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Subject[PATCH v2 5/6] sched/rt: Better manage pushing unfit tasks on wakeup
Date
On wakeup, if a task doesn't fit the CPU it is running on (due to its
uclamp_min value), then we trigger the push mechanism to try to find a
more suitable CPU.

But the logic introduced in commit 804d402fb6f6 ("sched/rt: Make RT capacity-aware")
was incomplete. If the rq isn't overloaded, push_rt_task() will bail out
immediately.

Steve suggested using the overloaded flag to force the push, but as
Pavan pointed out this could cause a lot of unnecessary IPIs in case of
HAVE_RT_PUSH_IPI.

To still allow pushing unfitting task ASAP, but without causing a lot of
disturbance in case this is not possible (no available CPU that is
running at a lower priority level), introduce a new rt_nr_unfitting in
struct rt_rq and use that to manage how hard we try to push an unfitting
task in push_rt_task().

If the task is pinned to a single CPU, we won't inc rt_nr_unfitting,
hence skipping the push in this case.

Also there's no need to force a push on switched_to_rt(). On the next
wakeup we should handle it which should suffice.

Fixes: 804d402fb6f6 ("sched/rt: Make RT capacity-aware")
LINK: https://lore.kernel.org/lkml/20200221080701.GF28029@codeaurora.org/
Signed-off-by: Qais Yousef <qais.yousef@arm.com>
---
kernel/sched/rt.c | 100 +++++++++++++++++++++++++++++++++++++++----
kernel/sched/sched.h | 3 ++
2 files changed, 95 insertions(+), 8 deletions(-)

diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index 9ae8a9fabe8b..b35e49cdafcc 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -11,6 +11,7 @@ int sched_rr_timeslice = RR_TIMESLICE;
int sysctl_sched_rr_timeslice = (MSEC_PER_SEC / HZ) * RR_TIMESLICE;

static int do_sched_rt_period_timer(struct rt_bandwidth *rt_b, int overrun);
+static bool rt_task_fits_capacity(struct task_struct *p, int cpu);

struct rt_bandwidth def_rt_bandwidth;

@@ -313,6 +314,27 @@ static void update_rt_migration(struct rt_rq *rt_rq)
}
}

+#ifdef CONFIG_UCLAMP_TASK
+static void inc_rt_unfit_tasks(struct task_struct *p, struct rt_rq *rt_rq)
+{
+ int cpu = cpu_of(rq_of_rt_rq(rt_rq));
+
+ if (!rt_task_fits_capacity(p, cpu))
+ rt_rq->rt_nr_unfit++;
+}
+
+static void dec_rt_unfit_tasks(struct task_struct *p, struct rt_rq *rt_rq)
+{
+ int cpu = cpu_of(rq_of_rt_rq(rt_rq));
+
+ if (!rt_task_fits_capacity(p, cpu))
+ rt_rq->rt_nr_unfit--;
+}
+#else
+static void inc_rt_unfit_tasks(struct task_struct *p, struct rt_rq *rt_rq) {}
+static void dec_rt_unfit_tasks(struct task_struct *p, struct rt_rq *rt_rq) {}
+#endif
+
static void inc_rt_migration(struct sched_rt_entity *rt_se, struct rt_rq *rt_rq)
{
struct task_struct *p;
@@ -324,9 +346,17 @@ static void inc_rt_migration(struct sched_rt_entity *rt_se, struct rt_rq *rt_rq)
rt_rq = &rq_of_rt_rq(rt_rq)->rt;

rt_rq->rt_nr_total++;
- if (p->nr_cpus_allowed > 1)
+ if (p->nr_cpus_allowed > 1) {
rt_rq->rt_nr_migratory++;

+ /*
+ * The task is dequeued and queue again on set_cpus_allowed(),
+ * so we can't end up with a unbalanced inc/dec if
+ * p->nr_cpus_allowed has changed.
+ */
+ inc_rt_unfit_tasks(p, rt_rq);
+ }
+
update_rt_migration(rt_rq);
}

@@ -341,12 +371,29 @@ static void dec_rt_migration(struct sched_rt_entity *rt_se, struct rt_rq *rt_rq)
rt_rq = &rq_of_rt_rq(rt_rq)->rt;

rt_rq->rt_nr_total--;
- if (p->nr_cpus_allowed > 1)
+ if (p->nr_cpus_allowed > 1) {
rt_rq->rt_nr_migratory--;

