sched/fair: Remove update of blocked load from newidle_balance

newidle_balance runs with both preempt and irq disabled which prevent
local irq to run during this period. The duration for updating the
blocked load of CPUs varies according to the number of CPU cgroups
with non-decayed load and extends this critical period to an uncontrolled
level.

Remove the update from newidle_balance and trigger a normal ILB that
will take care of the update instead.

This reduces the IRQ latency from O(nr_cgroups * nr_nohz_cpus) to
O(nr_cgroups).

Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>
Link: https://lkml.kernel.org/r/20210224133007.28644-2-vincent.guittot@linaro.org

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 794c2cb945f8b4dfb96b52e992bf0d6e8a6ea9c9..806e16fe35bad743cc4a8c10fcb97b444cf01fa3 100644 (file)
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7392,8 +7392,6 @@ enum migration_type {
 #define LBF_NEED_BREAK 0x02
 #define LBF_DST_PINNED  0x04
 #define LBF_SOME_PINNED        0x08
-#define LBF_NOHZ_STATS 0x10
-#define LBF_NOHZ_AGAIN 0x20
 
 struct lb_env {
        struct sched_domain     *sd;
@@ -8397,9 +8395,6 @@ static inline void update_sg_lb_stats(struct lb_env *env,
        for_each_cpu_and(i, sched_group_span(group), env->cpus) {
                struct rq *rq = cpu_rq(i);
 
-               if ((env->flags & LBF_NOHZ_STATS) && update_nohz_stats(rq, false))
-                       env->flags |= LBF_NOHZ_AGAIN;
-
                sgs->group_load += cpu_load(rq);
                sgs->group_util += cpu_util(i);
                sgs->group_runnable += cpu_runnable(rq);
@@ -8940,11 +8935,6 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd
        struct sg_lb_stats tmp_sgs;
        int sg_status = 0;
 
-#ifdef CONFIG_NO_HZ_COMMON
-       if (env->idle == CPU_NEWLY_IDLE && READ_ONCE(nohz.has_blocked))
-               env->flags |= LBF_NOHZ_STATS;
-#endif
-
        do {
                struct sg_lb_stats *sgs = &tmp_sgs;
                int local_group;
@@ -8981,14 +8971,6 @@ next_group:
        /* Tag domain that child domain prefers tasks go to siblings first */
        sds->prefer_sibling = child && child->flags & SD_PREFER_SIBLING;
 
-#ifdef CONFIG_NO_HZ_COMMON
-       if ((env->flags & LBF_NOHZ_AGAIN) &&
-           cpumask_subset(nohz.idle_cpus_mask, sched_domain_span(env->sd))) {
-
-               WRITE_ONCE(nohz.next_blocked,
-                          jiffies + msecs_to_jiffies(LOAD_AVG_PERIOD));
-       }
-#endif
 
        if (env->sd->flags & SD_NUMA)
                env->fbq_type = fbq_classify_group(&sds->busiest_stat);
@@ -10517,16 +10499,11 @@ static void nohz_newidle_balance(struct rq *this_rq)
            time_before(jiffies, READ_ONCE(nohz.next_blocked)))
                return;
 
-       raw_spin_unlock(&this_rq->lock);
        /*
-        * This CPU is going to be idle and blocked load of idle CPUs
-        * need to be updated. Run the ilb locally as it is a good
-        * candidate for ilb instead of waking up another idle CPU.
-        * Kick an normal ilb if we failed to do the update.
+        * Blocked load of idle CPUs need to be updated.
+        * Kick an ILB to update statistics.
         */
-       if (!_nohz_idle_balance(this_rq, NOHZ_STATS_KICK, CPU_NEWLY_IDLE))
-               kick_ilb(NOHZ_STATS_KICK);
-       raw_spin_lock(&this_rq->lock);
+       kick_ilb(NOHZ_STATS_KICK);
 }
 
 #else /* !CONFIG_NO_HZ_COMMON */
@@ -10587,8 +10564,6 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
                        update_next_balance(sd, &next_balance);
                rcu_read_unlock();
 
-               nohz_newidle_balance(this_rq);
-
                goto out;
        }
 
@@ -10654,6 +10629,8 @@ out:
 
        if (pulled_task)
                this_rq->idle_stamp = 0;
+       else
+               nohz_newidle_balance(this_rq);
 
        rq_repin_lock(this_rq, rf);