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

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

On 8/19/26 11:24 PM, Shrikanth Hegde wrote:
> 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.
> 

Ignore my comments. I clearly need to get rest.

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

Sorry for the noise.
Looks good to me.

Reviewed-by: Shrikanth Hegde <sshegde@linux.ibm.com>