diff --git a/include/linux/sched.h b/include/linux/sched.h
index 98888f1a03bc8250bf02b98c7f38b50459bc77eb..2c30ed860d66a44d4a2a9c51014ad2d925dffa18 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1070,6 +1070,7 @@ struct sched_group;
struct sched_domain_shared {
atomic_t ref;
atomic_t nr_busy_cpus;
+ int has_idle_cores;
};
struct sched_domain {
@@ -1102,6 +1103,8 @@ struct sched_domain {
u64 max_newidle_lb_cost;
unsigned long next_decay_max_lb_cost;
+ u64 avg_scan_cost; /* select_idle_sibling */
+
#ifdef CONFIG_SCHEDSTATS
/* load_balance() stats */
unsigned int lb_count[CPU_MAX_IDLE_TYPES];
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 26826eac5545f5dcfcc9235cba5040cf0888637e..75ecd4f29199aafa08d185435b286ee2d2092a86 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7478,6 +7478,7 @@ static struct kmem_cache *task_group_cache __read_mostly;
#endif
DECLARE_PER_CPU(cpumask_var_t, load_balance_mask);
+DECLARE_PER_CPU(cpumask_var_t, select_idle_mask);
void __init sched_init(void)
{
@@ -7514,6 +7515,8 @@ void __init sched_init(void)
for_each_possible_cpu(i) {
per_cpu(load_balance_mask, i) = (cpumask_var_t)kzalloc_node(
cpumask_size(), GFP_KERNEL, cpu_to_node(i));
+ per_cpu(select_idle_mask, i) = (cpumask_var_t)kzalloc_node(
+ cpumask_size(), GFP_KERNEL, cpu_to_node(i));
}
#endif /* CONFIG_CPUMASK_OFFSTACK */
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 15902cdb27b3d2412b0c13bf74122479749233fa..6b41589c41e43b993a2e64799a9447918b3fa35a 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1582,9 +1582,16 @@ balance:
* One idle CPU per node is evaluated for a task numa move.
* Call select_idle_sibling to maybe find a better one.
*/
- if (!cur)
+ if (!cur) {
+ /*
+ * select_idle_siblings() uses an per-cpu cpumask that
+ * can be used from IRQ context.
+ */
+ local_irq_disable();
env->dst_cpu = select_idle_sibling(env->p, env->src_cpu,
env->dst_cpu);
+ local_irq_enable();
+ }
assign:
task_numa_assign(env, cur, imp);
@@ -4616,6 +4623,11 @@ static void dequeue_task_fair(struct rq *rq, struct task_struct *p, int flags)
}
#ifdef CONFIG_SMP
+
+/* Working cpumask for: load_balance, load_balance_newidle. */
+DEFINE_PER_CPU(cpumask_var_t, load_balance_mask);
+DEFINE_PER_CPU(cpumask_var_t, select_idle_mask);
+
#ifdef CONFIG_NO_HZ_COMMON
/*
* per rq 'load' arrray crap; XXX kill this.
@@ -5280,65 +5292,231 @@ find_idlest_cpu(struct sched_group *group, struct task_struct *p, int this_cpu)
}
/*
- * Try and locate an idle CPU in the sched_domain.
+ * Implement a for_each_cpu() variant that starts the scan at a given cpu
+ * (@start), and wraps around.
+ *
+ * This is used to scan for idle CPUs; such that not all CPUs looking for an
+ * idle CPU find the same CPU. The down-side is that tasks tend to cycle
+ * through the LLC domain.
+ *
+ * Especially tbench is found sensitive to this.
