[RFC v2 3/7] Improve the tracking of active utilisation

Luca Abeni
2016-04-01T15:20:41Z

This patch implements a more theoretically sound algorithm for
thracking the active utilisation: instead of decreasing it when a
task blocks, use a timer (the "inactive timer", named after the
"Inactive" task state of the GRUB algorithm) to decrease the
active utilisaation at the so called "0-lag time".

Signed-off-by: Luca Abeni <luca.abeni@unitn.it>
---
include/linux/sched.h | 1 +
kernel/sched/core.c | 1 +
kernel/sched/deadline.c | 158 +++++++++++++++++++++++++++++++++++++++++-------
kernel/sched/sched.h | 1 +
4 files changed, 139 insertions(+), 22 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index c617ea1..f285461 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1351,6 +1351,7 @@ struct sched_dl_entity {
* own bandwidth to be enforced, thus we need one timer per task.
*/
struct hrtimer dl_timer;
+ struct hrtimer inactive_timer;
};

union rcu_special {
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index de38077..23d235c 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2076,6 +2076,7 @@ static void __sched_fork(unsigned long clone_flags, struct task_struct *p)

RB_CLEAR_NODE(&p->dl.rb_node);
init_dl_task_timer(&p->dl);
+ init_inactive_task_timer(&p->dl);
__dl_clear_params(p);

INIT_LIST_HEAD(&p->rt.run_list);
diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 05cfccb..97cd5f2 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -47,6 +47,7 @@ static void add_running_bw(struct sched_dl_entity *dl_se, struct dl_rq *dl_rq)
{
u64 se_bw = dl_se->dl_bw;

+ lockdep_assert_held(&(rq_of_dl_rq(dl_rq))->lock);
dl_rq->running_bw += se_bw;
}

@@ -54,11 +55,59 @@ static void sub_running_bw(struct sched_dl_entity *dl_se, struct dl_rq *dl_rq)
{
u64 se_bw = dl_se->dl_bw;

+ lockdep_assert_held(&(rq_of_dl_rq(dl_rq))->lock);
dl_rq->running_bw -= se_bw;
if (WARN_ON(dl_rq->running_bw < 0))
dl_rq->running_bw = 0;
}

+static void task_go_inactive(struct task_struct *p)
+{
+ struct sched_dl_entity *dl_se = &p->dl;
+ struct hrtimer *timer = &dl_se->inactive_timer;
+ struct dl_rq *dl_rq = dl_rq_of_se(dl_se);
+ struct rq *rq = rq_of_dl_rq(dl_rq);
+ ktime_t now, act;
+ s64 delta;
+ u64 zerolag_time;
+
+ WARN_ON(dl_se->dl_runtime == 0);
+
+ /* If the inactive timer is already armed, return immediately */
+ if (hrtimer_active(&dl_se->inactive_timer))
+ return;
+
+
+ /*
+ * We want the timer to fire at the "0 lag time", but considering
+ * that it is actually coming from rq->clock and not from
+ * hrtimer's time base reading.
+ */
+ zerolag_time = dl_se->deadline -
+ div64_long((dl_se->runtime * dl_se->dl_period),
+ dl_se->dl_runtime);
+
+ act = ns_to_ktime(zerolag_time);
+ now = hrtimer_cb_get_time(timer);
+ delta = ktime_to_ns(now) - rq_clock(rq);
+ act = ktime_add_ns(act, delta);
+
+ /*
+ * If the "0-lag time" already passed, decrease the active
+ * utilization now, instead of starting a timer
+ */
+ if (ktime_us_delta(act, now) < 0) {
+ sub_running_bw(dl_se, dl_rq);
+ if (!dl_task(p))
+ __dl_clear_params(p);
+
+ return;
+ }
+
+ get_task_struct(p);
+ hrtimer_start(timer, act, HRTIMER_MODE_ABS);
+}
+
static inline int is_leftmost(struct task_struct *p, struct dl_rq *dl_rq)
{
struct sched_dl_entity *dl_se = &p->dl;
@@ -526,7 +575,18 @@ static void update_dl_entity(struct sched_dl_entity *dl_se,
struct dl_rq *dl_rq = dl_rq_of_se(dl_se);
struct rq *rq = rq_of_dl_rq(dl_rq);

