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SubjectRe: [RFC 1/2] sched: reduce migration cost between faster caches for idle_balance
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On Thu, 2018-02-08 at 14:19 -0800, Rohit Jain wrote:
> This patch makes idle_balance more dynamic as the sched_migration_cost
> is now accounted on a sched_domain level. This in turn is done in
> sd_init when we know what the topology relationships are.
>
> For introduction sakes cost of migration within the same core is set as
> 0, across cores is 50 usec and across sockets is 500 usec. sysctl for
> these variables are introduced in patch 2.
>
> Signed-off-by: Rohit Jain <rohit.k.jain@oracle.com>
> ---
> include/linux/sched/topology.h | 1 +
> kernel/sched/fair.c | 6 +++---
> kernel/sched/topology.c | 5 +++++
> 3 files changed, 9 insertions(+), 3 deletions(-)
>
> diff --git a/include/linux/sched/topology.h b/include/linux/sched/topology.h
> index cf257c2..bcb4db2 100644
> --- a/include/linux/sched/topology.h
> +++ b/include/linux/sched/topology.h
> @@ -104,6 +104,7 @@ struct sched_domain {
> u64 max_newidle_lb_cost;
> unsigned long next_decay_max_lb_cost;
>
> + u64 sched_migration_cost;
> u64 avg_scan_cost; /* select_idle_sibling */
>
> #ifdef CONFIG_SCHEDSTATS
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 2fe3aa8..61d3508 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -8782,8 +8782,7 @@ static int idle_balance(struct rq *this_rq, struct rq_flags *rf)
> */
> rq_unpin_lock(this_rq, rf);
>
> - if (this_rq->avg_idle < sysctl_sched_migration_cost ||
> - !this_rq->rd->overload) {
> + if (!this_rq->rd->overload) {
> rcu_read_lock();
> sd = rcu_dereference_check_sched_domain(this_rq->sd);
> if (sd)

Unexplained/unrelated change.

> @@ -8804,7 +8803,8 @@ static int idle_balance(struct rq *this_rq, struct rq_flags *rf)
> if (!(sd->flags & SD_LOAD_BALANCE))
> continue;
>
> - if (this_rq->avg_idle < curr_cost + sd->max_newidle_lb_cost) {
> + if (this_rq->avg_idle < curr_cost + sd->max_newidle_lb_cost +
> + sd->sched_migration_cost) {
> update_next_balance(sd, &next_balance);
> break;
> }

Ditto.

> diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> index 034cbed..bcd8c64 100644
> --- a/kernel/sched/topology.c
> +++ b/kernel/sched/topology.c
> @@ -1148,12 +1148,14 @@ sd_init(struct sched_domain_topology_level *tl,
> sd->flags |= SD_PREFER_SIBLING;
> sd->imbalance_pct = 110;
> sd->smt_gain = 1178; /* ~15% */
> + sd->sched_migration_cost = 0;
>
> } else if (sd->flags & SD_SHARE_PKG_RESOURCES) {
> sd->flags |= SD_PREFER_SIBLING;
> sd->imbalance_pct = 117;
> sd->cache_nice_tries = 1;
> sd->busy_idx = 2;
> + sd->sched_migration_cost = 500000UL;
>
> #ifdef CONFIG_NUMA
> } else if (sd->flags & SD_NUMA) {
> @@ -1162,6 +1164,7 @@ sd_init(struct sched_domain_topology_level *tl,
> sd->idle_idx = 2;
>
> sd->flags |= SD_SERIALIZE;
> + sd->sched_migration_cost = 5000000UL;

That's not 500us.

-Mike

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