Re: [PATCH 2/5] sched/topology: Wrappers for sched_domains_mutex

[Waiman Long <llong@xxxxxxxxxx>]

On 3/4/25 3:40 AM, Juri Lelli wrote:
> Create wrappers for sched_domains_mutex so that it can transparently be
> used on both CONFIG_SMP and !CONFIG_SMP, as some function will need to
> do.
>
> Reported-by: Jon Hunter <jonathanh@xxxxxxxxxx>
> Fixes: 53916d5fd3c0 ("sched/deadline: Check bandwidth overflow earlier for hotplug")
> Signed-off-by: Juri Lelli <juri.lelli@xxxxxxxxxx>
> ---
>   include/linux/sched.h   |  2 ++
>   kernel/cgroup/cpuset.c  |  4 ++--
>   kernel/sched/core.c     |  4 ++--
>   kernel/sched/debug.c    |  8 ++++----
>   kernel/sched/topology.c | 17 +++++++++++++++--
>   5 files changed, 25 insertions(+), 10 deletions(-)
>
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 9632e3318e0d..d5f8c161d852 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -383,6 +383,8 @@ enum uclamp_id {
>   extern struct root_domain def_root_domain;
>   extern struct mutex sched_domains_mutex;
>   #endif
> +extern void sched_domains_mutex_lock(void);
> +extern void sched_domains_mutex_unlock(void);
>   

If all access to sched_domains_mutex is through the wrappers, we may not 
need to expose sched_domains_mutex at all. Also it is more efficient for 
the non-SMP case to put the wrappers inside the CONFIG_SMP block and 
define the empty inline functions in the else part.


>   struct sched_param {
>   	int sched_priority;
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index 0f910c828973..f87526edb2a4 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -994,10 +994,10 @@ static void
>   partition_and_rebuild_sched_domains(int ndoms_new, cpumask_var_t doms_new[],
>   				    struct sched_domain_attr *dattr_new)
>   {
> -	mutex_lock(&sched_domains_mutex);
> +	sched_domains_mutex_lock();
>   	partition_sched_domains_locked(ndoms_new, doms_new, dattr_new);
>   	dl_rebuild_rd_accounting();
> -	mutex_unlock(&sched_domains_mutex);
> +	sched_domains_mutex_unlock();
>   }
>   
>   /*
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 9aecd914ac69..7b14500d731b 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -8424,9 +8424,9 @@ void __init sched_init_smp(void)
>   	 * CPU masks are stable and all blatant races in the below code cannot
>   	 * happen.
>   	 */
> -	mutex_lock(&sched_domains_mutex);
> +	sched_domains_mutex_lock();
>   	sched_init_domains(cpu_active_mask);
> -	mutex_unlock(&sched_domains_mutex);
> +	sched_domains_mutex_unlock();
>   
>   	/* Move init over to a non-isolated CPU */
>   	if (set_cpus_allowed_ptr(current, housekeeping_cpumask(HK_TYPE_DOMAIN)) < 0)
> diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
> index ef047add7f9e..a0893a483d35 100644
> --- a/kernel/sched/debug.c
> +++ b/kernel/sched/debug.c
> @@ -292,7 +292,7 @@ static ssize_t sched_verbose_write(struct file *filp, const char __user *ubuf,
>   	bool orig;
>   
>   	cpus_read_lock();
> -	mutex_lock(&sched_domains_mutex);
> +	sched_domains_mutex_lock();
>   
>   	orig = sched_debug_verbose;
>   	result = debugfs_write_file_bool(filp, ubuf, cnt, ppos);
> @@ -304,7 +304,7 @@ static ssize_t sched_verbose_write(struct file *filp, const char __user *ubuf,
>   		sd_dentry = NULL;
>   	}
>   
> -	mutex_unlock(&sched_domains_mutex);
> +	sched_domains_mutex_unlock();
>   	cpus_read_unlock();
>   
>   	return result;
> @@ -515,9 +515,9 @@ 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);
> +	sched_domains_mutex_lock();
>   	update_sched_domain_debugfs();
> -	mutex_unlock(&sched_domains_mutex);
> +	sched_domains_mutex_unlock();
>   #endif
>   
>   #ifdef CONFIG_NUMA_BALANCING
> diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> index c49aea8c1025..e2b879ec9458 100644
> --- a/kernel/sched/topology.c
> +++ b/kernel/sched/topology.c
> @@ -6,6 +6,19 @@
>   #include <linux/bsearch.h>
>   
>   DEFINE_MUTEX(sched_domains_mutex);
> +#ifdef CONFIG_SMP
> +void sched_domains_mutex_lock(void)
> +{
> +	mutex_lock(&sched_domains_mutex);
> +}
> +void sched_domains_mutex_unlock(void)
> +{
> +	mutex_unlock(&sched_domains_mutex);
> +}
> +#else
> +void sched_domains_mutex_lock(void) { }
> +void sched_domains_mutex_unlock(void) { }
> +#endif
>   
>   /* Protected by sched_domains_mutex: */
>   static cpumask_var_t sched_domains_tmpmask;
> @@ -2791,7 +2804,7 @@ void partition_sched_domains_locked(int ndoms_new, cpumask_var_t doms_new[],
>   void partition_sched_domains(int ndoms_new, cpumask_var_t doms_new[],
>   			     struct sched_domain_attr *dattr_new)
>   {
> -	mutex_lock(&sched_domains_mutex);
> +	sched_domains_mutex_lock();
>   	partition_sched_domains_locked(ndoms_new, doms_new, dattr_new);
> -	mutex_unlock(&sched_domains_mutex);
> +	sched_domains_mutex_unlock();
>   }

There are two "lockdep_assert_held(&sched_domains_mutex);" statements in 
topology.c file and one in cpuset.c. That can be problematic in the 
non-SMP case. Maybe another wrapper to do the assert?

Cheers,
Longman