On 12.12.23 11:10, Julien Grall wrote:
Hi Juergen,On 12/12/2023 09:47, Juergen Gross wrote:Looking at arch/Kconfig, it looks like we are limiting NR_CPUS to maximum 4096. So can you outline why we need this?Allow 16 bits per cpu number, which is the limit imposed by spinlock_tickets_t. This will allow up to 65535 cpus, while increasing only the size of recursive spinlocks in debug builds from 8 to 12 bytes.
The limit of 4096 cpus is dictated by the current limit of the spinlock implementation. So asking "why do we need to support more than 4096 cpus in spinlock_t when the current Xen limit is 4096" is kind of the wrong question. The correct question would be: why is Xen limited to 4096 cpus? Answer: because of the limit in spinlock_t. There are (x86) machines out there with more cpus than 4096, and Javi (added to Cc:) was already looking into rising the Xen limit.
Just to be clear is I am not against this change, but alone it seems a little bit odd to increase the size in debug when that limit can never be reached (at least today).
Juergen
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