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authorPeter Zijlstra <peterz@infradead.org>2019-05-29 20:36:44 +0000
committerPeter Zijlstra <peterz@infradead.org>2019-08-08 09:09:31 +0200
commit67692435c411e5c53a1c588ecca2037aebd81f2e (patch)
tree53d9c44e6e80654a60c652493d4b0cc948ce5818
parent5f2a45fc9e89e022233085e6f0f352eb6ff770bb (diff)
downloadlinux-67692435c411e5c53a1c588ecca2037aebd81f2e.tar.bz2
sched: Rework pick_next_task() slow-path
Avoid the RETRY_TASK case in the pick_next_task() slow path. By doing the put_prev_task() early, we get the rt/deadline pull done, and by testing rq->nr_running we know if we need newidle_balance(). This then gives a stable state to pick a task from. Since the fast-path is fair only; it means the other classes will always have pick_next_task(.prev=NULL, .rf=NULL) and we can simplify. Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Cc: Aaron Lu <aaron.lwe@gmail.com> Cc: Valentin Schneider <valentin.schneider@arm.com> Cc: mingo@kernel.org Cc: Phil Auld <pauld@redhat.com> Cc: Julien Desfossez <jdesfossez@digitalocean.com> Cc: Nishanth Aravamudan <naravamudan@digitalocean.com> Link: https://lkml.kernel.org/r/aa34d24b36547139248f32a30138791ac6c02bd6.1559129225.git.vpillai@digitalocean.com
-rw-r--r--kernel/sched/core.c19
-rw-r--r--kernel/sched/deadline.c30
-rw-r--r--kernel/sched/fair.c9
-rw-r--r--kernel/sched/idle.c4
-rw-r--r--kernel/sched/rt.c29
-rw-r--r--kernel/sched/sched.h13
-rw-r--r--kernel/sched/stop_task.c3
7 files changed, 34 insertions, 73 deletions
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 7bbe78a31ba5..a6661852907b 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3791,7 +3791,7 @@ pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
p = fair_sched_class.pick_next_task(rq, prev, rf);
if (unlikely(p == RETRY_TASK))
- goto again;
+ goto restart;
/* Assumes fair_sched_class->next == idle_sched_class */
if (unlikely(!p))
@@ -3800,14 +3800,19 @@ pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
return p;
}
-again:
+restart:
+ /*
+ * Ensure that we put DL/RT tasks before the pick loop, such that they
+ * can PULL higher prio tasks when we lower the RQ 'priority'.
+ */
+ prev->sched_class->put_prev_task(rq, prev, rf);
+ if (!rq->nr_running)
+ newidle_balance(rq, rf);
+
for_each_class(class) {
- p = class->pick_next_task(rq, prev, rf);
- if (p) {
- if (unlikely(p == RETRY_TASK))
- goto again;
+ p = class->pick_next_task(rq, NULL, NULL);
+ if (p)
return p;
- }
}
/* The idle class should always have a runnable task: */
diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 2872e15a87cd..0b9cbfb2b1d4 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -1761,39 +1761,13 @@ pick_next_task_dl(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
struct task_struct *p;
struct dl_rq *dl_rq;
- dl_rq = &rq->dl;
-
- if (need_pull_dl_task(rq, prev)) {
- /*
- * This is OK, because current is on_cpu, which avoids it being
- * picked for load-balance and preemption/IRQs are still
- * disabled avoiding further scheduler activity on it and we're
- * being very careful to re-start the picking loop.
- */
- rq_unpin_lock(rq, rf);
- pull_dl_task(rq);
- rq_repin_lock(rq, rf);
- /*
- * pull_dl_task() can drop (and re-acquire) rq->lock; this
- * means a stop task can slip in, in which case we need to
- * re-start task selection.
- */
- if (rq->stop && task_on_rq_queued(rq->stop))
- return RETRY_TASK;
- }
+ WARN_ON_ONCE(prev || rf);
- /*
- * When prev is DL, we may throttle it in put_prev_task().
- * So, we update time before we check for dl_nr_running.
