Re: [PATCH v3 08/13] sched: Store maximum per-cpu capacity in root domain

From: Dietmar Eggemann
Date: Mon Aug 01 2016 - 15:30:46 EST


On 25/07/16 14:34, Morten Rasmussen wrote:

[...]

> @@ -6923,11 +6924,22 @@ static int build_sched_domains(const struct cpumask *cpu_map,
> /* Attach the domains */
> rcu_read_lock();
> for_each_cpu(i, cpu_map) {
> + rq = cpu_rq(i);
> sd = *per_cpu_ptr(d.sd, i);
> +
> + /* Use READ_ONCE/WRITE_ONCE to avoid load/store tearing */
> + if (rq->cpu_capacity_orig > READ_ONCE(rq->rd->max_cpu_capacity))
> + WRITE_ONCE(rq->rd->max_cpu_capacity, rq->cpu_capacity_orig);

We have to use d.rd rather rq->rd here since from v3 on we have this if
condition in front of the cpu_attach_domain() call which replaces
rq->rd with d.rd. Fixed patch below.

> +
> cpu_attach_domain(sd, d.rd, i);
> }
> rcu_read_unlock();
>
> + if (rq) {
> + pr_info("span: %*pbl (max cpu_capacity = %lu)\n",
> + cpumask_pr_args(cpu_map), rq->rd->max_cpu_capacity);
> + }
> +
> ret = 0;
> error:
> __free_domain_allocs(&d, alloc_state, cpu_map);

[...]

-- >8 --

From: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>

To be able to compare the capacity of the target cpu with the highest
available cpu capacity, store the maximum per-cpu capacity in the root
domain.

The max per-cpu capacity should be 1024 for all systems except SMT,
where the capacity is currently based on smt_gain and the number of
hardware threads and is <1024. If SMT can be brought to work with a
per-thread capacity of 1024, this patch can be dropped and replaced by a
hard-coded max capacity of 1024 (=SCHED_CAPACITY_SCALE).

cc: Ingo Molnar <mingo@xxxxxxxxxx>
cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>

Signed-off-by: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>
Signed-off-by: Morten Rasmussen <morten.rasmussen@xxxxxxx>
---
kernel/sched/core.c | 12 ++++++++++++
kernel/sched/sched.h | 2 ++
2 files changed, 14 insertions(+)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index a0a74b2d9f41..db03e6226d54 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -6873,6 +6873,7 @@ static int build_sched_domains(const struct cpumask *cpu_map,
enum s_alloc alloc_state;
struct sched_domain *sd;
struct s_data d;
+ struct rq *rq = NULL;
int i, ret = -ENOMEM;

alloc_state = __visit_domain_allocation_hell(&d, cpu_map);
@@ -6923,11 +6924,22 @@ static int build_sched_domains(const struct cpumask *cpu_map,
/* Attach the domains */
rcu_read_lock();
for_each_cpu(i, cpu_map) {
+ rq = cpu_rq(i);
sd = *per_cpu_ptr(d.sd, i);
+
+ /* Use READ_ONCE/WRITE_ONCE to avoid load/store tearing */
+ if (rq->cpu_capacity_orig > READ_ONCE(d.rd->max_cpu_capacity))
+ WRITE_ONCE(d.rd->max_cpu_capacity, rq->cpu_capacity_orig);
+
cpu_attach_domain(sd, d.rd, i);
}
rcu_read_unlock();

+ if (rq) {
+ pr_info("span: %*pbl (max cpu_capacity = %lu)\n",
+ cpumask_pr_args(cpu_map), rq->rd->max_cpu_capacity);
+ }
+
ret = 0;
error:
__free_domain_allocs(&d, alloc_state, cpu_map);
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index f44da95c70cd..444d8f38743f 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -565,6 +565,8 @@ struct root_domain {
*/
cpumask_var_t rto_mask;
struct cpupri cpupri;
+
+ unsigned long max_cpu_capacity;
};

extern struct root_domain def_root_domain;
--
1.9.1