Messages in this thread Patch in this message | | | Date | Wed, 6 Feb 2019 17:04:13 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH 4/5] sched/fair: Tune down misfit nohz kicks |
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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 <valentin.schneider@arm.com>
> +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;
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