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

[Julien Grall <julien@xxxxxxx>]

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