[RFC][PATCH 2/5] sched/fair: Make select_idle_cpu() proportional to cores

From: Peter Zijlstra
Date: Mon Dec 14 2020 - 12:12:54 EST


Instead of calculating how many (logical) CPUs to scan, compute how
many cores to scan.

This changes behaviour for anything !SMT2.

Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
kernel/sched/core.c | 19 ++++++++++++++-----
kernel/sched/fair.c | 12 ++++++++++--
2 files changed, 24 insertions(+), 7 deletions(-)

--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7454,11 +7454,20 @@ int sched_cpu_activate(unsigned int cpu)
balance_push_set(cpu, false);

#ifdef CONFIG_SCHED_SMT
- /*
- * When going up, increment the number of cores with SMT present.
- */
- if (cpumask_weight(cpu_smt_mask(cpu)) == 2)
- static_branch_inc_cpuslocked(&sched_smt_present);
+ do {
+ int weight = cpumask_weight(cpu_smt_mask(cpu));
+ extern int sched_smt_weight;
+
+ if (weight > sched_smt_weight)
+ sched_smt_weight = weight;
+
+ /*
+ * When going up, increment the number of cores with SMT present.
+ */
+ if (weight == 2)
+ static_branch_inc_cpuslocked(&sched_smt_present);
+
+ } while (0);
#endif
set_cpu_active(cpu, true);

--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6010,6 +6010,8 @@ static inline int find_idlest_cpu(struct
DEFINE_STATIC_KEY_FALSE(sched_smt_present);
EXPORT_SYMBOL_GPL(sched_smt_present);

+int sched_smt_weight = 1;
+
static inline void set_idle_cores(int cpu, int val)
{
struct sched_domain_shared *sds;
@@ -6124,6 +6126,8 @@ static int select_idle_smt(struct task_s

#else /* CONFIG_SCHED_SMT */

+#define sched_smt_weight 1
+
static inline int select_idle_core(struct task_struct *p, struct sched_domain *sd, int target)
{
return -1;
@@ -6136,6 +6140,8 @@ static inline int select_idle_smt(struct

#endif /* CONFIG_SCHED_SMT */

+#define sis_min_cores 2
+
/*
* Scan the LLC domain for idle CPUs; this is dynamically regulated by
* comparing the average scan cost (tracked in sd->avg_scan_cost) against the
@@ -6166,10 +6172,12 @@ static int select_idle_cpu(struct task_s
avg_cost = this_sd->avg_scan_cost + 1;

span_avg = sd->span_weight * avg_idle;
- if (span_avg > 4*avg_cost)
+ if (span_avg > sis_min_cores * avg_cost)
nr = div_u64(span_avg, avg_cost);
else
- nr = 4;
+ nr = sis_min_cores;
+
+ nr *= sched_smt_weight;

time = cpu_clock(this);
}