- */
- if (prev->sched_class == &dl_sched_class)
- update_curr_dl(rq);
+ dl_rq = &rq->dl;
if (unlikely(!dl_rq->dl_nr_running))
return NULL;
- put_prev_task(rq, prev);
-
dl_se = pick_next_dl_entity(rq, dl_rq);
BUG_ON(!dl_se);
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 4418c1998e69..19c58599e967 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6770,7 +6770,7 @@ again:
goto idle;
#ifdef CONFIG_FAIR_GROUP_SCHED
- if (prev->sched_class != &fair_sched_class)
+ if (!prev || prev->sched_class != &fair_sched_class)
goto simple;
/*
@@ -6847,8 +6847,8 @@ again:
goto done;
simple:
#endif
-
- put_prev_task(rq, prev);
+ if (prev)
+ put_prev_task(rq, prev);
do {
se = pick_next_entity(cfs_rq, NULL);
@@ -6876,6 +6876,9 @@ done: __maybe_unused;
return p;
idle:
+ if (!rf)
+ return NULL;
+
new_tasks = newidle_balance(rq, rf);
/*
diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
index 8d59de2e4a6e..7c54550dda6a 100644
--- a/kernel/sched/idle.c
+++ b/kernel/sched/idle.c
@@ -389,7 +389,9 @@ pick_next_task_idle(struct rq *rq, struct task_struct *prev, struct rq_flags *rf
{
struct task_struct *next = rq->idle;
- put_prev_task(rq, prev);
+ if (prev)
+ put_prev_task(rq, prev);
+
set_next_task_idle(rq, next);
return next;
diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index dbdabd76f192..858c4cc6f99b 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -1553,38 +1553,11 @@ pick_next_task_rt(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
struct task_struct *p;
struct rt_rq *rt_rq = &rq->rt;
- if (need_pull_rt_task(rq, prev)) {
- /*
- * This is OK, because current is on_cpu, which avoids it being
- * picked for load-balance and preemption/IRQs are still
- * disabled avoiding further scheduler activity on it and we're
- * being very careful to re-start the picking loop.
- */
- rq_unpin_lock(rq, rf);
- pull_rt_task(rq);
- rq_repin_lock(rq, rf);
- /*
- * pull_rt_task() can drop (and re-acquire) rq->lock; this
- * means a dl or stop task can slip in, in which case we need
- * to re-start task selection.
- */
- if (unlikely((rq->stop && task_on_rq_queued(rq->stop)) ||
- rq->dl.dl_nr_running))
- return RETRY_TASK;
- }
-
- /*
- * We may dequeue prev's rt_rq in put_prev_task().
- * So, we update time before rt_queued check.
- */
- if (prev->sched_class == &rt_sched_class)
- update_curr_rt(rq);
+ WARN_ON_ONCE(prev || rf);
if (!rt_rq->rt_queued)
return NULL;
- put_prev_task(rq, prev);
-
p = _pick_next_task_rt(rq);
set_next_task_rt(rq, p);
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index e085cffb8004..7111e3a1eeb4 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1700,12 +1700,15 @@ struct sched_class {
void (*check_preempt_curr)(struct rq *rq, struct task_struct *p, int flags);
/*
- * It is the responsibility of the pick_next_task() method that will
- * return the next task to call put_prev_task() on the @prev task or
- * something equivalent.
+ * Both @prev and @rf are optional and may be NULL, in which case the
+ * caller must already have invoked put_prev_task(rq, prev, rf).
*
- * May return RETRY_TASK when it finds a higher prio class has runnable
- * tasks.
+ * Otherwise it is the responsibility of the pick_next_task() to call
+ * put_prev_task() on the @prev task or something equivalent, IFF it
+ * returns a next task.
+ *
+ * In that case (@rf != NULL) it may return RETRY_TASK when it finds a
+ * higher prio class has runnable tasks.
*/
struct task_struct * (*pick_next_task)(struct rq *rq,
struct task_struct *prev,
diff --git a/kernel/sched/stop_task.c b/kernel/sched/stop_task.c
index 8f414018d5e0..7e1cee4e65b2 100644
--- a/kernel/sched/stop_task.c
+++ b/kernel/sched/stop_task.c
@@ -33,10 +33,11 @@ pick_next_task_stop(struct rq *rq, struct task_struct *prev, struct rq_flags *rf
{
struct task_struct *stop = rq->stop;
+ WARN_ON_ONCE(prev || rf);
+
if (!stop || !task_on_rq_queued(stop))
return NULL;
- put_prev_task(rq, prev);
set_next_task_stop(rq, stop);
return stop;