Re: [PATCH v3] ARM64: Dynamically allocate cpumasks and increase supported CPUs to 512

[Mark Rutland]

On Fri, Mar 08, 2024 at 09:08:59AM -0800, Christoph Lameter (Ampere) wrote:
> On Fri, 8 Mar 2024, Marek Szyprowski wrote:
> 
> > > > 
> > > > It looks that cpufreq-dt and/or opp drivers needs some adjustments
> > > > related with this change.
> > > That's strange. Is this with defconfig? I wonder whether NR_CPUS being
> > > larger caused the issue with this specific code. Otherwise
> > > CPUMASK_OFFSTACK may not work that well on arm64.
> 
> cpumask handling must use the accessor functions provided in
> include/linux/cpumask.h for declaring and accessing cpumasks. It is likely
> related to the driver opencoding one of the accessors.

I took a look at both the OPP code and the cpufreq-dt code and it looks like
those are doign the right thing w.r.t. cpumask manipulation (i.e. they only use
the cpumask accessors, and use the cpumask_var_*() functions to dynamically
allocate/free cpumasks). Maybe I've missed something, but superficially those
look right.

Marek, can you try reverting this commit and trying defconfig + NR_CPUS=512?
That'll have CPUMASK_OFFSTACK=n, and:

* If that blows up, we know the problem is independent of CPUMASK_OFFSTACK, and
  has something to do with large cpumasks (either a driver bug, or elsewhere).

* If that doesn't blow up, it suggests the problem is related to
  CPUMASK_OFFSTACK rather than with large cpumasks specifically.

Either way, we probably need to revert this patch for now, as this won't have
enough time to soak in linux-next in time for v6.9.

Mark.