Re: [PATCH] sched/topology: Add a cpus_read_lock to partition_sched_domains()

[Tim Chen]

On Wed, 2026-08-12 at 20:54 +0800, Chen, Yu C wrote:
> Hi Sebastian,
> 
> On 8/12/2026 5:58 PM, Sebastian Andrzej Siewior wrote:
> > A read from /proc/sys/kernel/sched_rt_runtime_us leads to backtrace due
> > to missing cpu_hotplug_lock. The callchain is sched_rt_handler() ->
> > partition_sched_domains() -> sched_cache_set() ->
> > static_key_enable_cpuslocked(&sched_cache_present).
> > 
> > sched_cache_set() itself is also invoked from sched_init_domains() which
> > is early during the boot, holding just the sched_domains_mutex_lock().
> > Here is no warning because it happens before user space is running (and
> > hotplug operations are not possible).
> > 
> > There is also sched_cache_active_set() which acquires the hotplug lock
> > via before invoking any of the _cpuslocked() functions.
> > 
> > Acquire CPU hotplug lock before in partition_sched_domains(), before
> > sched_domains_mutex before the _cpuslocked() functions are invoked.
> > 
> > Fixes: a7660ce1590fc ("sched/cache: Fix has_multi_llcs iff at least one partition has multiple LLCs")
> > Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
> > ---
> >   kernel/sched/topology.c | 1 +
> >   1 file changed, 1 insertion(+)
> > 
> > diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> > index 622e2e01974c4..ef037d6664d98 100644
> > --- a/kernel/sched/topology.c
> > +++ b/kernel/sched/topology.c
> > @@ -3498,6 +3498,7 @@ static 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)
> >   {
> > +	guard(cpus_read_lock)();
> >   	sched_domains_mutex_lock();
> >   	partition_sched_domains_locked(ndoms_new, doms_new, dattr_new);
> >   	sched_domains_mutex_unlock();
> 
> Thanks for taking a look at this issue. I found that there is a comment 
> around
> partition_sched_domains() says: "Call with hotplug lock held"
> Not sure if the caller of partition_sched_domains() rather than 
> partition_sched_domains()
> should grab the cpuhotplug lock? I guess the issue is triggered when
> CONFIG_CPUSETS=n, in this case rebuild_sched_domains() will not grab
> cpuhotplug lock, should we add guard(cpus_read_lock) in 
> rebuild_sched_domains()
> instead?

When CONFIG_CPUSETS=n, rebuild_sched_domains() is stubbed to

static inline void rebuild_sched_domains(void)
{
        partition_sched_domains(1, NULL, NULL);
}

without cpus_read_lock.  Are you suggesting adding the lock
here?  We do acquire the lock for CONFIG_CPUSETS=y.

If we do follow this convention, cpuset_reset_sched_domains()
and cpuset_reset_sched_domains() also stubbed to
partition_sched_domains(1, NULL, NULL);
Probably should add cpus_read_lock there too for consistency.

Tim

> 
> thanks,
> Chenyu
>