On Thu, Jan 17, 2019 at 03:34:10PM +0000, Valentin Schneider wrote:
> In
>
> commmit 3b1baa6496e6 ("sched/fair: Add 'group_misfit_task' load-balance
> type")
>
> we set rq->misfit_task_load whenever the current running task has a
> utilization greater than 80% of rq->cpu_capacity. A non-zero value in
> this field enables misfit load balancing.
>
> However, if the task being looked at is already running on a CPU of
> highest capacity, there's nothing more we can do for it. We can
> currently spot this in update_sd_pick_busiest(), which prevents us
> from selecting a sched_group of group_type == group_misfit_task as the
> busiest group, but we don't do any of that in nohz_balancer_kick().
>
> This means that we could repeatedly kick nohz CPUs when there's no
> improvements in terms of load balance to be done.
>
> Introduce a check_misfit_status() helper that returns true iff there
> is a CPU in the system that could give more CPU capacity to a rq's
> misfit task - IOW, there exists a CPU of higher capacity_orig or the
> rq's CPU is severely pressured by rt/IRQ.
>
> Signed-off-by: Valentin Schneider <[email protected]>
> +static inline int check_misfit_status(struct rq *rq, struct sched_domain *sd)
> +{
> + return rq->misfit_task_load &&
> + (rq->cpu_capacity_orig < rq->rd->max_cpu_capacity ||
> + check_cpu_capacity(rq, sd));
> +}
> @@ -9527,7 +9539,7 @@ static void nohz_balancer_kick(struct rq *rq)
> if (time_before(now, nohz.next_balance))
> goto out;
>
> - if (rq->nr_running >= 2 || rq->misfit_task_load) {
> + if (rq->nr_running >= 2) {
> flags = NOHZ_KICK_MASK;
> goto out;
> }
> @@ -9561,6 +9573,14 @@ static void nohz_balancer_kick(struct rq *rq)
sd = rcu_dereference(rq->sd);
if (sd) {
if ((rq->cfs.h_nr_running >= 1) &&
check_cpu_capacity(rq, sd)) {
flags = NOHZ_KICK_MASK;
goto unlock;
> }
> }
>
> + sd = rcu_dereference(per_cpu(sd_asym_cpucapacity, cpu));
> + if (sd) {
> + if (check_misfit_status(rq, sd)) {
> + flags = NOHZ_KICK_MASK;
> + goto unlock;
> + }
> + }
So while the exact @sd to use for check_cpu_capacity() likely doesn't
matter; this is a 'implicit' test for actually having asym_capacity.
Fair enough I suppose. However, now that you wrote such a nice comment
for the sd_llc_shared case, these other two cases are sad to not have a
comment.
So how about you add something like:
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -9589,8 +9589,12 @@ static void nohz_balancer_kick(struct rq
sd = rcu_dereference(rq->sd);
if (sd) {
- if ((rq->cfs.h_nr_running >= 1) &&
- check_cpu_capacity(rq, sd)) {
+ /*
+ * If there's a CFS task and the current CPU has reduced
+ * capacity; kick the ILB to see if there's a better CPU to run
+ * on.
+ */
+ if (rq->cfs.h_nr_running >= 1 && check_cpu_capacity(rq, sd)) {
flags = NOHZ_KICK_MASK;
goto unlock;
}
@@ -9598,6 +9602,10 @@ static void nohz_balancer_kick(struct rq
sd = rcu_dereference(per_cpu(sd_asym_cpucapacity, cpu));
if (sd) {
+ /*
+ * When ASYM_CAPACITY; see if there's a higher capacity CPU to
+ * run the misfit task on.
+ */
if (check_misfit_status(rq, sd)) {
flags = NOHZ_KICK_MASK;
goto unlock;
@@ -9606,6 +9614,10 @@ static void nohz_balancer_kick(struct rq
sd = rcu_dereference(per_cpu(sd_asym_packing, cpu));
if (sd) {
+ /*
+ * When ASYM_PACKING; see if there's a more preferred CPU going
+ * idle; in which case, kick the ILB to move tasks around.
+ */
for_each_cpu_and(i, sched_domain_span(sd), nohz.idle_cpus_mask)
{
if (sched_asym_prefer(i, cpu)) {
flags = NOHZ_KICK_MASK;
