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Re: [PATCH v4 12/12] xen/spinlock: support higher number of cpus



Hi,

On 12/12/2023 12:11, Juergen Gross wrote:
On 12.12.23 12:40, Julien Grall wrote:
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).

Okay, are you fine with the following addition:

   The current Xen limit of 4095 cpus is imposed by SPINLOCK_CPU_BITS
   being 12. There are machines available with more cpus than the current
   Xen limit, so it makes sense to have the possibility to use more cpus.

Yes. That makes clearer.

Cheers,

--
Julien Grall



 


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