- add_running_bw(dl_se, dl_rq);
+ /*
+ * If the "inactive timer" is still active, stop it and leave
+ * the active utilisation unchanged.
+ * Otherwise, increase the active utilisation.
+ * If the timer cannot be cancelled, inactive_task_timer() will
+ * find the task state as TASK_RUNNING, and will do nothing, so
+ * we are still safe.
+ */
+ if (hrtimer_active(&dl_se->inactive_timer))
+ hrtimer_try_to_cancel(&dl_se->inactive_timer);
+ else
+ add_running_bw(dl_se, dl_rq);

if (dl_time_before(dl_se->deadline, rq_clock(rq)) ||
dl_entity_overflow(dl_se, pi_se, rq_clock(rq))) {
@@ -614,14 +674,8 @@ static enum hrtimer_restart dl_task_timer(struct hrtimer *timer)

rq = task_rq_lock(p, &flags);

- /*
- * The task might have changed its scheduling policy to something
- * different than SCHED_DEADLINE (through switched_fromd_dl()).
- */
- if (!dl_task(p)) {
- __dl_clear_params(p);
+ if (!dl_task(p))
goto unlock;
- }

/*
* The task might have been boosted by someone else and might be in the
@@ -800,6 +854,44 @@ throttle:
}
}

+static enum hrtimer_restart inactive_task_timer(struct hrtimer *timer)
+{
+ struct sched_dl_entity *dl_se = container_of(timer,
+ struct sched_dl_entity,
+ inactive_timer);
+ struct task_struct *p = dl_task_of(dl_se);
+ unsigned long flags;
+ struct rq *rq;
+
+ rq = task_rq_lock(p, &flags);
+
+ if (!dl_task(p)) {
+ __dl_clear_params(p);
+
+ goto unlock;
+ }
+ if (p->state == TASK_RUNNING)
+ goto unlock;
+
+ sched_clock_tick();
+ update_rq_clock(rq);
+
+ sub_running_bw(dl_se, &rq->dl);
+unlock:
+ task_rq_unlock(rq, p, &flags);
+ put_task_struct(p);
+
+ return HRTIMER_NORESTART;
+}
+
+void init_inactive_task_timer(struct sched_dl_entity *dl_se)
+{
+ struct hrtimer *timer = &dl_se->inactive_timer;
+
+ hrtimer_init(timer, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
+ timer->function = inactive_task_timer;
+}
+
#ifdef CONFIG_SMP

static void inc_dl_deadline(struct dl_rq *dl_rq, u64 deadline)
@@ -976,7 +1068,8 @@ static void enqueue_task_dl(struct rq *rq, struct task_struct *p, int flags)
* run yet) will take care of this.
*/
if (p->dl.dl_throttled && !(flags & ENQUEUE_REPLENISH)) {
- add_running_bw(&p->dl, &rq->dl);
+ if (hrtimer_try_to_cancel(&p->dl.inactive_timer) < 0)
+ add_running_bw(&p->dl, &rq->dl);
return;
}

@@ -997,7 +1090,7 @@ static void dequeue_task_dl(struct rq *rq, struct task_struct *p, int flags)
update_curr_dl(rq);
__dequeue_task_dl(rq, p, flags);
if (flags & DEQUEUE_SLEEP)
- sub_running_bw(&p->dl, &rq->dl);
+ task_go_inactive(p);
}

/*
@@ -1071,6 +1164,23 @@ select_task_rq_dl(struct task_struct *p, int cpu, int sd_flag, int flags)
}
rcu_read_unlock();

