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Subject[PATCH v4 10/10] sched/fair: Provide idle search schedstats
Date
Add schedstats to measure the effectiveness of searching for idle CPUs
and stealing tasks. This is a temporary patch intended for use during
development only. SCHEDSTAT_VERSION is bumped to 16, and the following
fields are added to the per-CPU statistics of /proc/schedstat:

field 10: # of times select_idle_sibling "easily" found an idle CPU --
prev or target is idle.
field 11: # of times select_idle_sibling searched and found an idle cpu.
field 12: # of times select_idle_sibling searched and found an idle core.
field 13: # of times select_idle_sibling failed to find anything idle.
field 14: time in nanoseconds spent in functions that search for idle
CPUs and search for tasks to steal.
field 15: # of times an idle CPU steals a task from another CPU.
field 16: # of times try_steal finds overloaded CPUs but no task is
migratable.

Signed-off-by: Steve Sistare <steven.sistare@oracle.com>
---
kernel/sched/core.c | 31 ++++++++++++++++++++++++++++--
kernel/sched/fair.c | 54 ++++++++++++++++++++++++++++++++++++++++++++++------
kernel/sched/sched.h | 9 +++++++++
kernel/sched/stats.c | 11 ++++++++++-
kernel/sched/stats.h | 13 +++++++++++++
5 files changed, 109 insertions(+), 9 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index f12225f..14ee88b 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2220,17 +2220,44 @@ int sysctl_numa_balancing(struct ctl_table *table, int write,
DEFINE_STATIC_KEY_FALSE(sched_schedstats);
static bool __initdata __sched_schedstats = false;

+unsigned long schedstat_skid;
+
+static void compute_skid(void)
+{
+ int i, n = 0;
+ s64 t;
+ int skid = 0;
+
+ for (i = 0; i < 100; i++) {
+ t = local_clock();
+ t = local_clock() - t;
+ if (t > 0 && t < 1000) { /* only use sane samples */
+ skid += (int) t;
+ n++;
+ }
+ }
+
+ if (n > 0)
+ schedstat_skid = skid / n;
+ else
+ schedstat_skid = 0;
+ pr_info("schedstat_skid = %lu\n", schedstat_skid);
+}
+
static void set_schedstats(bool enabled)
{
- if (enabled)
+ if (enabled) {
+ compute_skid();
static_branch_enable(&sched_schedstats);
- else
+ } else {
static_branch_disable(&sched_schedstats);
+ }
}

void force_schedstat_enabled(void)
{
if (!schedstat_enabled()) {
+ compute_skid();
pr_info("kernel profiling enabled schedstats, disable via kernel.sched_schedstats.\n");
static_branch_enable(&sched_schedstats);
}
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 1efd9c4..73e9873 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -3748,29 +3748,35 @@ static inline bool steal_enabled(void)
static void overload_clear(struct rq *rq)
{
struct sparsemask *overload_cpus;
+ unsigned long time;

if (!steal_enabled())
return;

+ time = schedstat_start_time();
rcu_read_lock();
overload_cpus = rcu_dereference(rq->cfs_overload_cpus);
if (overload_cpus)
sparsemask_clear_elem(overload_cpus, rq->cpu);
rcu_read_unlock();
+ schedstat_end_time(rq->find_time, time);
}

static void overload_set(struct rq *rq)
{
struct sparsemask *overload_cpus;
+ unsigned long time;

if (!steal_enabled())
return;

+ time = schedstat_start_time();
rcu_read_lock();
overload_cpus = rcu_dereference(rq->cfs_overload_cpus);
if (overload_cpus)
sparsemask_set_elem(overload_cpus, rq->cpu);
rcu_read_unlock();
+ schedstat_end_time(rq->find_time, time);
}

static int try_steal(struct rq *this_rq, struct rq_flags *rf);
@@ -6191,6 +6197,16 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
return cpu;
}

+#define SET_STAT(STAT) \
+ do { \
+ if (schedstat_enabled()) { \
+ struct rq *rq = this_rq(); \
+ \
+ if (rq) \
+ __schedstat_inc(rq->STAT); \
+ } \
+ } while (0)
+
/*
* Try and locate an idle core/thread in the LLC cache domain.
*/
@@ -6199,14 +6215,18 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
struct sched_domain *sd;
int i, recent_used_cpu;

