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Message-ID: <82baaf6c-f3d9-4c3e-be69-24389eadb18c@linux.ibm.com>
Date: Thu, 17 Apr 2025 14:49:23 +0530
From: Shrikanth Hegde <sshegde@...ux.ibm.com>
To: Tim Chen <tim.c.chen@...ux.intel.com>
Cc: Vincent Guittot <vincent.guittot@...aro.org>,
Doug Nelson <doug.nelson@...el.com>,
Mohini Narkhede <mohini.narkhede@...el.com>,
linux-kernel@...r.kernel.org, Peter Zijlstra <peterz@...radead.org>,
Ingo Molnar <mingo@...nel.org>, "Chen, Yu C" <yu.c.chen@...el.com>
Subject: Re: [PATCH] sched: Skip useless sched_balance_running acquisition if
load balance is not due
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;
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