Messages in this thread Patch in this message | | | Date | Fri, 15 Oct 2021 19:40:45 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH v2 3/4] sched/fair: Wait before decaying max_newidle_lb_cost |
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On Fri, Oct 15, 2021 at 02:46:53PM +0200, Vincent Guittot wrote: > Decay max_newidle_lb_cost only when it has not been updated for a while > and ensure to not decay a recently changed value.
I was more thinking something long these lines; ofcourse, no idea how well it actually behaves.
Index: linux-2.6/include/linux/sched/topology.h =================================================================== --- linux-2.6.orig/include/linux/sched/topology.h +++ linux-2.6/include/linux/sched/topology.h @@ -98,7 +98,6 @@ struct sched_domain { /* idle_balance() stats */ u64 max_newidle_lb_cost; - unsigned long next_decay_max_lb_cost; u64 avg_scan_cost; /* select_idle_sibling */ Index: linux-2.6/kernel/sched/fair.c =================================================================== --- linux-2.6.orig/kernel/sched/fair.c +++ linux-2.6/kernel/sched/fair.c @@ -10241,6 +10241,17 @@ void update_max_interval(void) } /* + * Asymmetric IIR filter, 1/4th down, 3/4th up. + */ +static void update_newidle_cost(u64 *cost, u64 new) +{ + s64 diff = new - *cost; + if (diff > 0) + diff *= 3; + *cost += diff / 4; +} + +/* * It checks each scheduling domain to see if it is due to be balanced, * and initiates a balancing operation if so. * @@ -10256,33 +10267,18 @@ static void rebalance_domains(struct rq /* Earliest time when we have to do rebalance again */ unsigned long next_balance = jiffies + 60*HZ; int update_next_balance = 0; - int need_serialize, need_decay = 0; - u64 max_cost = 0; + int need_serialize; rcu_read_lock(); for_each_domain(cpu, sd) { - /* - * Decay the newidle max times here because this is a regular - * visit to all the domains. Decay ~1% per second. - */ - if (time_after(jiffies, sd->next_decay_max_lb_cost)) { - sd->max_newidle_lb_cost = - (sd->max_newidle_lb_cost * 253) / 256; - sd->next_decay_max_lb_cost = jiffies + HZ; - need_decay = 1; - } - max_cost += sd->max_newidle_lb_cost; /* * Stop the load balance at this level. There is another * CPU in our sched group which is doing load balancing more * actively. */ - if (!continue_balancing) { - if (need_decay) - continue; + if (!continue_balancing) break; - } interval = get_sd_balance_interval(sd, busy); @@ -10313,14 +10309,7 @@ out: update_next_balance = 1; } } - if (need_decay) { - /* - * Ensure the rq-wide value also decays but keep it at a - * reasonable floor to avoid funnies with rq->avg_idle. - */ - rq->max_idle_balance_cost = - max((u64)sysctl_sched_migration_cost, max_cost); - } + rcu_read_unlock(); /* @@ -10909,8 +10898,7 @@ static int newidle_balance(struct rq *th t1 = sched_clock_cpu(this_cpu); domain_cost = t1 - t0; - if (domain_cost > sd->max_newidle_lb_cost) - sd->max_newidle_lb_cost = domain_cost; + update_newidle_cost(&sd->max_newidle_lb_cost, domain_cost); curr_cost += domain_cost; t0 = t1; @@ -10930,8 +10918,7 @@ static int newidle_balance(struct rq *th raw_spin_rq_lock(this_rq); - if (curr_cost > this_rq->max_idle_balance_cost) - this_rq->max_idle_balance_cost = curr_cost; + update_newidle_cost(&this_rq->max_idle_balance_cost, curr_cost); /* * While browsing the domains, we released the rq lock, a task could
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