I noticed that tg_shares_up() unconditionally takes rq-locks for all cpus
in the sched_domain. This hurts.
We need the rq-locks whenever we change the weight of the per-cpu group sched
entities. To allevate this a little, only change the weight when the new
weight is at least shares_thresh away from the old value.
This avoids the rq-lock for the top level entries, since those will never
be re-weighted, and fuzzes the lower level entries a little to gain performance
in semi-stable situations.
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
diff --git a/kernel/sched.c b/kernel/sched.c
index c530b84..11ca390 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -818,6 +818,13 @@
unsigned int sysctl_sched_shares_ratelimit = 250000;
/*
+ * Inject some fuzzyness into changing the per-cpu group shares
+ * this avoids remote rq-locks at the expense of fairness.
+ * default: 4
+ */
+unsigned int sysctl_sched_shares_thresh = 4;
+
+/*
* period over which we measure -rt task cpu usage in us.
* default: 1s
*/
@@ -1453,8 +1460,8 @@
* Calculate and set the cpu's group shares.
*/
static void
-__update_group_shares_cpu(struct task_group *tg, int cpu,
- unsigned long sd_shares, unsigned long sd_rq_weight)
+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;
@@ -1485,19 +1492,23 @@
*
*/
shares = (sd_shares * rq_weight) / (sd_rq_weight + 1);
+ shares = clamp_t(unsigned long, shares, MIN_SHARES, MAX_SHARES);
- /*
- * 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;
+ if (abs(shares - tg->se[cpu]->load.weight) >
+ sysctl_sched_shares_thresh) {
+ struct rq *rq = cpu_rq(cpu);
+ unsigned long flags;
- if (shares < MIN_SHARES)
- shares = MIN_SHARES;
- else if (shares > MAX_SHARES)
- shares = MAX_SHARES;
+ 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;
- __set_se_shares(tg->se[cpu], shares);
+ __set_se_shares(tg->se[cpu], shares);
+ spin_unlock_irqrestore(&rq->lock, flags);
+ }
}
/*
@@ -1526,14 +1537,8 @@
if (!rq_weight)
rq_weight = cpus_weight(sd->span) * NICE_0_LOAD;
- for_each_cpu_mask(i, sd->span) {
- struct rq *rq = cpu_rq(i);
- unsigned long flags;
-
- spin_lock_irqsave(&rq->lock, flags);
- __update_group_shares_cpu(tg, i, shares, rq_weight);
- spin_unlock_irqrestore(&rq->lock, flags);
- }
+ for_each_cpu_mask(i, sd->span)
+ update_group_shares_cpu(tg, i, shares, rq_weight);
return 0;
}