linux-kernel - [PATCH RFC] sched/fair: bump sd->max_newidle_lb

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Message-ID: <20250624204942.715206-1-clm@fb.com>
Date: Tue, 24 Jun 2025 13:48:08 -0700
From: Chris Mason <clm@...com>
To: <peterz@...radead.org>, <vincent.guittot@...aro.org>,
        <linux-kernel@...r.kernel.org>
CC: Chris Mason <clm@...com>
Subject: [PATCH RFC] sched/fair: bump sd->max_newidle_lb_cost when newidle balance fails

schbench (https://github.com/masoncl/schbench.git) is showing a
regression from previous production kernels that bisected down to:

sched/fair: Remove sysctl_sched_migration_cost condition (c5b0a7eefc)

The schbench command line was:

schbench -L -m 4 -M auto -t 256 -n 0 -r 0 -s 0

This creates 4 message threads pinned to CPUs 0-3, and 256x4 worker
threads spread across the rest of the CPUs.  Neither the worker threads
or the message threads do any work, they just wake each other up and go
back to sleep as soon as possible.

The end result is the first 4 CPUs are pegged waking up those 1024
workers, and the rest of the CPUs are constantly banging in and out of
idle.  If I take a v6.9 Linus kernel and revert that one commit,
performance goes from 3.4M RPS to 5.4M RPS.

schedstat shows there are ~100x  more new idle balance operations, and
profiling shows the worker threads are spending ~20% of their CPU time
on new idle balance.  schedstats also shows that almost all of these new
idle balance attemps are failing to find busy groups.

The fix used here is to crank up the cost of the balance whenever it
fails to find a busy group or busy queue.

Signed-off-by: Chris Mason <clm@...com>
---
 kernel/sched/fair.c | 52 ++++++++++++++++++++++++---------------------
 1 file changed, 28 insertions(+), 24 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 7a14da5396fb2..0c4701564ce01 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -11744,6 +11744,32 @@ static void update_lb_imbalance_stat(struct lb_env *env, struct sched_domain *sd
 	}
 }
 
+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.   Add a bit to the min so we don't
+		 * have to worry about <= vs <, and to handle the sysctl set at
+		 * zero.
+		 */
+		sd->max_newidle_lb_cost = min(cost, sysctl_sched_migration_cost + 200);
+		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;
+}
+
 /*
  * Check this_cpu to ensure it is balanced within domain. Attempt to move
  * tasks if there is an imbalance.
@@ -11782,12 +11808,14 @@ static int sched_balance_rq(int this_cpu, struct rq *this_rq,
 
 	group = sched_balance_find_src_group(&env);
 	if (!group) {
+		update_newidle_cost(sd, sd->max_newidle_lb_cost + sd->max_newidle_lb_cost / 2);
 		schedstat_inc(sd->lb_nobusyg[idle]);
 		goto out_balanced;
 	}
 
 	busiest = sched_balance_find_src_rq(&env, group);
 	if (!busiest) {
+		update_newidle_cost(sd, sd->max_newidle_lb_cost + sd->max_newidle_lb_cost / 2);
 		schedstat_inc(sd->lb_nobusyq[idle]);
 		goto out_balanced;
 	}
@@ -12168,30 +12196,6 @@ 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.
-- 
2.47.1