+ */
+
+static int cpumask_next_wrap(int n, const struct cpumask *mask, int start, int *wrapped)
+{
+ int next;
+
+again:
+ next = find_next_bit(cpumask_bits(mask), nr_cpumask_bits, n+1);
+
+ if (*wrapped) {
+ if (next >= start)
+ return nr_cpumask_bits;
+ } else {
+ if (next >= nr_cpumask_bits) {
+ *wrapped = 1;
+ n = -1;
+ goto again;
+ }
+ }
+
+ return next;
+}
+
+#define for_each_cpu_wrap(cpu, mask, start, wrap) \
+ for ((wrap) = 0, (cpu) = (start)-1; \
+ (cpu) = cpumask_next_wrap((cpu), (mask), (start), &(wrap)), \
+ (cpu) < nr_cpumask_bits; )
+
+#ifdef CONFIG_SCHED_SMT
+
+static inline void set_idle_cores(int cpu, int val)
+{
+ struct sched_domain_shared *sds;
+
+ sds = rcu_dereference(per_cpu(sd_llc_shared, cpu));
+ if (sds)
+ WRITE_ONCE(sds->has_idle_cores, val);
+}
+
+static inline bool test_idle_cores(int cpu, bool def)
+{
+ struct sched_domain_shared *sds;
+
+ sds = rcu_dereference(per_cpu(sd_llc_shared, cpu));
+ if (sds)
+ return READ_ONCE(sds->has_idle_cores);
+
+ return def;
+}
+
+/*
+ * Scans the local SMT mask to see if the entire core is idle, and records this
+ * information in sd_llc_shared->has_idle_cores.
+ *
+ * Since SMT siblings share all cache levels, inspecting this limited remote
+ * state should be fairly cheap.
+ */
+void update_idle_core(struct rq *rq)
+{
+ int core = cpu_of(rq);
+ int cpu;
+
+ rcu_read_lock();
+ if (test_idle_cores(core, true))
+ goto unlock;
+
+ for_each_cpu(cpu, cpu_smt_mask(core)) {
+ if (cpu == core)
+ continue;
+
+ if (!idle_cpu(cpu))
+ goto unlock;
+ }
+
+ set_idle_cores(core, 1);
+unlock:
+ rcu_read_unlock();
+}
+
+/*
+ * Scan the entire LLC domain for idle cores; this dynamically switches off if
+ * there are no idle cores left in the system; tracked through
+ * sd_llc->shared->has_idle_cores and enabled through update_idle_core() above.
+ */
+static int select_idle_core(struct task_struct *p, struct sched_domain *sd, int target)
+{
+ struct cpumask *cpus = this_cpu_cpumask_var_ptr(select_idle_mask);
+ int core, cpu, wrap;
+
+ if (!test_idle_cores(target, false))
+ return -1;
+
+ cpumask_and(cpus, sched_domain_span(sd), tsk_cpus_allowed(p));
+
+ for_each_cpu_wrap(core, cpus, target, wrap) {
+ bool idle = true;
+
+ for_each_cpu(cpu, cpu_smt_mask(core)) {
+ cpumask_clear_cpu(cpu, cpus);
+ if (!idle_cpu(cpu))
+ idle = false;
+ }
+
+ if (idle)
+ return core;
+ }
+
+ /*
+ * Failed to find an idle core; stop looking for one.
+ */
+ set_idle_cores(target, 0);
+
+ return -1;
+}
+
+/*
+ * Scan the local SMT mask for idle CPUs.
+ */
+static int select_idle_smt(struct task_struct *p, struct sched_domain *sd, int target)
+{
+ int cpu;
+
+ for_each_cpu(cpu, cpu_smt_mask(target)) {
+ if (!cpumask_test_cpu(cpu, tsk_cpus_allowed(p)))
+ continue;
+ if (idle_cpu(cpu))
+ return cpu;
+ }
+
+ return -1;
+}
+
+#else /* CONFIG_SCHED_SMT */
+
+static inline int select_idle_core(struct task_struct *p, struct sched_domain *sd, int target)
+{
+ return -1;
+}
+
+static inline int select_idle_smt(struct task_struct *p, struct sched_domain *sd, int target)
+{
+ return -1;
+}
+
+#endif /* CONFIG_SCHED_SMT */
+
+/*
+ * 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
+ * average idle time for this rq (as found in rq->avg_idle).
+ */
+static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int target)
+{
+ struct sched_domain *this_sd = rcu_dereference(*this_cpu_ptr(&sd_llc));
+ u64 avg_idle = this_rq()->avg_idle;
+ u64 avg_cost = this_sd->avg_scan_cost;
+ u64 time, cost;
+ s64 delta;
+ int cpu, wrap;
+
+ /*
+ * Due to large variance we need a large fuzz factor; hackbench in
+ * particularly is sensitive here.
