[RFC PATCH v0 1/1] powerpc/percpu: Use 2MB atom_size in percpu allocator on radix

[Bharata B Rao]

On Mon, Jul 12, 2021 at 01:00:10PM +1000, Nicholas Piggin wrote:
> Excerpts from Bharata B Rao's message of July 8, 2021 3:29 pm:
> > The atom_size used by percpu allocator on powerpc is currently
> > determined by mmu_linear_psize which is initialized to 4K and
> > mmu_linear_psize is modified only by hash. Till now for radix
> > the atom_size was defaulting to PAGE_SIZE(64K).
> 
> Looks like it was 1MB to me?

Was it hash? Because atom_size will get set to 1MB on hash.
And both on baremetal and KVM radix, I see 64K atom_size.

> 
> > Go for 2MB
> > atom_size on radix if support for 2MB pages exist.
> > 
> > 2MB atom_size on radix will allow using PMD mappings in the
> > vmalloc area if and when support for higher sized vmalloc
> > mappings is enabled for the pecpu allocator. However right now
> 
> That would be nice.
> 
> > this change will result in more number of units to be allocated
> > within one allocation due to increased upa(units per allocation).
> 
> In that case is there any reason to do it until then?

Not strictly. I observed a similar setting on x86 which has
been there for long, so was just checking if it makes sense
here too.

> 
> > 
> > Signed-off-by: Bharata B Rao <bharata at linux.ibm.com>
> > ---
> >  arch/powerpc/kernel/setup_64.c | 34 +++++++++++++++++++++++++---------
> >  1 file changed, 25 insertions(+), 9 deletions(-)
> > 
> > diff --git a/arch/powerpc/kernel/setup_64.c b/arch/powerpc/kernel/setup_64.c
> > index 1ff258f6c76c..45ce2d6e8112 100644
> > --- a/arch/powerpc/kernel/setup_64.c
> > +++ b/arch/powerpc/kernel/setup_64.c
> > @@ -871,6 +871,30 @@ static void __init pcpu_populate_pte(unsigned long addr)
> >  	      __func__, PAGE_SIZE, PAGE_SIZE, PAGE_SIZE);
> >  }
> >  
> > +static size_t pcpu_atom_size(void)
> > +{
> > +	size_t atom_size = PAGE_SIZE;
> > +
> > +	/*
> > +	 * Radix: Use PAGE_SIZE by default or 2M if available.
> > +	 */
> > +	if (radix_enabled()) {
> > +		if (mmu_psize_defs[MMU_PAGE_2M].shift)
> > +			atom_size = 1 << mmu_psize_defs[MMU_PAGE_2M].shift;
> 
> Looks like this changes behaviour for radix.

Yes, it does as it increases the atom_size which results in higher
upa as noted. Did you mean some other behaviour change?

> 
> Also mmu_psize_defs is a pretty horrible interface you only need it in 
> some low level instruction encodings. You already explicitly know it's
> 2MB there, so you can just PMD_SHIFT.

Ok.

> 
> If you want to know whether huge PMD is supported and enabled in vmalloc
> memory, you would have to add some check which also accounts for
> vmap_allow_huge, so that would be another patch.

Yes makes sense if we want to tie the setting of higher atom_size
to actual availability of PMD mappings in vmalloc.

Regards,
Bharata.