- if (available_idle_cpu(target))
+ if (available_idle_cpu(target)) {
+ SET_STAT(found_idle_cpu_easy);
return target;
+ }

/*
* If the previous CPU is cache affine and idle, don't be stupid:
*/
- if (prev != target && cpus_share_cache(prev, target) && available_idle_cpu(prev))
+ if (prev != target && cpus_share_cache(prev, target) && available_idle_cpu(prev)) {
+ SET_STAT(found_idle_cpu_easy);
return prev;
+ }

/* Check a recently used CPU as a potential idle candidate: */
recent_used_cpu = p->recent_used_cpu;
@@ -6219,26 +6239,36 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
* Replace recent_used_cpu with prev as it is a potential
* candidate for the next wake:
*/
+ SET_STAT(found_idle_cpu_easy);
p->recent_used_cpu = prev;
return recent_used_cpu;
}

sd = rcu_dereference(per_cpu(sd_llc, target));
- if (!sd)
+ if (!sd) {
+ SET_STAT(nofound_idle_cpu);
return target;
+ }

i = select_idle_core(p, sd, target);
- if ((unsigned)i < nr_cpumask_bits)
+ if ((unsigned)i < nr_cpumask_bits) {
+ SET_STAT(found_idle_core);
return i;
+ }

i = select_idle_cpu(p, sd, target);
- if ((unsigned)i < nr_cpumask_bits)
+ if ((unsigned)i < nr_cpumask_bits) {
+ SET_STAT(found_idle_cpu);
return i;
+ }

i = select_idle_smt(p, sd, target);
- if ((unsigned)i < nr_cpumask_bits)
+ if ((unsigned)i < nr_cpumask_bits) {
+ SET_STAT(found_idle_cpu);
return i;
+ }

+ SET_STAT(nofound_idle_cpu);
return target;
}

@@ -6392,6 +6422,7 @@ static int wake_cap(struct task_struct *p, int cpu, int prev_cpu)
static int
select_task_rq_fair(struct task_struct *p, int prev_cpu, int sd_flag, int wake_flags)
{
+ unsigned long time = schedstat_start_time();
struct sched_domain *tmp, *sd = NULL;
int cpu = smp_processor_id();
int new_cpu = prev_cpu;
@@ -6440,6 +6471,7 @@ static int wake_cap(struct task_struct *p, int cpu, int prev_cpu)
current->recent_used_cpu = cpu;
}
rcu_read_unlock();
+ schedstat_end_time(cpu_rq(cpu)->find_time, time);

return new_cpu;
}
@@ -6686,6 +6718,7 @@ static void check_preempt_wakeup(struct rq *rq, struct task_struct *p, int wake_
struct sched_entity *se;
struct task_struct *p;
int new_tasks;
+ unsigned long time;

again:
if (!cfs_rq->nr_running)
@@ -6800,6 +6833,8 @@ static void check_preempt_wakeup(struct rq *rq, struct task_struct *p, int wake_
idle:
update_misfit_status(NULL, rq);

+ time = schedstat_start_time();
+
/*
* We must set idle_stamp _before_ calling try_steal() or
* idle_balance(), such that we measure the duration as idle time.
@@ -6813,6 +6848,8 @@ static void check_preempt_wakeup(struct rq *rq, struct task_struct *p, int wake_
if (new_tasks)
rq_idle_stamp_clear(rq);

+ schedstat_end_time(rq->find_time, time);
+
/*
* Because try_steal() and idle_balance() release (and re-acquire)
* rq->lock, it is possible for any higher priority task to appear.
@@ -9886,6 +9923,7 @@ static int steal_from(struct rq *dst_rq, struct rq_flags *dst_rf, bool *locked,
update_rq_clock(dst_rq);
attach_task(dst_rq, p);
stolen = 1;
+ schedstat_inc(dst_rq->steal);
}
local_irq_restore(rf.flags);

