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

[Juri Lelli]

Hi Waiman,

Thanks for the review!

On 04/03/25 10:05, Waiman Long wrote:
> 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?

Yes, makes sense. Will modify as you suggest here and above.

Best,
Juri