+ /*
+ * The task is dequeued and queue again on set_cpus_allowed(),
+ * so we can't end up with a unbalanced inc/dec if
+ * p->nr_cpus_allowed has changed.
+ */
+ dec_rt_unfit_tasks(p, rt_rq);
+ }
+
update_rt_migration(rt_rq);
}

+static inline int has_unfit_tasks(struct rq *rq)
+{
+#ifdef CONFIG_UCLAMP_TASK
+ return rq->rt.rt_nr_unfit;
+#else
+ return 0;
+#endif
+}
+
static inline int has_pushable_tasks(struct rq *rq)
{
return !plist_head_empty(&rq->rt.pushable_tasks);
@@ -1862,8 +1909,9 @@ static int push_rt_task(struct rq *rq)
struct task_struct *next_task;
struct rq *lowest_rq;
int ret = 0;
+ bool fit;

- if (!rq->rt.overloaded)
+ if (!rq->rt.overloaded && !has_unfit_tasks(rq))
return 0;

next_task = pick_next_pushable_task(rq);
@@ -1874,12 +1922,21 @@ static int push_rt_task(struct rq *rq)
if (WARN_ON(next_task == rq->curr))
return 0;

+ /*
+ * The rq could be overloaded because it has unfitting task, if that's
+ * the case then we need to try harder to find a better fitting CPU.
+ */
+ fit = rt_task_fits_capacity(next_task, cpu_of(rq));
+
/*
* It's possible that the next_task slipped in of
* higher priority than current. If that's the case
* just reschedule current.
+ *
+ * Unless next_task doesn't fit in this cpu, then continue with the
+ * attempt to push it.
*/
- if (unlikely(next_task->prio < rq->curr->prio)) {
+ if (unlikely(next_task->prio < rq->curr->prio && fit)) {
resched_curr(rq);
return 0;
}
@@ -1922,6 +1979,35 @@ static int push_rt_task(struct rq *rq)
goto retry;
}

+ /*
+ * Bail out if the task doesn't fit on either CPUs.
+ *
+ * Unless..
+ *
+ * * The rq is already overloaded, then push anyway.
+ *
+ * * The priority of next_task is higher than current, then we
+ * resched_curr(). We forced skipping this condition above if the rq
+ * was overloaded but the task didn't fit.
+ */
+ if (!fit && !rt_task_fits_capacity(next_task, cpu_of(lowest_rq))) {
+
+ /*
+ * If the system wasn't overloaded, then pretend we didn't run.
+ */
+ if (!rq->rt.overloaded)
+ goto out_unlock;
+
+ /*
+ * If the system is overloaded, we forced skipping this
+ * condition, so re-evaluate it.
+ */
+ if (unlikely(next_task->prio < rq->curr->prio)) {
+ resched_curr(rq);
+ goto out_unlock;
+ }
+ }
+
deactivate_task(rq, next_task, 0);
set_task_cpu(next_task, lowest_rq->cpu);
activate_task(lowest_rq, next_task, 0);
@@ -1929,6 +2015,7 @@ static int push_rt_task(struct rq *rq)

resched_curr(lowest_rq);

+out_unlock:
double_unlock_balance(rq, lowest_rq);

out:
@@ -2297,10 +2384,7 @@ static void switched_to_rt(struct rq *rq, struct task_struct *p)
*/
if (task_on_rq_queued(p) && rq->curr != p) {
#ifdef CONFIG_SMP
- bool need_to_push = rq->rt.overloaded ||
- !rt_task_fits_capacity(p, cpu_of(rq));
-
- if (p->nr_cpus_allowed > 1 && need_to_push)
+ if (p->nr_cpus_allowed > 1 && rq->rt.overloaded)
rt_queue_push_tasks(rq);
#endif /* CONFIG_SMP */
if (p->prio < rq->curr->prio && cpu_online(cpu_of(rq)))
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 1a88dc8ad11b..7dea81ccd49a 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -603,6 +603,9 @@ struct rt_rq {
#ifdef CONFIG_SMP
unsigned long rt_nr_migratory;
unsigned long rt_nr_total;
+#ifdef CONFIG_UCLAMP_TASK
+ unsigned long rt_nr_unfit;
+#endif
int overloaded;
struct plist_head pushable_tasks;

--
2.17.1
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