Re: [PATCH] sched: Skip useless sched_balance_running acquisition if load balance is not due

[Shrikanth Hegde <sshegde@linux.ibm.com>]

On 4/16/25 21:49, Tim Chen wrote:
> On Wed, 2025-04-16 at 14:46 +0530, Shrikanth Hegde wrote:
>>

>>
> You mean doing a should_we_balance() check?  I think we should not
> even consider that if balance time is not due and this balance due check should
> come first.
> 
Hi Tim.

This is the code I was suggesting.

---

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 0c19459c8042..e712934973e7 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -11723,6 +11723,21 @@ static void update_lb_imbalance_stat(struct lb_env *env, struct sched_domain *sd
                 break;
         }
  }
+/*
+ * This flag serializes load-balancing passes over large domains
+ * (above the NODE topology level) - only one load-balancing instance
+ * may run at a time, to reduce overhead on very large systems with
+ * lots of CPUs and large NUMA distances.
+ *
+ * - Note that load-balancing passes triggered while another one
+ *   is executing are skipped and not re-tried.
+ *
+ * - Also note that this does not serialize rebalance_domains()
+ *   execution, as non-SD_SERIALIZE domains will still be
+ *   load-balanced in parallel.
+ */
+static atomic_t sched_balance_running = ATOMIC_INIT(0);
+
  
  /*
   * Check this_cpu to ensure it is balanced within domain. Attempt to move
@@ -11738,6 +11753,7 @@ static int sched_balance_rq(int this_cpu, struct rq *this_rq,
         struct rq *busiest;
         struct rq_flags rf;
         struct cpumask *cpus = this_cpu_cpumask_var_ptr(load_balance_mask);
+       int need_serialize = 0;
         struct lb_env env = {
                 .sd             = sd,
                 .dst_cpu        = this_cpu,
@@ -11760,6 +11776,12 @@ static int sched_balance_rq(int this_cpu, struct rq *this_rq,
                 goto out_balanced;
         }
  
+       need_serialize = (idle!=CPU_NEWLY_IDLE) && sd->flags & SD_SERIALIZE;
+       if (need_serialize) {
+               if (atomic_cmpxchg_acquire(&sched_balance_running, 0, 1))
+                       return ld_moved;
+       }
+
         group = sched_balance_find_src_group(&env);
         if (!group) {
                 schedstat_inc(sd->lb_nobusyg[idle]);
@@ -11884,6 +11906,8 @@ static int sched_balance_rq(int this_cpu, struct rq *this_rq,
                         if (!cpumask_subset(cpus, env.dst_grpmask)) {
                                 env.loop = 0;
                                 env.loop_break = SCHED_NR_MIGRATE_BREAK;
+                               if (need_serialize)
+                                       atomic_set_release(&sched_balance_running, 0);
                                 goto redo;
                         }
                         goto out_all_pinned;
@@ -12000,6 +12024,9 @@ static int sched_balance_rq(int this_cpu, struct rq *this_rq,
             sd->balance_interval < sd->max_interval)
                 sd->balance_interval *= 2;
  out:
+       if (need_serialize)
+               atomic_set_release(&sched_balance_running, 0);
+
         return ld_moved;
  }
  
@@ -12124,21 +12151,6 @@ static int active_load_balance_cpu_stop(void *data)
         return 0;
  }
  
-/*
- * This flag serializes load-balancing passes over large domains
- * (above the NODE topology level) - only one load-balancing instance
- * may run at a time, to reduce overhead on very large systems with
- * lots of CPUs and large NUMA distances.
- *
- * - Note that load-balancing passes triggered while another one
- *   is executing are skipped and not re-tried.
- *
- * - Also note that this does not serialize rebalance_domains()
- *   execution, as non-SD_SERIALIZE domains will still be
- *   load-balanced in parallel.
- */
-static atomic_t sched_balance_running = ATOMIC_INIT(0);
-
  /*
   * Scale the max sched_balance_rq interval with the number of CPUs in the system.
   * This trades load-balance latency on larger machines for less cross talk.
@@ -12188,7 +12200,7 @@ static void sched_balance_domains(struct rq *rq, enum cpu_idle_type idle)
         /* 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;
+       int need_decay = 0;
         u64 max_cost = 0;
  
         rcu_read_lock();
@@ -12213,12 +12225,6 @@ static void sched_balance_domains(struct rq *rq, enum cpu_idle_type idle)
  
                 interval = get_sd_balance_interval(sd, busy);
  
-               need_serialize = sd->flags & SD_SERIALIZE;
-               if (need_serialize) {
-                       if (atomic_cmpxchg_acquire(&sched_balance_running, 0, 1))
-                               goto out;
-               }
-
                 if (time_after_eq(jiffies, sd->last_balance + interval)) {
                         if (sched_balance_rq(cpu, rq, sd, idle, &continue_balancing)) {
                                 /*
@@ -12232,9 +12238,7 @@ static void sched_balance_domains(struct rq *rq, enum cpu_idle_type idle)
                         sd->last_balance = jiffies;
                         interval = get_sd_balance_interval(sd, busy);
                 }
-               if (need_serialize)
-                       atomic_set_release(&sched_balance_running, 0);
-out:
+
                 if (time_after(next_balance, sd->last_balance + interval)) {
                         next_balance = sd->last_balance + interval;
                         update_next_balance = 1;