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SubjectRe: [PATCH RFC 1/2] sched: Minimize the idle cpu selection race window.
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On Fri, 2017-11-24 at 19:46 +0100, Mike Galbraith wrote:
>
> My view is you're barking up the wrong tree: you're making the idle
> data SIS is using more accurate, but I question the benefit.  That it
> makes an imperfect placement decision occasionally due to raciness is
> nearly meaningless compared to the cost of frequent bounce.

Playing with SIS (yet again), below is a hack that I think illustrates
why I think the occasional races are nearly meaningless.

Box = i4790 desktop.

master masterx
TCP_STREAM-1 Avg: 70495 71295
TCP_STREAM-2 Avg: 54388 66202
TCP_STREAM-4 Avg: 19316 21413
TCP_STREAM-8 Avg: 9678 8894
TCP_STREAM-16 Avg: 4286 4360
TCP_MAERTS-1 Avg: 69238 71799
TCP_MAERTS-2 Avg: 50729 65612
TCP_MAERTS-4 Avg: 19095 21984
TCP_MAERTS-8 Avg: 9405 8888
TCP_MAERTS-16 Avg: 4891 4371
TCP_RR-1 Avg: 198617 203291
TCP_RR-2 Avg: 152862 191761
TCP_RR-4 Avg: 112241 117888
TCP_RR-8 Avg: 104453 113260
TCP_RR-16 Avg: 50897 55280
UDP_RR-1 Avg: 250738 264214
UDP_RR-2 Avg: 196250 253352
UDP_RR-4 Avg: 152862 158819
UDP_RR-8 Avg: 143781 154071
UDP_RR-16 Avg: 68605 76492


tbench 1 2 4 8 16
master 772 1207 1829 3516 3440
masterx 811 1466 1959 3737 3670

hackbench -l 10000
5.917 5.990 5.957 avg 5.954 NO_SIS_DEBOUNCE
5.886 5.808 5.826 avg 5.840 SIS_DEBOUNCE


echo 0 > tracing_on
echo 1 > events/sched/sched_migrate_task/enable
start endless tbench 2

for i in `seq 3`
do
echo > trace
echo 1 > tracing_on
sleep 10
echo 0 > tracing_on
cat trace|grep tbench|wc -l
done

kde desktop idling

NO_SIS_DEBOUNCE 261 208 199
SIS_DEBOUNCE 8 6 0

add firefox playing youtube documentary

NO_SIS_DEBOUNCE 10906 10094 10774
SIS_DEBOUNCE 34 34 34

tbench 2 throughput as firefox runs

NO_SIS_DEBOUNCE 1129.63 MB/sec
SIS_DEBOUNCE 1462.53 MB/sec

Advisory: welding goggles.

---
include/linux/sched.h | 3 ++-
kernel/sched/fair.c | 41 +++++++++++++++++++++++++++++++++++++++--
kernel/sched/features.h | 1 +
3 files changed, 42 insertions(+), 3 deletions(-)

--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -541,7 +541,6 @@ struct task_struct {
unsigned int ptrace;

#ifdef CONFIG_SMP
- struct llist_node wake_entry;
int on_cpu;
#ifdef CONFIG_THREAD_INFO_IN_TASK
/* Current CPU: */
@@ -549,8 +548,10 @@ struct task_struct {
#endif
unsigned int wakee_flips;
unsigned long wakee_flip_decay_ts;
+ unsigned long wakee_placed;
struct task_struct *last_wakee;

+ struct llist_node wake_entry;
int wake_cpu;
#endif
int on_rq;
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6174,6 +6174,9 @@ static int select_idle_sibling(struct ta
if ((unsigned)i < nr_cpumask_bits)
return i;

+ if (sched_feat(SIS_DEBOUNCE))
+ p->wakee_placed = jiffies;
+
return target;
}

@@ -6258,6 +6261,22 @@ static int wake_cap(struct task_struct *
return min_cap * 1024 < task_util(p) * capacity_margin;
}

+static bool task_placed(struct task_struct *p)
+{
+ return p->wakee_placed == jiffies;
+}
+
+static bool task_llc_affine_and_cold(struct task_struct *p, int cpu, int prev)
+{
+ int cold = sysctl_sched_migration_cost;
+
+ if (!cpus_share_cache(cpu, prev))
+ return false;
+ if (cold > 0 && rq_clock_task(cpu_rq(prev)) - p->se.exec_start > cold)
+ return true;
+ return false;
+}
+
/*
* select_task_rq_fair: Select target runqueue for the waking task in domains
* that have the 'sd_flag' flag set. In practice, this is SD_BALANCE_WAKE,
@@ -6276,16 +6295,26 @@ select_task_rq_fair(struct task_struct *
struct sched_domain *tmp, *affine_sd = NULL, *sd = NULL;
int cpu = smp_processor_id();
int new_cpu = prev_cpu;
- int want_affine = 0;
+ int want_affine = 0, want_debounce = 0;
int sync = wake_flags & WF_SYNC;

+ rcu_read_lock();
if (sd_flag & SD_BALANCE_WAKE) {
record_wakee(p);
+ want_debounce = sched_feat(SIS_DEBOUNCE);
+ if (task_placed(p))
+ goto out_unlock;
+ /* Balance cold tasks to reduce hot task bounce tendency. */
+ if (want_debounce && task_llc_affine_and_cold(p, cpu, prev_cpu)) {
+ sd_flag |= SD_SHARE_PKG_RESOURCES;
+ sd = highest_flag_domain(prev_cpu, SD_SHARE_PKG_RESOURCES);
+ p->wakee_placed = jiffies;
+ goto pick_cpu_cold;
+ }
want_affine = !wake_wide(p) && !wake_cap(p, cpu, prev_cpu)
&& cpumask_test_cpu(cpu, &p->cpus_allowed);
}

- rcu_read_lock();
for_each_domain(cpu, tmp) {
if (!(tmp->flags & SD_LOAD_BALANCE))
break;
@@ -6315,6 +6344,7 @@ select_task_rq_fair(struct task_struct *
new_cpu = cpu;
}

+pick_cpu_cold:
if (sd && !(sd_flag & SD_BALANCE_FORK)) {
/*
* We're going to need the task's util for capacity_spare_wake
@@ -6329,9 +6359,13 @@ select_task_rq_fair(struct task_struct *
if (sd_flag & SD_BALANCE_WAKE) /* XXX always ? */
new_cpu = select_idle_sibling(p, prev_cpu, new_cpu);

+ if (want_debounce && new_cpu == prev_cpu)
+ p->wakee_placed = jiffies;
+
} else {
new_cpu = find_idlest_cpu(sd, p, cpu, prev_cpu, sd_flag);
}
+out_unlock:
rcu_read_unlock();

return new_cpu;
@@ -6952,6 +6986,9 @@ static int task_hot(struct task_struct *
if (sysctl_sched_migration_cost == 0)
return 0;

+ if (task_placed(p))
+ return 1;
+
delta = rq_clock_task(env->src_rq) - p->se.exec_start;

return delta < (s64)sysctl_sched_migration_cost;
--- a/kernel/sched/features.h
+++ b/kernel/sched/features.h
@@ -57,6 +57,7 @@ SCHED_FEAT(TTWU_QUEUE, true)
*/
SCHED_FEAT(SIS_AVG_CPU, false)
SCHED_FEAT(SIS_PROP, true)
+SCHED_FEAT(SIS_DEBOUNCE, true)

/*
* Issue a WARN when we do multiple update_rq_clock() calls
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