Commit a0fe2cf0 authored Feb 11, 2019 by Valentin Schneider Committed by Ingo Molnar Mar 19, 2019

sched/fair: Tune down misfit NOHZ kicks



In this commit:

  3b1baa64 ("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>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Dietmar.Eggemann@arm.com
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rik van Riel <riel@surriel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: morten.rasmussen@arm.com
Cc: vincent.guittot@linaro.org
Link: https://lkml.kernel.org/r/20190211175946.4961-3-valentin.schneider@arm.com


Signed-off-by: Ingo Molnar <mingo@kernel.org>

parent e25a7a94

kernel/sched/fair.c

+25 −1

Original line number	Diff line number	Diff line
		@@ -8058,6 +8058,18 @@ check_cpu_capacity(struct rq rq, struct sched_domain sd)
		(rq->cpu_capacity_orig * 100));
		}

		/*
		* Check whether a rq has a misfit task and if it looks like we can actually
		* help that task: we can migrate the task to a CPU of higher capacity, or
		* the task's current CPU is heavily pressured.
		*/
		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));
		}

		/*
		* Group imbalance indicates (and tries to solve) the problem where balancing
		* groups is inadequate due to ->cpus_allowed constraints.
		@@ -9585,7 +9597,7 @@ 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;
		}
		@@ -9623,6 +9635,18 @@ static void nohz_balancer_kick(struct rq *rq)
		}
		}

		sd = rcu_dereference(per_cpu(sd_asym_cpucapacity, cpu));
		if (sd) {
		/*
		* When ASYM_CPUCAPACITY; 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;
		}
		}

		sd = rcu_dereference(per_cpu(sd_asym_packing, cpu));
		if (sd) {
		/*

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