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

[Juergen Gross <jgross@xxxxxxxx>]

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.


Juergen

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