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SubjectRe: [PATCH 8/9] sched/fair: Optimize cgroup pick_next_task_fair
On Tue, Jan 21, 2014 at 12:03:38PM -0800, bsegall@google.com wrote:

> > Indeed, in that case we can miss something... we could try to call
> > check_cfs_rq_runtime() from the initial top-down selection loop? When
> > true, just put the entire stack and don't pretend to be smart?
>
> Yeah, I think that should work. I wasn't sure if there could be a
> problem of doing throttle_cfs_rq(parent); throttle_cfs_rq(child);, but
> thinking about, that has to be ok, because schedule can do that if
> deactivate throttled the parent, schedule calls update_rq_clock, and
> then put_prev throttled the child.

Maybe something like the below? Completely untested and everything.

There's the obviuos XXX fail that was also in the previous version; not
sure how to deal with that yet, either we need to change the interface
to take struct task_struct **prev, or get smarter :-)

Any other obvious fails in here?

---
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -2736,17 +2736,36 @@ wakeup_preempt_entity(struct sched_entit
* 3) pick the "last" process, for cache locality
* 4) do not run the "skip" process, if something else is available
*/
-static struct sched_entity *pick_next_entity(struct cfs_rq *cfs_rq)
+static struct sched_entity *
+pick_next_entity(struct cfs_rq *cfs_rq, struct sched_entity *curr)
{
- struct sched_entity *se = __pick_first_entity(cfs_rq);
- struct sched_entity *left = se;
+ struct sched_entity *left = __pick_first_entity(cfs_rq);
+ struct sched_entity *se;
+
+ /*
+ * If curr is set we have to see if its left of the leftmost entity
+ * still in the tree, provided there was anything in the tree at all.
+ */
+ if (!left || (curr && entity_before(curr, left)))
+ left = curr;
+
+ se = left; /* ideally we run the leftmost entity */

/*
* Avoid running the skip buddy, if running something else can
* be done without getting too unfair.
*/
if (cfs_rq->skip == se) {
- struct sched_entity *second = __pick_next_entity(se);
+ struct sched_entity *second;
+
+ if (se == curr) {
+ second = __pick_first_entity(cfs_rq);
+ } else {
+ second = __pick_next_entity(se);
+ if (!second || (curr && entity_before(curr, second)))
+ second = curr;
+ }
+
if (second && wakeup_preempt_entity(second, left) < 1)
se = second;
}
@@ -2768,7 +2787,7 @@ static struct sched_entity *pick_next_en
return se;
}

-static void check_cfs_rq_runtime(struct cfs_rq *cfs_rq);
+static bool check_cfs_rq_runtime(struct cfs_rq *cfs_rq);

static void put_prev_entity(struct cfs_rq *cfs_rq, struct sched_entity *prev)
{
@@ -3423,22 +3442,23 @@ static void check_enqueue_throttle(struc
}

/* conditionally throttle active cfs_rq's from put_prev_entity() */
-static void check_cfs_rq_runtime(struct cfs_rq *cfs_rq)
+static bool check_cfs_rq_runtime(struct cfs_rq *cfs_rq)
{
if (!cfs_bandwidth_used())
- return;
+ return false;

if (likely(!cfs_rq->runtime_enabled || cfs_rq->runtime_remaining > 0))
- return;
+ return false;

/*
* it's possible for a throttled entity to be forced into a running
* state (e.g. set_curr_task), in this case we're finished.
*/
if (cfs_rq_throttled(cfs_rq))
- return;
+ return true;

throttle_cfs_rq(cfs_rq);
+ return true;
}

static enum hrtimer_restart sched_cfs_slack_timer(struct hrtimer *timer)
@@ -3548,7 +3568,7 @@ static inline u64 cfs_rq_clock_task(stru
}

static void account_cfs_rq_runtime(struct cfs_rq *cfs_rq, u64 delta_exec) {}
-static void check_cfs_rq_runtime(struct cfs_rq *cfs_rq) {}
+static bool check_cfs_rq_runtime(struct cfs_rq *cfs_rq) { return false; }
static void check_enqueue_throttle(struct cfs_rq *cfs_rq) {}
static __always_inline void return_cfs_rq_runtime(struct cfs_rq *cfs_rq) {}

