From 23fb06d9602b0fcfa1a16357a5d7a7f7076a39e2 Mon Sep 17 00:00:00 2001 From: Valentin Schneider Date: Wed, 7 Apr 2021 23:06:27 +0100 Subject: [PATCH] sched/fair: Clean up active balance nr_balance_failed trickery 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 Signed-off-by: Peter Zijlstra (Intel) Reviewed-by: Dietmar Eggemann Reviewed-by: Vincent Guittot Link: https://lkml.kernel.org/r/20210407220628.3798191-3-valentin.schneider@arm.com --- kernel/sched/fair.c | 31 +++++++++++++++---------------- 1 file changed, 15 insertions(+), 16 deletions(-) diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 1ad929b..356637a 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -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 @@ more_balance: 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); -- 2.7.4