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sched/fair: Clean up active balance nr_balance_failed trickery

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When triggering an active load balance, sd->nr_balance_failed is set to
such a value that any further can_migrate_task() using said sd will ignore
the output of task_hot().

This behaviour makes sense, as active load balance intentionally preempts a
rq's running task to migrate it right away, but this asynchronous write is
a bit shoddy, as the stopper thread might run active_load_balance_cpu_stop
before the sd->nr_balance_failed write either becomes visible to the
stopper's CPU or even happens on the CPU that appended the stopper work.

Add a struct lb_env flag to denote active balancing, and use it in
can_migrate_task(). Remove the sd->nr_balance_failed write that served the
same purpose. Cleanup the LBF_DST_PINNED active balance special case.

Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
Link: https://lkml.kernel.org/r/20210407220628.3798191-3-valentin.schneider@arm.com
This commit is contained in:
Valentin Schneider 2021-04-07 23:06:27 +01:00 committed by Peter Zijlstra
parent 9bcb959d05
commit 23fb06d960
1 changed files with 15 additions and 16 deletions
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31
kernel/sched/fair.c
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@ -7453,6 +7453,7 @@ enum migration_type {
#define LBF_NEED_BREAK 0x02
#define LBF_DST_PINNED 0x04
#define LBF_SOME_PINNED 0x08
#define LBF_ACTIVE_LB 0x10
struct lb_env {
struct sched_domain *sd;
@ -7614,10 +7615,13 @@ int can_migrate_task(struct task_struct *p, struct lb_env *env)
* our sched_group. We may want to revisit it if we couldn't
* meet load balance goals by pulling other tasks on src_cpu.
*
* Avoid computing new_dst_cpu for NEWLY_IDLE or if we have
* already computed one in current iteration.
* Avoid computing new_dst_cpu
* - for NEWLY_IDLE
* - if we have already computed one in current iteration
* - if it's an active balance
*/
if (env->idle == CPU_NEWLY_IDLE || (env->flags & LBF_DST_PINNED))
if (env->idle == CPU_NEWLY_IDLE ||
env->flags & (LBF_DST_PINNED | LBF_ACTIVE_LB))
return 0;
/* Prevent to re-select dst_cpu via env's CPUs: */
@ -7642,10 +7646,14 @@ int can_migrate_task(struct task_struct *p, struct lb_env *env)
/*
* Aggressive migration if:
* 1) destination numa is preferred
* 2) task is cache cold, or
* 3) too many balance attempts have failed.
* 1) active balance
* 2) destination numa is preferred
* 3) task is cache cold, or
* 4) too many balance attempts have failed.
*/
if (env->flags & LBF_ACTIVE_LB)
return 1;
tsk_cache_hot = migrate_degrades_locality(p, env);
if (tsk_cache_hot == -1)
tsk_cache_hot = task_hot(p, env);
@ -9836,9 +9844,6 @@ static int load_balance(int this_cpu, struct rq *this_rq,
active_load_balance_cpu_stop, busiest,
&busiest->active_balance_work);
}
/* We've kicked active balancing, force task migration. */
sd->nr_balance_failed = sd->cache_nice_tries+1;
}
} else {
sd->nr_balance_failed = 0;
@ -9988,13 +9993,7 @@ static int active_load_balance_cpu_stop(void *data)
.src_cpu = busiest_rq->cpu,
.src_rq = busiest_rq,
.idle = CPU_IDLE,
/*
* can_migrate_task() doesn't need to compute new_dst_cpu
* for active balancing. Since we have CPU_IDLE, but no
* @dst_grpmask we need to make that test go away with lying
* about DST_PINNED.
*/
.flags = LBF_DST_PINNED,
.flags = LBF_ACTIVE_LB,
};
schedstat_inc(sd->alb_count);