+ if (rq != cpu_rq(cpu)) {
+ int migrate_active;
+
+ raw_spin_lock(&rq->lock);
+ migrate_active = hrtimer_active(&p->dl.inactive_timer);
+ if (migrate_active)
+ sub_running_bw(&p->dl, &rq->dl);
+ raw_spin_unlock(&rq->lock);
+ if (migrate_active) {
+ rq = cpu_rq(cpu);
+ raw_spin_lock(&rq->lock);
+ add_running_bw(&p->dl, &rq->dl);
+ raw_spin_unlock(&rq->lock);
+ }
+ }
+
+
out:
return cpu;
}
@@ -1232,8 +1342,6 @@ static void task_fork_dl(struct task_struct *p)
static void task_dead_dl(struct task_struct *p)
{
struct dl_bw *dl_b = dl_bw_of(task_cpu(p));
- struct dl_rq *dl_rq = dl_rq_of_se(&p->dl);
- struct rq *rq = rq_of_dl_rq(dl_rq);

/*
* Since we are TASK_DEAD we won't slip out of the domain!
@@ -1242,9 +1350,6 @@ static void task_dead_dl(struct task_struct *p)
/* XXX we should retain the bw until 0-lag */
dl_b->total_bw -= p->dl.dl_bw;
raw_spin_unlock_irq(&dl_b->lock);
-
- if (task_on_rq_queued(p))
- sub_running_bw(&p->dl, &rq->dl);
}

static void set_curr_task_dl(struct rq *rq)
@@ -1720,15 +1825,23 @@ void __init init_sched_dl_class(void)
static void switched_from_dl(struct rq *rq, struct task_struct *p)
{
/*
- * Start the deadline timer; if we switch back to dl before this we'll
- * continue consuming our current CBS slice. If we stay outside of
- * SCHED_DEADLINE until the deadline passes, the timer will reset the
- * task.
+ * task_go_inactive() can start the "inactive timer" (if the 0-lag
+ * time is in the future). If the task switches back to dl before
+ * the "inactive timer" fires, it can continue to consume its current
+ * runtime using its current deadline. If it stays outside of
+ * SCHED_DEADLINE until the 0-lag time passes, inactive_task_timer()
+ * will reset the task parameters.
*/
- if (!start_dl_timer(p))
- __dl_clear_params(p);
+ if (task_on_rq_queued(p) && p->dl.dl_runtime)
+ task_go_inactive(p);

- if (task_on_rq_queued(p))
+ /*
+ * We cannot use inactive_task_timer() to invoke sub_running_bw()
+ * at the 0-lag time, because the task could have been migrated
+ * while SCHED_OTHER in the meanwhile.
+ */
+ if (hrtimer_active(&p->dl.inactive_timer) &&
+ !hrtimer_callback_running(&p->dl.inactive_timer))
sub_running_bw(&p->dl, &rq->dl);

/*
@@ -1750,6 +1863,7 @@ static void switched_to_dl(struct rq *rq, struct task_struct *p)
{
if (dl_time_before(p->dl.deadline, rq_clock(rq)))
setup_new_dl_entity(&p->dl, &p->dl);
+ add_running_bw(&p->dl, &rq->dl);

if (task_on_rq_queued(p) && rq->curr != p) {
#ifdef CONFIG_SMP
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index bc05c29..22d36b2 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1315,6 +1315,7 @@ extern void init_rt_bandwidth(struct rt_bandwidth *rt_b, u64 period, u64 runtime
extern struct dl_bandwidth def_dl_bandwidth;
extern void init_dl_bandwidth(struct dl_bandwidth *dl_b, u64 period, u64 runtime);
extern void init_dl_task_timer(struct sched_dl_entity *dl_se);
+extern void init_inactive_task_timer(struct sched_dl_entity *dl_se);

unsigned long to_ratio(u64 period, u64 runtime);

--
2.5.0