Re: [RFC PATCH v3 07/13] s390/topology: Initialize sbm topology during topology_init_early()

[K Prateek Nayak]

Hello Mete,

On 10/7/2026 3:49 PM, Mete Durlu wrote:
> On 01/10/2026 21:28, K Prateek Nayak wrote:
>> When the topology mode is TOPOLOGY_MODE_HW, topology_init_early()
>> already parses the set of socket present on the system.
>>
>> Use the socket_info to configure the sparsebitmask (sbm) properties for
>> the system - namely the number of sockets and maximum threads in a
>> socket instance.
>>
>> arch_sbm_cpu_instance_id() traverses all socket instances to find
>> a matching CPU which is not optimal but instance ID is only checked
>> when the CPU is coming online. Since this is a rare event, the
>> inefficiency is tolerable.
>>
>> Signed-off-by: K Prateek Nayak <kprateek.nayak@xxxxxxx>
>> ---
>> Build tested with LKP's s390 randconfig with QEMU cmdline:
>>
>>    qemu-system-s390x \
>>    -cpu max \
>>    -smp 2 \
>>    -m 2048 \
>>    -kernel ./arch/s390/boot/vmlinux  \
>>    -append "console=ttyAMA0 earlycon ignore_loglevel log_buf_len=10M print_fatal_signals=1 LOGLEVEL=8 sched_debug" \
>>    -nographic
>>
>> Note: Since ctop needs KVM acceleration to emulate TOPOLOGY_MODE_HW, the
>> actual spasemask setting is only build tested at the moment.
>>
>> XXX: Any way around this using cross-compile and QEMU?
>> ---
>>   arch/s390/kernel/topology.c | 52 +++++++++++++++++++++++++++++++++++++
>>   1 file changed, 52 insertions(+)
>>
>> diff --git a/arch/s390/kernel/topology.c b/arch/s390/kernel/topology.c
>> index 1377c6f3f670..80dc51f5ce6f 100644
>> --- a/arch/s390/kernel/topology.c
>> +++ b/arch/s390/kernel/topology.c
>> @@ -20,6 +20,7 @@
>>   #include <linux/init.h>
>>   #include <linux/slab.h>
>>   #include <linux/cpu.h>
>> +#include <linux/sbm.h>
>>   #include <linux/smp.h>
>>   #include <linux/mm.h>
>>   #include <linux/nodemask.h>
>> @@ -561,6 +562,55 @@ static int __init detect_polarization(union topology_entry *tle)
>>       return tl_core->pp != POLARIZATION_HRZ;
>>   }
> 
> topology_init_early() is called once during early boot and never again.
> Considering that s390's topology can change during runtime this
> implementation won't work as sbm won't be able to adapt.
> On s390 max_threads_per_instance can change during runtime.
> 
> Like sashiko pointed out, this approach also does not consider
> any newly added cpus.
> 
> IMHO, there is a slight redesign required so that smb_set_topology()
> and smb_init() can be called each time a topology rebuild is necessary.

So based on my limited understanding, doesn't store_topology() populate
the mask for the entire system on s390? Are you saying that can change
or that stsi() may return a partial topology for only the online CPUs?

Since s390 default enables hotplug, I couldn't easily grep the hotplug
bits so please forgive if I missed something.

-- 
Thanks and Regards,
Prateek