@@ -9910,6 +9948,7 @@ static int try_steal(struct rq *dst_rq, struct rq_flags *dst_rf)
int dst_cpu = dst_rq->cpu;
bool locked = true;
int stolen = 0;
+ bool any_overload = false;
struct sparsemask *overload_cpus;

if (!steal_enabled())
@@ -9952,6 +9991,7 @@ static int try_steal(struct rq *dst_rq, struct rq_flags *dst_rf)
stolen = 1;
goto out;
}
+ any_overload = true;
}

out:
@@ -9963,6 +10003,8 @@ static int try_steal(struct rq *dst_rq, struct rq_flags *dst_rf)
stolen |= (dst_rq->cfs.h_nr_running > 0);
if (dst_rq->nr_running != dst_rq->cfs.h_nr_running)
stolen = -1;
+ if (!stolen && any_overload)
+ schedstat_inc(dst_rq->steal_fail);
return stolen;
}

diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 2a28340..f61f640 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -915,6 +915,15 @@ struct rq {
/* try_to_wake_up() stats */
unsigned int ttwu_count;
unsigned int ttwu_local;
+
+ /* Idle search stats */
+ unsigned int found_idle_core;
+ unsigned int found_idle_cpu;
+ unsigned int found_idle_cpu_easy;
+ unsigned int nofound_idle_cpu;
+ unsigned long find_time;
+ unsigned int steal;
+ unsigned int steal_fail;
#endif

#ifdef CONFIG_SMP
diff --git a/kernel/sched/stats.c b/kernel/sched/stats.c
index 750fb3c..00b3de5 100644
--- a/kernel/sched/stats.c
+++ b/kernel/sched/stats.c
@@ -10,7 +10,7 @@
* Bump this up when changing the output format or the meaning of an existing
* format, so that tools can adapt (or abort)
*/
-#define SCHEDSTAT_VERSION 15
+#define SCHEDSTAT_VERSION 16

static int show_schedstat(struct seq_file *seq, void *v)
{
@@ -37,6 +37,15 @@ static int show_schedstat(struct seq_file *seq, void *v)
rq->rq_cpu_time,
rq->rq_sched_info.run_delay, rq->rq_sched_info.pcount);

+ seq_printf(seq, " %u %u %u %u %lu %u %u",
+ rq->found_idle_cpu_easy,
+ rq->found_idle_cpu,
+ rq->found_idle_core,
+ rq->nofound_idle_cpu,
+ rq->find_time,
+ rq->steal,
+ rq->steal_fail);
+
seq_printf(seq, "\n");

#ifdef CONFIG_SMP
diff --git a/kernel/sched/stats.h b/kernel/sched/stats.h
index 4904c46..63ba3c2 100644
--- a/kernel/sched/stats.h
+++ b/kernel/sched/stats.h
@@ -39,6 +39,17 @@
#define schedstat_set(var, val) do { if (schedstat_enabled()) { var = (val); } } while (0)
#define schedstat_val(var) (var)
#define schedstat_val_or_zero(var) ((schedstat_enabled()) ? (var) : 0)
+#define schedstat_start_time() schedstat_val_or_zero(local_clock())
+#define schedstat_end_time(stat, time) \
+ do { \
+ unsigned long endtime; \
+ \
+ if (schedstat_enabled() && (time)) { \
+ endtime = local_clock() - (time) - schedstat_skid; \
+ schedstat_add((stat), endtime); \
+ } \
+ } while (0)
+extern unsigned long schedstat_skid;

#else /* !CONFIG_SCHEDSTATS: */
static inline void rq_sched_info_arrive (struct rq *rq, unsigned long long delta) { }
@@ -53,6 +64,8 @@ static inline void rq_sched_info_depart (struct rq *rq, unsigned long long delt
# define schedstat_set(var, val) do { } while (0)
# define schedstat_val(var) 0
# define schedstat_val_or_zero(var) 0
+# define schedstat_start_time() 0
+# define schedstat_end_time(stat, t) do { } while (0)
#endif /* CONFIG_SCHEDSTATS */

#ifdef CONFIG_PSI
--
1.8.3.1
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