Re: [PATCH v2] sched/topology: Add a cpus_read_lock to rebuild_sched_domains()

[Shrikanth Hegde <sshegde@linux.ibm.com>]

Hi Sebastian.

On 8/13/26 1:08 PM, Sebastian Andrzej Siewior wrote:
> A read from /proc/sys/kernel/sched_rt_runtime_us leads to backtrace due
> to missing cpu_hotplug_lock with CONFIG_CPUSETS=n. 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
> before invoking any of the _cpuslocked() functions.
> 
> This is only a problem with CONFIG_CPUSETS=n because in the =y case the
> other implementation of rebuild_sched_domains acquires the CPU-hotplug
> lock.
> 
> Acquire CPU hotplug lock before in rebuild_sched_domains(), before
> partition_sched_domains() is invoked for the CONFIG_CPUSETS=n case.
> 
> Fixes: a7660ce1590fc ("sched/cache: Fix has_multi_llcs iff at least one partition has multiple LLCs")
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> ---
> v1…v2: https://lore.kernel.org/all/20260812095800.gl06ANul@linutronix.de/
>    - Move the lock from partition_sched_domains() to
>      rebuild_sched_domains() in the CONFIG_CPUSETS=n since this is the
>      only affected case. Noticed by Yu C Chen and Tim Chen.
> 
>   include/linux/cpuset.h | 1 +
>   1 file changed, 1 insertion(+)
> 
> diff --git a/include/linux/cpuset.h b/include/linux/cpuset.h
> index 65d76a38974ba..bf3999daa080a 100644
> --- a/include/linux/cpuset.h
> +++ b/include/linux/cpuset.h
> @@ -273,6 +273,7 @@ static inline void dl_rebuild_rd_accounting(void)
>   
>   static inline void rebuild_sched_domains(void)
>   {
> +	guard(cpus_read_lock)();
>   	partition_sched_domains(1, NULL, NULL);
>   }
>   

Now, both rebuild_sched_domains have the same code. I guess it is worth
moving it out of CONFIG_CPUSETS.

kernel/cgroup/cpuset.c:
void rebuild_sched_domains(void)
{
         cpus_read_lock();
         rebuild_sched_domains_cpuslocked();
         cpus_read_unlock();
}