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Subject[PATCH v2 1/3] sched/fair: Call newidle_balance() from balance_callback on PREEMPT_RT
Date
This is required in order to be able to enable interrupts in the next
patch. This is limited to PREEMPT_RT to avoid adding potentially
measurable overhead to the non-RT case (requiring a double switch when
pulling a task onto a newly idle cpu).

update_misfit_status() is factored out for the PREEMPT_RT case, to ensure
that the misfit status is kept consistent before dropping the lock.

Signed-off-by: Scott Wood <swood@redhat.com>
---
v2: Use a balance callback, and limit to PREEMPT_RT

kernel/sched/fair.c | 34 ++++++++++++++++++++++++++++++----
1 file changed, 30 insertions(+), 4 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 794c2cb945f8..ff369c38a5b5 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5660,6 +5660,9 @@ static void dequeue_task_fair(struct rq *rq, struct task_struct *p, int flags)

#ifdef CONFIG_SMP

+static const bool newidle_balance_in_callback = IS_ENABLED(CONFIG_PREEMPT_RT);
+static DEFINE_PER_CPU(struct callback_head, rebalance_head);
+
/* 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);
@@ -10549,7 +10552,7 @@ static inline void nohz_newidle_balance(struct rq *this_rq) { }
* 0 - failed, no new tasks
* > 0 - success, new (fair) tasks present
*/
-static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
+static int do_newidle_balance(struct rq *this_rq, struct rq_flags *rf)
{
unsigned long next_balance = jiffies + HZ;
int this_cpu = this_rq->cpu;
@@ -10557,7 +10560,9 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
int pulled_task = 0;
u64 curr_cost = 0;

- update_misfit_status(NULL, this_rq);
+ if (!newidle_balance_in_callback)
+ update_misfit_status(NULL, this_rq);
+
/*
* We must set idle_stamp _before_ calling idle_balance(), such that we
* measure the duration of idle_balance() as idle time.
@@ -10576,7 +10581,8 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
* further scheduler activity on it and we're being very careful to
* re-start the picking loop.
*/
- rq_unpin_lock(this_rq, rf);
+ if (!newidle_balance_in_callback)
+ rq_unpin_lock(this_rq, rf);

if (this_rq->avg_idle < sysctl_sched_migration_cost ||
!READ_ONCE(this_rq->rd->overload)) {
@@ -10655,11 +10661,31 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
if (pulled_task)
this_rq->idle_stamp = 0;

- rq_repin_lock(this_rq, rf);
+ if (!newidle_balance_in_callback)
+ rq_repin_lock(this_rq, rf);

return pulled_task;
}

+static void newidle_balance_cb(struct rq *this_rq)
+{
+ update_rq_clock(this_rq);
+ do_newidle_balance(this_rq, NULL);
+}
+
+static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
+{
+ if (newidle_balance_in_callback) {
+ update_misfit_status(NULL, this_rq);
+ queue_balance_callback(this_rq,
+ &per_cpu(rebalance_head, this_rq->cpu),
+ newidle_balance_cb);
+ return 0;
+ }
+
+ return do_newidle_balance(this_rq, rf);
+}
+
/*
* run_rebalance_domains is triggered when needed from the scheduler tick.
* Also triggered for nohz idle balancing (with nohz_balancing_kick set).
--
2.27.0
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