@@ -4484,26 +4504,109 @@ static void check_preempt_wakeup(struct
set_last_buddy(se);
}

+/*
+ * Account for a descheduled task:
+ */
+static void put_prev_task_fair(struct rq *rq, struct task_struct *prev)
+{
+ struct sched_entity *se = &prev->se;
+ struct cfs_rq *cfs_rq;
+
+ for_each_sched_entity(se) {
+ cfs_rq = cfs_rq_of(se);
+ put_prev_entity(cfs_rq, se);
+ }
+}
+
static struct task_struct *
pick_next_task_fair(struct rq *rq, struct task_struct *prev)
{
- struct task_struct *p;
struct cfs_rq *cfs_rq = &rq->cfs;
struct sched_entity *se;
+ struct task_struct *p;

if (!cfs_rq->nr_running)
return NULL;

- if (prev)
+ if (!IS_ENABLED(CONFIG_FAIR_GROUP_SCHED) ||
+ (prev->sched_class != &fair_sched_class)) {
prev->sched_class->put_prev_task(rq, prev);
+ prev = NULL;
+ }
+
+#ifdef CONFIG_FAIR_GROUP_SCHED
+ if (!prev)
+ goto simple;

do {
- se = pick_next_entity(cfs_rq);
+ struct sched_entity *curr = cfs_rq->curr;
+
+ /*
+ * Since we got here without doing put_prev_entity() we also
+ * have to consider cfs_rq->curr. If it is still a runnable
+ * entity, update_curr() will update its vruntime, otherwise
+ * forget we've ever seen it.
+ */
+ if (curr->on_rq)
+ update_curr(cfs_rq);
+ else
+ curr = NULL;
+
+ if (unlikely(check_cfs_rq_runtime(cfs_rq))) {
+ put_prev_task_fair(rq, prev);
+ goto simple;
+ }
+
+ se = pick_next_entity(cfs_rq, curr);
+ cfs_rq = group_cfs_rq(se);
+ } while (cfs_rq);
+
+ p = task_of(se);
+
+ /*
+ * Since we haven't yet done put_prev_entity and if the selected task
+ * is a different task than we started out with, try and touch the
+ * least amount of cfs_rqs.
+ */
+ if (prev != p) {
+ struct sched_entity *pse = &prev->se;
+
+ while (!(cfs_rq = is_same_group(se, pse))) {
+ int se_depth = se->depth;
+ int pse_depth = pse->depth;
+
+ if (se_depth <= pse_depth) {
+ put_prev_entity(cfs_rq_of(pse), pse);
+ pse = parent_entity(pse);
+ }
+ if (se_depth >= pse_depth) {
+ set_next_entity(cfs_rq_of(se), se);
+ se = parent_entity(se);
+ }
+ }
+
+ put_prev_entity(cfs_rq, pse);
+ set_next_entity(cfs_rq, se);
+ }
+
+ return p;
+simple:
+#endif
+ cfs_rq = &rq->cfs;
+
+ if (!cfs_rq->nr_running) {
+ // XXX FAIL we should never return NULL after putting @prev
+ return NULL;
+ }
+
+ do {
+ se = pick_next_entity(cfs_rq, NULL);
set_next_entity(cfs_rq, se);
cfs_rq = group_cfs_rq(se);
} while (cfs_rq);

p = task_of(se);
+
if (hrtick_enabled(rq))
hrtick_start_fair(rq, p);

@@ -4511,20 +4614,6 @@ pick_next_task_fair(struct rq *rq, struc
}

/*
- * Account for a descheduled task:
- */
-static void put_prev_task_fair(struct rq *rq, struct task_struct *prev)
-{
- struct sched_entity *se = &prev->se;
- struct cfs_rq *cfs_rq;
-
- for_each_sched_entity(se) {
- cfs_rq = cfs_rq_of(se);
- put_prev_entity(cfs_rq, se);
- }
-}
-
-/*
* sched_yield() is very simple
*
* The magic of dealing with the ->skip buddy is in pick_next_entity.

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