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SubjectRe: [PATCH v4] sched/freq: move call to cpufreq_update_util
On Fri, Nov 15, 2019 at 02:37:27PM +0100, Vincent Guittot wrote:
> On Fri, 15 Nov 2019 at 14:25, Peter Zijlstra <peterz@infradead.org> wrote:

> > Should not all 3 have their windows aligned and thus alway return the
> > exact same value?
>
> rt and dl yes but not irq
>
> But having aligned window doesn't mean that they will all decay.
> One can have been updated just before (during a dequeue as an example)
> or at least less than 1ms before

Now, the thing is, if that update happened in sched/rt, then it wouldn't
have called cpufreq anyway. And once we're idle longer than a period,
they'll all decay at once.

Except indeed that IRQ stuff, which runs out of sync.

That is, I'm just not convinced it matters much if we keep rq->cfs
on the list forever (like UP). Because we'll only stop calling when
update_blocked_averages() when everything hit 0, and up until that
point, we'll get one update per period from rq->cfs.

For good measure we can force an update when @done, at that point we
know all 0s.

How is something like this?

---
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 545bcb90b4de..a99ac2aa4a23 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -3508,9 +3508,6 @@ update_cfs_rq_load_avg(u64 now, struct cfs_rq *cfs_rq)
cfs_rq->load_last_update_time_copy = sa->last_update_time;
#endif

- if (decayed)
- cfs_rq_util_change(cfs_rq, 0);
-
return decayed;
}

@@ -3620,8 +3617,12 @@ static inline void update_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *s
attach_entity_load_avg(cfs_rq, se, SCHED_CPUFREQ_MIGRATION);
update_tg_load_avg(cfs_rq, 0);

- } else if (decayed && (flags & UPDATE_TG))
- update_tg_load_avg(cfs_rq, 0);
+ } else if (decayed) {
+ cfs_rq_util_change(cfs_rq, 0);
+
+ if (flags & UPDATE_TG)
+ update_tg_load_avg(cfs_rq, 0);
+ }
}

#ifndef CONFIG_64BIT
@@ -7453,7 +7454,7 @@ static void update_blocked_averages(int cpu)
struct cfs_rq *cfs_rq, *pos;
const struct sched_class *curr_class;
struct rq_flags rf;
- bool done = true;
+ bool done = true, decayed = false;

rq_lock_irqsave(rq, &rf);
update_rq_clock(rq);
@@ -7476,10 +7477,14 @@ static void update_blocked_averages(int cpu)
* list_add_leaf_cfs_rq() for details.
*/
for_each_leaf_cfs_rq_safe(rq, cfs_rq, pos) {
+ bool last = cfs_rq == &rq->cfs;
struct sched_entity *se;

- if (update_cfs_rq_load_avg(cfs_rq_clock_pelt(cfs_rq), cfs_rq))
+ if (update_cfs_rq_load_avg(cfs_rq_clock_pelt(cfs_rq), cfs_rq)) {
update_tg_load_avg(cfs_rq, 0);
+ if (last)
+ decayed = true;
+ }

/* Propagate pending load changes to the parent, if any: */
se = cfs_rq->tg->se[cpu];
@@ -7490,7 +7495,7 @@ static void update_blocked_averages(int cpu)
* There can be a lot of idle CPU cgroups. Don't let fully
* decayed cfs_rqs linger on the list.
*/
- if (cfs_rq_is_decayed(cfs_rq))
+ if (!last && cfs_rq_is_decayed(cfs_rq))
list_del_leaf_cfs_rq(cfs_rq);

/* Don't need periodic decay once load/util_avg are null */
@@ -7498,6 +7503,9 @@ static void update_blocked_averages(int cpu)
done = false;
}

+ if (decayed || done)
+ cpufreq_update_util(rq, 0);
+
update_blocked_load_status(rq, !done);
rq_unlock_irqrestore(rq, &rf);
}
@@ -7555,6 +7563,7 @@ static inline void update_blocked_averages(int cpu)
struct cfs_rq *cfs_rq = &rq->cfs;
const struct sched_class *curr_class;
struct rq_flags rf;
+ bool done, decayed;

rq_lock_irqsave(rq, &rf);
update_rq_clock(rq);
@@ -7568,9 +7577,13 @@ static inline void update_blocked_averages(int cpu)
update_dl_rq_load_avg(rq_clock_pelt(rq), rq, curr_class == &dl_sched_class);
update_irq_load_avg(rq, 0);

- update_cfs_rq_load_avg(cfs_rq_clock_pelt(cfs_rq), cfs_rq);
+ decayed = update_cfs_rq_load_avg(cfs_rq_clock_pelt(cfs_rq), cfs_rq);
+ done = !(cfs_rq_has_blocked(cfs_rq) || others_have_blocked(rq));

- update_blocked_load_status(rq, cfs_rq_has_blocked(cfs_rq) || others_have_blocked(rq));
+ if (decayed || done)
+ cpufreq_update_util(rq, 0);
+
+ update_blocked_load_status(rq, !done);
rq_unlock_irqrestore(rq, &rf);
}

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