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

[Mete Durlu <meted@linux.ibm.com>]

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@amd.com>
> ---
> 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.