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

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 <[email protected]>
> ---
>   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?

thanks,
Chenyu