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Subject[PATCH v2 9/9] sched/topology: Define and use shortcut pointers for wakeup sd_flag scan
Date
Reworking select_task_rq_fair()'s domain walk exposed that !want_affine
wakeups only look for highest sched_domain with the required sd_flag
set. This is something we can cache at sched domain build time to slightly
optimize select_task_rq_fair(). Note that this isn't a "free" optimization:
it costs us 3 pointers per CPU.

Add cached per-CPU pointers for the highest domains with SD_BALANCE_WAKE,
SD_BALANCE_EXEC and SD_BALANCE_FORK. Use them in select_task_rq_fair().

Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
---
kernel/sched/fair.c | 25 +++++++++++++------------
kernel/sched/sched.h | 3 +++
kernel/sched/topology.c | 12 ++++++++++++
3 files changed, 28 insertions(+), 12 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index a6fca6817e92..40fb97062157 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6595,17 +6595,6 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags)
int want_affine = 0;
int sd_flag;

- switch (wake_flags & (WF_TTWU | WF_FORK | WF_EXEC)) {
- case WF_TTWU:
- sd_flag = SD_BALANCE_WAKE;
- break;
- case WF_FORK:
- sd_flag = SD_BALANCE_FORK;
- break;
- default:
- sd_flag = SD_BALANCE_EXEC;
- }
-
if (wake_flags & WF_TTWU) {
record_wakee(p);

@@ -6621,7 +6610,19 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags)

rcu_read_lock();

- sd = highest_flag_domain(cpu, sd_flag);
+ switch (wake_flags & (WF_TTWU | WF_FORK | WF_EXEC)) {
+ case WF_TTWU:
+ sd_flag = SD_BALANCE_WAKE;
+ sd = rcu_dereference(per_cpu(sd_balance_wake, cpu));
+ break;
+ case WF_FORK:
+ sd_flag = SD_BALANCE_FORK;
+ sd = rcu_dereference(per_cpu(sd_balance_fork, cpu));
+ break;
+ default:
+ sd_flag = SD_BALANCE_EXEC;
+ sd = rcu_dereference(per_cpu(sd_balance_exec, cpu));
+ }

/*
* If !want_affine, we just look for the highest domain where
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index ad2106245e12..3a0e38f2f713 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1393,6 +1393,9 @@ DECLARE_PER_CPU(int, sd_llc_size);
DECLARE_PER_CPU(int, sd_llc_id);
DECLARE_PER_CPU(struct sched_domain_shared __rcu *, sd_llc_shared);
DECLARE_PER_CPU(struct sched_domain __rcu *, sd_numa);
+DECLARE_PER_CPU(struct sched_domain __rcu *, sd_balance_wake);
+DECLARE_PER_CPU(struct sched_domain __rcu *, sd_balance_fork);
+DECLARE_PER_CPU(struct sched_domain __rcu *, sd_balance_exec);
DECLARE_PER_CPU(struct sched_domain __rcu *, sd_asym_packing);
DECLARE_PER_CPU(struct sched_domain __rcu *, sd_asym_cpucapacity);
extern struct static_key_false sched_asym_cpucapacity;
diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index 6077b23f9723..0270252a964b 100644
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -610,6 +610,9 @@ DEFINE_PER_CPU(int, sd_llc_size);
DEFINE_PER_CPU(int, sd_llc_id);
DEFINE_PER_CPU(struct sched_domain_shared __rcu *, sd_llc_shared);
DEFINE_PER_CPU(struct sched_domain __rcu *, sd_numa);
+DEFINE_PER_CPU(struct sched_domain __rcu *, sd_balance_wake);
+DEFINE_PER_CPU(struct sched_domain __rcu *, sd_balance_fork);
+DEFINE_PER_CPU(struct sched_domain __rcu *, sd_balance_exec);
DEFINE_PER_CPU(struct sched_domain __rcu *, sd_asym_packing);
DEFINE_PER_CPU(struct sched_domain __rcu *, sd_asym_cpucapacity);
DEFINE_STATIC_KEY_FALSE(sched_asym_cpucapacity);
@@ -636,6 +639,15 @@ static void update_top_cache_domain(int cpu)
sd = lowest_flag_domain(cpu, SD_NUMA);
rcu_assign_pointer(per_cpu(sd_numa, cpu), sd);

+ sd = highest_flag_domain(cpu, SD_BALANCE_WAKE);
+ rcu_assign_pointer(per_cpu(sd_balance_wake, cpu), sd);
+
+ sd = highest_flag_domain(cpu, SD_BALANCE_FORK);
+ rcu_assign_pointer(per_cpu(sd_balance_fork, cpu), sd);
+
+ sd = highest_flag_domain(cpu, SD_BALANCE_EXEC);
+ rcu_assign_pointer(per_cpu(sd_balance_exec, cpu), sd);
+
sd = highest_flag_domain(cpu, SD_ASYM_PACKING);
rcu_assign_pointer(per_cpu(sd_asym_packing, cpu), sd);

--
2.24.0
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