Subject: Re: [RFCv2 5/9] arch/powerpc: Split hash page table sizing heuristic into a helper

From: David Gibson <[email-masked]>
Date: Tue, 2 Feb 2016 12:04:47 +1100

On Mon, Feb 01, 2016 at 12:34:32PM +0530, Anshuman Khandual wrote:
> On 01/29/2016 10:53 AM, David Gibson wrote:
> > htab_get_table_size() either retrieve the size of the hash page table (HPT)
> > from the device tree - if the HPT size is determined by firmware - or
> > uses a heuristic to determine a good size based on RAM size if the kernel
> > is responsible for allocating the HPT.
> > 
> > To support a PAPR extension allowing resizing of the HPT, we're going to
> > want the memory size -> HPT size logic elsewhere, so split it out into a
> > helper function.
> > 
> > Signed-off-by: David Gibson <[email-masked]>
> > ---
> >  arch/powerpc/include/asm/mmu-hash64.h |  3 +++
> >  arch/powerpc/mm/hash_utils_64.c       | 30 +++++++++++++++++-------------
> >  2 files changed, 20 insertions(+), 13 deletions(-)
> > 
> > diff --git a/arch/powerpc/include/asm/mmu-hash64.h b/arch/powerpc/include/asm/mmu-hash64.h
> > index 7352d3f..cf070fd 100644
> > --- a/arch/powerpc/include/asm/mmu-hash64.h
> > +++ b/arch/powerpc/include/asm/mmu-hash64.h
> > @@ -607,6 +607,9 @@ static inline unsigned long get_kernel_vsid(unsigned long ea, int ssize)
> >  	context = (MAX_USER_CONTEXT) + ((ea >> 60) - 0xc) + 1;
> >  	return get_vsid(context, ea, ssize);
> >  }
> > +
> > +unsigned htab_shift_for_mem_size(unsigned long mem_size);
> > +
> >  #endif /* __ASSEMBLY__ */
> >  
> >  #endif /* _ASM_POWERPC_MMU_HASH64_H_ */
> > diff --git a/arch/powerpc/mm/hash_utils_64.c b/arch/powerpc/mm/hash_utils_64.c
> > index e88a86e..d63f7dc 100644
> > --- a/arch/powerpc/mm/hash_utils_64.c
> > +++ b/arch/powerpc/mm/hash_utils_64.c
> > @@ -606,10 +606,24 @@ static int __init htab_dt_scan_pftsize(unsigned long node,
> >  	return 0;
> >  }
> >  
> > -static unsigned long __init htab_get_table_size(void)
> > +unsigned htab_shift_for_mem_size(unsigned long mem_size)
> >  {
> > -	unsigned long mem_size, rnd_mem_size, pteg_count, psize;
> > +	unsigned memshift = __ilog2(mem_size);
> > +	unsigned pshift = mmu_psize_defs[mmu_virtual_psize].shift;
> > +	unsigned pteg_shift;
> > +
> > +	/* round mem_size up to next power of 2 */
> > +	if ((1UL << memshift) < mem_size)
> > +		memshift += 1;
> > +
> > +	/* aim for 2 pages / pteg */
> 
> While here I guess its a good opportunity to write couple of lines
> about why one PTE group for every two physical pages on the system,

Well, that don't really know, it's just copied from the existing code.

> why minimum (1UL << 11 = 2048) number of PTE groups required,

Ok.

> why
> (1U << 7 = 128) entries per PTE group

Um.. what?  Because that's how big a PTEG is, I don't think
re-explaining the HPT structure here is useful.

> and also remove the existing
> confusing comments above ? Just a suggestion.

Not sure which comment you mean.

> 
> > +	pteg_shift = memshift - (pshift + 1);
> > +
> > +	return max(pteg_shift + 7, 18U);
> > +}
> >  
> > +static unsigned long __init htab_get_table_size(void)
> > +{
> >  	/* If hash size isn't already provided by the platform, we try to
> >  	 * retrieve it from the device-tree. If it's not there neither, we
> >  	 * calculate it now based on the total RAM size
> > @@ -619,17 +633,7 @@ static unsigned long __init htab_get_table_size(void)
> >  	if (ppc64_pft_size)
> >  		return 1UL << ppc64_pft_size;
> >  
> > -	/* round mem_size up to next power of 2 */
> > -	mem_size = memblock_phys_mem_size();
> > -	rnd_mem_size = 1UL << __ilog2(mem_size);
> > -	if (rnd_mem_size < mem_size)
> > -		rnd_mem_size <<= 1;
> > -
> > -	/* # pages / 2 */
> > -	psize = mmu_psize_defs[mmu_virtual_psize].shift;
> > -	pteg_count = max(rnd_mem_size >> (psize + 1), 1UL << 11);
> > -
> > -	return pteg_count << 7;
> > +	return htab_shift_for_mem_size(memblock_phys_mem_size());
> 
> Would it be 1UL << htab_shift_for_mem_size(memblock_phys_mem_size())
> instead ? It was returning the size of the HPT not the shift of HPT
> originally or I am missing something here.

Oops, yes.  That would have broken all non-LPAR platforms.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson