linux-kernel - Re: sched/debug: CPU hotplug operation suffers in a large cpu systems

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Message-ID: <Y2oozs/YgqqRV5hq@li-05afa54c-330e-11b2-a85c-e3f3aa0db1e9.ibm.com>
Date:   Tue, 8 Nov 2022 15:30:46 +0530
From:   Vishal Chourasia <vishalc@...ux.vnet.ibm.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org, mingo@...hat.com,
        vincent.guittot@...aro.org, vschneid@...hat.com,
        srikar@...ux.vnet.ibm.com, sshegde@...ux.ibm.com,
        linuxppc-dev@...ts.ozlabs.org, ritesh.list@...il.com,
        aneesh.kumar@...ux.ibm.com
Subject: Re: sched/debug: CPU hotplug operation suffers in a large cpu systems


Thanks Greg & Peter for your direction. 

While we pursue the idea of having debugfs based on kernfs, we thought about
having a boot time parameter which would disable creating and updating of the
sched_domain debugfs files and this would also be useful even when the kernfs
solution kicks in, as users who may not care about these debugfs files would
benefit from a faster CPU hotplug operation.

However, these sched_domain debugfs files are created by default.

-- vishal.c

------>8-----------------------------------------------------8<--------------

From f66f66ee05a9f719b58822d13e501d65391dd9d3 Mon Sep 17 00:00:00 2001
From: Vishal Chourasia <vishalc@...ux.vnet.ibm.com>
Date: Tue, 8 Nov 2022 14:21:15 +0530
Subject: [PATCH] Add kernel parameter to disable creation of sched_domain
 files

For large systems, creation of sched_domain debug files takes unusually long
time. In which case, sched_sd_export can be passed as kernel command line
parameter during boot time to prevent kernel from creating sched_domain files.

This commit adds a kernel command line parameter, sched_sd_export, which can be
used to, optionally, disable the creation of sched_domain debug files. 
---
 kernel/sched/debug.c    |  9 ++++++---
 kernel/sched/sched.h    |  1 +
 kernel/sched/topology.c | 11 ++++++++++-
 3 files changed, 17 insertions(+), 4 deletions(-)

diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
index bb3d63bdf4ae..bd307847b76a 100644
--- a/kernel/sched/debug.c
+++ b/kernel/sched/debug.c
@@ -279,6 +279,7 @@ static const struct file_operations sched_dynamic_fops = {
 #endif /* CONFIG_PREEMPT_DYNAMIC */
 
 __read_mostly bool sched_debug_verbose;
+__read_mostly int sched_debug_export = 1;
 
 static const struct seq_operations sched_debug_sops;
 
@@ -321,9 +322,11 @@ static __init int sched_init_debug(void)
 	debugfs_create_u32("migration_cost_ns", 0644, debugfs_sched, &sysctl_sched_migration_cost);
 	debugfs_create_u32("nr_migrate", 0644, debugfs_sched, &sysctl_sched_nr_migrate);
 
-	mutex_lock(&sched_domains_mutex);
-	update_sched_domain_debugfs();
-	mutex_unlock(&sched_domains_mutex);
+	if (likely(sched_debug_export)) {
+		mutex_lock(&sched_domains_mutex);
+		update_sched_domain_debugfs();
+		mutex_unlock(&sched_domains_mutex);
+	}
 #endif
 
 #ifdef CONFIG_NUMA_BALANCING
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index e26688d387ae..a4d06588d876 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -2738,6 +2738,7 @@ extern struct sched_entity *__pick_last_entity(struct cfs_rq *cfs_rq);
 
 #ifdef	CONFIG_SCHED_DEBUG
 extern bool sched_debug_verbose;
+extern int sched_debug_export;
 
 extern void print_cfs_stats(struct seq_file *m, int cpu);
 extern void print_rt_stats(struct seq_file *m, int cpu);
diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index 8739c2a5a54e..7bcdbc2f856d 100644
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -19,6 +19,13 @@ static int __init sched_debug_setup(char *str)
 }
 early_param("sched_verbose", sched_debug_setup);
 
+static int __init sched_debug_disable_export(char *str)
+{
+	sched_debug_export = 0;
+	return 0;
+}
+early_param("sched_sd_export", sched_debug_disable_export);
+
 static inline bool sched_debug(void)
 {
 	return sched_debug_verbose;
@@ -152,6 +159,7 @@ static void sched_domain_debug(struct sched_domain *sd, int cpu)
 #else /* !CONFIG_SCHED_DEBUG */
 
 # define sched_debug_verbose 0
+# define sched_debug_export 1
 # define sched_domain_debug(sd, cpu) do { } while (0)
 static inline bool sched_debug(void)
 {
@@ -2632,7 +2640,8 @@ void partition_sched_domains_locked(int ndoms_new, cpumask_var_t doms_new[],
 	dattr_cur = dattr_new;
 	ndoms_cur = ndoms_new;
 
-	update_sched_domain_debugfs();
+	if (likely(sched_debug_export))
+		update_sched_domain_debugfs();
 }
 
 /*

base-commit: 7e18e42e4b280c85b76967a9106a13ca61c16179
-- 
2.31.1

  

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