Hi Juergen,
On 12/12/2023 11:09, Juergen Gross wrote:
On 12.12.23 11:10, Julien Grall wrote:
Hi Juergen,
On 12/12/2023 09:47, Juergen Gross wrote:
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.
Looking at arch/Kconfig, it looks like we are limiting NR_CPUS to
maximum 4096. So can you outline why we need this?
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.
I thought there was also some lock contention issue in Xen. Hence why I
asked the question because the commit message doesn't really give any
clue why we are raising the limit... (This is a hint that it probably
needs to be expanded a bit).
Cheers,
--
Julien Grall
