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Date:   Tue, 31 Jul 2018 12:59:16 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Morten Rasmussen <morten.rasmussen@arm.com>
Cc:     mingo@redhat.com, valentin.schneider@arm.com,
        dietmar.eggemann@arm.com, vincent.guittot@linaro.org,
        gaku.inami.xh@renesas.com, linux-kernel@vger.kernel.org
Subject: Re: [PATCHv4 01/12] sched: Add static_key for asymmetric cpu
 capacity optimizations
Message-ID: <20180731105916.GR2476@hirez.programming.kicks-ass.net>
References: <1530699470-29808-1-git-send-email-morten.rasmussen@arm.com>
 <1530699470-29808-2-git-send-email-morten.rasmussen@arm.com>
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Combined with that SD_ASYM.. rework I ended up with the below.

Holler if you want it changed :-)


---


Subject: sched: Add static_key for asymmetric cpu capacity optimizations
From: Morten Rasmussen <morten.rasmussen@arm.com>
Date: Wed, 4 Jul 2018 11:17:39 +0100

The existing asymmetric cpu capacity code should cause minimal overhead
for others. Putting it behind a static_key, it has been done for SMT
optimizations, would make it easier to extend and improve without
causing harm to others moving forward.

cc: Ingo Molnar <mingo@redhat.com>
cc: Peter Zijlstra <peterz@infradead.org>

Cc: valentin.schneider@arm.com
Cc: mingo@redhat.com
Cc: vincent.guittot@linaro.org
Cc: dietmar.eggemann@arm.com
Cc: gaku.inami.xh@renesas.com
Signed-off-by: Morten Rasmussen <morten.rasmussen@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1530699470-29808-2-git-send-email-morten.rasmussen@arm.com
---
 kernel/sched/fair.c     |    3 +++
 kernel/sched/sched.h    |    1 +
 kernel/sched/topology.c |    9 ++++++++-
 3 files changed, 12 insertions(+), 1 deletion(-)

--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6186,6 +6186,9 @@ static int wake_cap(struct task_struct *
 {
 	long min_cap, max_cap;
 
+	if (!static_branch_unlikely(&sched_asym_cpucapacity))
+		return 0;
+
 	min_cap = min(capacity_orig_of(prev_cpu), capacity_orig_of(cpu));
 	max_cap = cpu_rq(cpu)->rd->max_cpu_capacity;
 
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1185,6 +1185,7 @@ DECLARE_PER_CPU(int, sd_llc_id);
 DECLARE_PER_CPU(struct sched_domain_shared *, sd_llc_shared);
 DECLARE_PER_CPU(struct sched_domain *, sd_numa);
 DECLARE_PER_CPU(struct sched_domain *, sd_asym);
+extern struct static_key_false sched_asym_cpucapacity;
 
 struct sched_group_capacity {
 	atomic_t		ref;
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -398,6 +398,7 @@ DEFINE_PER_CPU(int, sd_llc_id);
 DEFINE_PER_CPU(struct sched_domain_shared *, sd_llc_shared);
 DEFINE_PER_CPU(struct sched_domain *, sd_numa);
 DEFINE_PER_CPU(struct sched_domain *, sd_asym);
+DEFINE_STATIC_KEY_FALSE(sched_asym_cpucapacity);
 
 static void update_top_cache_domain(int cpu)
 {
@@ -1708,6 +1709,7 @@ build_sched_domains(const struct cpumask
 	struct rq *rq = NULL;
 	int i, ret = -ENOMEM;
 	struct sched_domain_topology_level *tl_asym;
+	bool has_asym = false;
 
 	alloc_state = __visit_domain_allocation_hell(&d, cpu_map);
 	if (alloc_state != sa_rootdomain)
@@ -1723,8 +1725,10 @@ build_sched_domains(const struct cpumask
 		for_each_sd_topology(tl) {
 			int dflags = 0;
 
-			if (tl == tl_asym)
+			if (tl == tl_asym) {
 				dflags |= SD_ASYM_CPUCAPACITY;
+				has_asym = true;
+			}
 
 			sd = build_sched_domain(tl, cpu_map, attr, sd, dflags, i);
 
@@ -1776,6 +1780,9 @@ build_sched_domains(const struct cpumask
 	}
 	rcu_read_unlock();
 
+	if (has_asym)
+		static_branch_enable_cpuslocked(&sched_asym_cpucapacity);
+
 	if (rq && sched_debug_enabled) {
 		pr_info("root domain span: %*pbl (max cpu_capacity = %lu)\n",
 			cpumask_pr_args(cpu_map), rq->rd->max_cpu_capacity);