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    Subject[tip: sched/core] sched/fair: Wait before decaying max_newidle_lb_cost
    The following commit has been merged into the sched/core branch of tip:

    Commit-ID: e60b56e46b384cee1ad34e6adc164d883049c6c3
    Gitweb: https://git.kernel.org/tip/e60b56e46b384cee1ad34e6adc164d883049c6c3
    Author: Vincent Guittot <vincent.guittot@linaro.org>
    AuthorDate: Tue, 19 Oct 2021 14:35:35 +02:00
    Committer: Peter Zijlstra <peterz@infradead.org>
    CommitterDate: Sun, 31 Oct 2021 11:11:38 +01:00

    sched/fair: Wait before decaying max_newidle_lb_cost

    Decay max_newidle_lb_cost only when it has not been updated for a while
    and ensure to not decay a recently changed value.

    Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
    Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
    Acked-by: Mel Gorman <mgorman@suse.de>
    Link: https://lore.kernel.org/r/20211019123537.17146-4-vincent.guittot@linaro.org
    ---
    include/linux/sched/topology.h | 2 +-
    kernel/sched/fair.c | 36 ++++++++++++++++++++++++---------
    kernel/sched/topology.c | 2 +-
    3 files changed, 29 insertions(+), 11 deletions(-)

    diff --git a/include/linux/sched/topology.h b/include/linux/sched/topology.h
    index 2f9166f..c07bfa2 100644
    --- a/include/linux/sched/topology.h
    +++ b/include/linux/sched/topology.h
    @@ -105,7 +105,7 @@ struct sched_domain {

    /* idle_balance() stats */
    u64 max_newidle_lb_cost;
    - unsigned long next_decay_max_lb_cost;
    + unsigned long last_decay_max_lb_cost;

    u64 avg_scan_cost; /* select_idle_sibling */

    diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
    index c4c3686..e50fd75 100644
    --- a/kernel/sched/fair.c
    +++ b/kernel/sched/fair.c
    @@ -10239,6 +10239,30 @@ void update_max_interval(void)
    max_load_balance_interval = HZ*num_online_cpus()/10;
    }

    +static inline bool update_newidle_cost(struct sched_domain *sd, u64 cost)
    +{
    + if (cost > sd->max_newidle_lb_cost) {
    + /*
    + * Track max cost of a domain to make sure to not delay the
    + * next wakeup on the CPU.
    + */
    + sd->max_newidle_lb_cost = cost;
    + sd->last_decay_max_lb_cost = jiffies;
    + } else if (time_after(jiffies, sd->last_decay_max_lb_cost + HZ)) {
    + /*
    + * Decay the newidle max times by ~1% per second to ensure that
    + * it is not outdated and the current max cost is actually
    + * shorter.
    + */
    + sd->max_newidle_lb_cost = (sd->max_newidle_lb_cost * 253) / 256;
    + sd->last_decay_max_lb_cost = jiffies;
    +
    + return true;
    + }
    +
    + return false;
    +}
    +
    /*
    * It checks each scheduling domain to see if it is due to be balanced,
    * and initiates a balancing operation if so.
    @@ -10262,14 +10286,9 @@ static void rebalance_domains(struct rq *rq, enum cpu_idle_type idle)
    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.
    + * visit to all the domains.
    */
    - 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;
    - }
    + need_decay = update_newidle_cost(sd, 0);
    max_cost += sd->max_newidle_lb_cost;

    /*
    @@ -10911,8 +10930,7 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)

    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, domain_cost);

    curr_cost += domain_cost;
    t0 = t1;
    diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
    index e812467..30169c7 100644
    --- a/kernel/sched/topology.c
    +++ b/kernel/sched/topology.c
    @@ -1568,7 +1568,7 @@ sd_init(struct sched_domain_topology_level *tl,
    .last_balance = jiffies,
    .balance_interval = sd_weight,
    .max_newidle_lb_cost = 0,
    - .next_decay_max_lb_cost = jiffies,
    + .last_decay_max_lb_cost = jiffies,
    .child = child,
    #ifdef CONFIG_SCHED_DEBUG
    .name = tl->name,
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