maxcpus behavior in arm64

[catalin.marinas@arm.com (Catalin Marinas)]

On Wed, May 14, 2014 at 01:10:17AM +0100, Rohit Vaswani wrote:
> On 5/13/2014 2:02 PM, Catalin Marinas wrote:
> > On Tue, May 13, 2014 at 07:56:01PM +0100, Rohit Vaswani wrote:
> >> I notice that the maxcpus behavior is different in arm64 than from how
> >> arm uses it.
> >> in arm64/kernel/smp.c - in smp_prepare_cpus, maxcpus is used to limit
> >> the cpu_present_mask.
> >> However in arm/kernel/smp.c - maxcpus is not used as a decision maker to
> >> set the cpu_preset_mask.
> >>
> >> Is this behavior expected and intentionally different in arm and arm64 ?
> >> This also means that in arm64 (unlike arm)- maxcpus cannot be used to
> >> boot a subset of total cpus with the
> >> option of getting the secondary cores online at a later point from
> >> userspace using hotplug.
> >> It seems like maxcpus is being treated like nr_cpus in arm64 ?
> > I don't think there is any particular reason, only that the code has
> > been derived from arm long time ago and it probably inherited the
> > original behaviour. In the meantime, arm got commit 7fa22bd5460 (ARM:
> > 6993/1: platsmp: Allow secondary cpu hotplug with maxcpus=1).
> >
> > I'm happy to change the behaviour for arm64. Basically we still call
> > cpu_prepare() for max_cpus but we initialise the present mask with
> > init_cpu_present(cpu_possible_mask) as we don't have physical hotplug
> > for the time being.
> 
> Thanks. Initializing the present mask with possible mask is good.
> But, how would one call cpu_prepare on the other CPUS then ?
> Currently cpu_prepare is called only from smp_prepare_cpus. I was going 
> to suggest calling cpu_prepare for each possible CPU.
> We could have the for_each_possible_cpu loop in smp_prepare_cpus not 
> depend on max_cpus and call cpu_prepare for the possible cpus.

For PSCI this would be fine since cpu_prepare() does not bring the CPU
into the kernel. With spin-table, cpu_prepare brings the CPU up to the
holding_pen loop. But I don't see a reason why we couldn't do all steps
in smp_spin_table_cpu_boot() and simply ignore prepare (and we could get
rid of cpu_prepare altogether).

> I didn't really understand the part of physical hotplug - we have 
> config_hotplug enabled

What I meant is that possible != present in case of physical hotplug.

-- 
Catalin