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SubjectRe: [PATCH] fix scheduler regression from "sched/fair: Rework load_balance()"
Le lundi 26 oct. 2020 à 12:04:45 (-0400), Rik van Riel a écrit :
> On Mon, 26 Oct 2020 16:42:14 +0100
> Vincent Guittot <vincent.guittot@linaro.org> wrote:
> > On Mon, 26 Oct 2020 at 16:04, Rik van Riel <riel@surriel.com> wrote:
>
> > > Could utilization estimates be off, either lagging or
> > > simply having a wrong estimate for a task, resulting
> > > in no task getting pulled sometimes, while doing a
> > > migrate_task imbalance always moves over something?
> >
> > task and cpu utilization are not always up to fully synced and may lag
> > a bit which explains that sometimes LB can fail to migrate for a small
> > diff
>
> OK, running with this little snippet below, I see latencies
> improve back to near where they used to be:
>
> Latency percentiles (usec) runtime 150 (s)
> 50.0th: 13
> 75.0th: 31
> 90.0th: 69
> 95.0th: 90
> *99.0th: 761
> 99.5th: 2268
> 99.9th: 9104
> min=1, max=16158
>
> I suspect the right/cleaner approach might be to use
> migrate_task more in !CPU_NOT_IDLE cases?
>
> Running a task to an idle CPU immediately, instead of refusing
> to have the load balancer move it, improves latencies for fairly
> obvious reasons.
>
> I am not entirely clear on why the load balancer should need to
> be any more conservative about moving tasks than the wakeup
> path is in eg. select_idle_sibling.


what you are suggesting is something like:
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 4978964e75e5..3b6fbf33abc2 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -9156,7 +9156,8 @@ static inline void calculate_imbalance(struct lb_env *env, struct sd_lb_stats *s
* emptying busiest.
*/
if (local->group_type == group_has_spare) {
- if (busiest->group_type > group_fully_busy) {
+ if ((busiest->group_type > group_fully_busy) &&
+ !(env->sd->flags & SD_SHARE_PKG_RESOURCES)) {
/*
* If busiest is overloaded, try to fill spare
* capacity. This might end up creating spare capacity
which also fixes the problem for me and alignes LB with wakeup path regarding the migration
in the LLC

>
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 35bdc0cccfa6..60acf71a2d39 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -7415,7 +7415,7 @@ static int detach_tasks(struct lb_env *env)
> case migrate_util:
> util = task_util_est(p);
>
> - if (util > env->imbalance)
> + if (util > env->imbalance && env->idle == CPU_NOT_IDLE)

we don't want to be so agressive outside LLC and such bypass will defeat the purpose of
using utilization instead of nr_running.


> goto next;
>
> env->imbalance -= util;

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