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

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Date:	Mon, 1 Aug 2016 19:53:35 +0100
From:	Dietmar Eggemann <dietmar.eggemann@....com>
To:	Morten Rasmussen <morten.rasmussen@....com>, peterz@...radead.org,
	mingo@...hat.com
Cc:	yuyang.du@...el.com, vincent.guittot@...aro.org,
	mgalbraith@...e.de, sgurrappadi@...dia.com,
	freedom.tan@...iatek.com, keita.kobayashi.ym@...esas.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 08/13] sched: Store maximum per-cpu capacity in root
 domain

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@....com>

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@...hat.com>
cc: Peter Zijlstra <peterz@...radead.org>

Signed-off-by: Dietmar Eggemann <dietmar.eggemann@....com>
Signed-off-by: Morten Rasmussen <morten.rasmussen@....com>
---
 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