Commit 3318544b authored by Vincent Guittot's avatar Vincent Guittot Committed by Ingo Molnar
Browse files

sched/fair: Fix rework of find_idlest_group() 



The task, for which the scheduler looks for the idlest group of CPUs, must
be discounted from all statistics in order to get a fair comparison
between groups. This includes utilization, load, nr_running and idle_cpus.

Such unfairness can be easily highlighted with the unixbench execl 1 task.
This test continuously call execve() and the scheduler looks for the idlest
group/CPU on which it should place the task. Because the task runs on the
local group/CPU, the latter seems already busy even if there is nothing
else running on it. As a result, the scheduler will always select another
group/CPU than the local one.

This recovers most of the performance regression on my system from the
recent load-balancer rewrite.

[ mingo: Minor cleanups. ]

Reported-by: default avatarkernel test robot <rong.a.chen@intel.com>
Tested-by: default avatarkernel test robot <rong.a.chen@intel.com>
Signed-off-by: default avatarVincent Guittot <vincent.guittot@linaro.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Morten.Rasmussen@arm.com
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: dietmar.eggemann@arm.com
Cc: hdanton@sina.com
Cc: parth@linux.ibm.com
Cc: pauld@redhat.com
Cc: quentin.perret@arm.com
Cc: riel@surriel.com
Cc: srikar@linux.vnet.ibm.com
Cc: valentin.schneider@arm.com
Fixes: 57abff06 ("sched/fair: Rework find_idlest_group()")
Link: https://lkml.kernel.org/r/1571762798-25900-1-git-send-email-vincent.guittot@linaro.org


Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent 7763baac

kernel/sched/fair.c

+84 −7
Original line number Diff line number Diff line
@@ -5391,6 +5391,37 @@ static unsigned long cpu_load(struct rq *rq) 
	return cfs_rq_load_avg(&rq->cfs);

}



/*

 * cpu_load_without - compute CPU load without any contributions from *p

 * @cpu: the CPU which load is requested

 * @p: the task which load should be discounted

 *

 * The load of a CPU is defined by the load of tasks currently enqueued on that

 * CPU as well as tasks which are currently sleeping after an execution on that

 * CPU.

 *

 * This method returns the load of the specified CPU by discounting the load of

 * the specified task, whenever the task is currently contributing to the CPU

 * load.

 */

static unsigned long cpu_load_without(struct rq *rq, struct task_struct *p)

{

	struct cfs_rq *cfs_rq;

	unsigned int load;



	/* Task has no contribution or is new */

	if (cpu_of(rq) != task_cpu(p) || !READ_ONCE(p->se.avg.last_update_time))

		return cpu_load(rq);



	cfs_rq = &rq->cfs;

	load = READ_ONCE(cfs_rq->avg.load_avg);



	/* Discount task's util from CPU's util */

	lsub_positive(&load, task_h_load(p));



	return load;

}



static unsigned long capacity_of(int cpu)

{

	return cpu_rq(cpu)->cpu_capacity;
@@ -8141,11 +8172,56 @@ static inline enum fbq_type fbq_classify_rq(struct rq *rq) 


struct sg_lb_stats;



/*

 * task_running_on_cpu - return 1 if @p is running on @cpu.

 */



static unsigned int task_running_on_cpu(int cpu, struct task_struct *p)

{

	/* Task has no contribution or is new */

	if (cpu != task_cpu(p) || !READ_ONCE(p->se.avg.last_update_time))

		return 0;



	if (task_on_rq_queued(p))

		return 1;



	return 0;

}



/**

 * idle_cpu_without - would a given CPU be idle without p ?

 * @cpu: the processor on which idleness is tested.

 * @p: task which should be ignored.

 *

 * Return: 1 if the CPU would be idle. 0 otherwise.

 */

static int idle_cpu_without(int cpu, struct task_struct *p)

{

	struct rq *rq = cpu_rq(cpu);



	if (rq->curr != rq->idle && rq->curr != p)

		return 0;



	/*

	 * rq->nr_running can't be used but an updated version without the

	 * impact of p on cpu must be used instead. The updated nr_running

	 * be computed and tested before calling idle_cpu_without().

	 */



#ifdef CONFIG_SMP

	if (!llist_empty(&rq->wake_list))

		return 0;

#endif



	return 1;

}



/*

 * update_sg_wakeup_stats - Update sched_group's statistics for wakeup.

 * @denv: The ched_domain level to look for idlest group.

 * @sd: The sched_domain level to look for idlest group.

 * @group: sched_group whose statistics are to be updated.

 * @sgs: variable to hold the statistics for this group.

 * @p: The task for which we look for the idlest group/CPU.

 */

static inline void update_sg_wakeup_stats(struct sched_domain *sd,

					  struct sched_group *group,
@@ -8158,21 +8234,22 @@ static inline void update_sg_wakeup_stats(struct sched_domain *sd, 


	for_each_cpu(i, sched_group_span(group)) {

		struct rq *rq = cpu_rq(i);

		unsigned int local;



		sgs->group_load += cpu_load(rq);

		sgs->group_load += cpu_load_without(rq, p);

		sgs->group_util += cpu_util_without(i, p);

		sgs->sum_h_nr_running += rq->cfs.h_nr_running;

		local = task_running_on_cpu(i, p);

		sgs->sum_h_nr_running += rq->cfs.h_nr_running - local;



		nr_running = rq->nr_running;

		nr_running = rq->nr_running - local;

		sgs->sum_nr_running += nr_running;



		/*

		 * No need to call idle_cpu() if nr_running is not 0

		 * No need to call idle_cpu_without() if nr_running is not 0

		 */

		if (!nr_running && idle_cpu(i))

		if (!nr_running && idle_cpu_without(i, p))

			sgs->idle_cpus++;





	}



	/* Check if task fits in the group */

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