+ */
+ if ((avg_idle / 512) < avg_cost)
+ return -1;
+
+ time = local_clock();
+
+ for_each_cpu_wrap(cpu, sched_domain_span(sd), target, wrap) {
+ if (!cpumask_test_cpu(cpu, tsk_cpus_allowed(p)))
+ continue;
+ if (idle_cpu(cpu))
+ break;
+ }
+
+ time = local_clock() - time;
+ cost = this_sd->avg_scan_cost;
+ delta = (s64)(time - cost) / 8;
+ this_sd->avg_scan_cost += delta;
+
+ return cpu;
+}
+
+/*
+ * Try and locate an idle core/thread in the LLC cache domain.
*/
static int select_idle_sibling(struct task_struct *p, int prev, int target)
{
struct sched_domain *sd;
- struct sched_group *sg;
+ int i;
if (idle_cpu(target))
return target;
/*
- * If the prevous cpu is cache affine and idle, don't be stupid.
+ * If the previous cpu is cache affine and idle, don't be stupid.
*/
if (prev != target && cpus_share_cache(prev, target) && idle_cpu(prev))
return prev;
- /*
- * Otherwise, iterate the domains and find an eligible idle cpu.
- *
- * A completely idle sched group at higher domains is more
- * desirable than an idle group at a lower level, because lower
- * domains have smaller groups and usually share hardware
- * resources which causes tasks to contend on them, e.g. x86
- * hyperthread siblings in the lowest domain (SMT) can contend
- * on the shared cpu pipeline.
- *
- * However, while we prefer idle groups at higher domains
- * finding an idle cpu at the lowest domain is still better than
- * returning 'target', which we've already established, isn't
- * idle.
- */
sd = rcu_dereference(per_cpu(sd_llc, target));
- for_each_lower_domain(sd) {
- sg = sd->groups;
- do {
- int i;
+ if (!sd)
+ return target;
- if (!cpumask_intersects(sched_group_cpus(sg),
- tsk_cpus_allowed(p)))
- goto next;
+ i = select_idle_core(p, sd, target);
+ if ((unsigned)i < nr_cpumask_bits)
+ return i;
- /* Ensure the entire group is idle */
- for_each_cpu(i, sched_group_cpus(sg)) {
- if (i == target || !idle_cpu(i))
- goto next;
- }
+ i = select_idle_cpu(p, sd, target);
+ if ((unsigned)i < nr_cpumask_bits)
+ return i;
+
+ i = select_idle_smt(p, sd, target);
+ if ((unsigned)i < nr_cpumask_bits)
+ return i;
- /*
- * It doesn't matter which cpu we pick, the
- * whole group is idle.
- */
- target = cpumask_first_and(sched_group_cpus(sg),
- tsk_cpus_allowed(p));
- goto done;
-next:
- sg = sg->next;
- } while (sg != sd->groups);
- }
-done:
return target;
}
@@ -7397,9 +7575,6 @@ static struct rq *find_busiest_queue(struct lb_env *env,
*/
#define MAX_PINNED_INTERVAL 512
-/* Working cpumask for load_balance and load_balance_newidle. */
-DEFINE_PER_CPU(cpumask_var_t, load_balance_mask);
-
static int need_active_balance(struct lb_env *env)
{
struct sched_domain *sd = env->sd;
diff --git a/kernel/sched/idle_task.c b/kernel/sched/idle_task.c
index dedc81ecbb2e2ad221f937584bc39070d1e3fe3d..5405d3feb112f54e3b4dabecbe573d59fee7a160 100644
--- a/kernel/sched/idle_task.c
+++ b/kernel/sched/idle_task.c
@@ -27,7 +27,7 @@ static struct task_struct *
pick_next_task_idle(struct rq *rq, struct task_struct *prev, struct pin_cookie cookie)
{
put_prev_task(rq, prev);
-
+ update_idle_core(rq);
schedstat_inc(rq->sched_goidle);
return rq->idle;
}
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 4fc6e9876d9c0470f04951fdd8f5a483ed4db1c8..c917dcad82addb4a0a2266239ed256b4ad64e039 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -36,6 +36,12 @@ extern void cpu_load_update_active(struct rq *this_rq);
static inline void cpu_load_update_active(struct rq *this_rq) { }
#endif
+#ifdef CONFIG_SCHED_SMT
+extern void update_idle_core(struct rq *rq);
+#else
+static inline void update_idle_core(struct rq *rq) { }
+#endif
+
/*
* Helpers for converting nanosecond timing to jiffy resolution
*/