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

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 7bbe78a..a666185 100644 (file)
--- 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 2872e15..0b9cbfb 100644 (file)
--- 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 4418c19..19c5859 100644 (file)
--- 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 8d59de2..7c54550 100644 (file)
--- 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 dbdabd7..858c4cc 100644 (file)
--- 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 e085cff..7111e3a 100644 (file)
--- 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 8f41401..7e1cee4 100644 (file)
--- 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;