Re: [PATCH v2 1/4] arm64: mm: Don't remap pgtables per-cont(pte|pmd) block

[Ryan Roberts]

On 10/04/2024 10:46, Mark Rutland wrote:
> On Thu, Apr 04, 2024 at 03:33:05PM +0100, Ryan Roberts wrote:
>> A large part of the kernel boot time is creating the kernel linear map
>> page tables. When rodata=full, all memory is mapped by pte. And when
>> there is lots of physical ram, there are lots of pte tables to populate.
>> The primary cost associated with this is mapping and unmapping the pte
>> table memory in the fixmap; at unmap time, the TLB entry must be
>> invalidated and this is expensive.
>>
>> Previously, each pmd and pte table was fixmapped/fixunmapped for each
>> cont(pte|pmd) block of mappings (16 entries with 4K granule). This means
>> we ended up issuing 32 TLBIs per (pmd|pte) table during the population
>> phase.
>>
>> Let's fix that, and fixmap/fixunmap each page once per population, for a
>> saving of 31 TLBIs per (pmd|pte) table. This gives a significant boot
>> speedup.
>>
>> Execution time of map_mem(), which creates the kernel linear map page
>> tables, was measured on different machines with different RAM configs:
>>
>>                | Apple M2 VM | Ampere Altra| Ampere Altra| Ampere Altra
>>                | VM, 16G     | VM, 64G     | VM, 256G    | Metal, 512G
>> ---------------|-------------|-------------|-------------|-------------
>>                |   ms    (%) |   ms    (%) |   ms    (%) |    ms    (%)
>> ---------------|-------------|-------------|-------------|-------------
>> before         |  153   (0%) | 2227   (0%) | 8798   (0%) | 17442   (0%)
>> after          |   77 (-49%) |  431 (-81%) | 1727 (-80%) |  3796 (-78%)
>>
>> Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx>
>> Tested-by: Itaru Kitayama <itaru.kitayama@xxxxxxxxxxx>
>> Tested-by: Eric Chanudet <echanude@xxxxxxxxxx>
>> ---
>>  arch/arm64/mm/mmu.c | 32 ++++++++++++++++++--------------
>>  1 file changed, 18 insertions(+), 14 deletions(-)
>>
>> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
>> index 495b732d5af3..fd91b5bdb514 100644
>> --- a/arch/arm64/mm/mmu.c
>> +++ b/arch/arm64/mm/mmu.c
>> @@ -172,12 +172,9 @@ bool pgattr_change_is_safe(u64 old, u64 new)
>>  	return ((old ^ new) & ~mask) == 0;
>>  }
>>  
>> -static void init_pte(pmd_t *pmdp, unsigned long addr, unsigned long end,
>> -		     phys_addr_t phys, pgprot_t prot)
>> +static pte_t *init_pte(pte_t *ptep, unsigned long addr, unsigned long end,
>> +		       phys_addr_t phys, pgprot_t prot)
>>  {
>> -	pte_t *ptep;
>> -
>> -	ptep = pte_set_fixmap_offset(pmdp, addr);
>>  	do {
>>  		pte_t old_pte = __ptep_get(ptep);
>>  
>> @@ -193,7 +190,7 @@ static void init_pte(pmd_t *pmdp, unsigned long addr, unsigned long end,
>>  		phys += PAGE_SIZE;
>>  	} while (ptep++, addr += PAGE_SIZE, addr != end);
>>  
>> -	pte_clear_fixmap();
>> +	return ptep;
>>  }
>>  
>>  static void alloc_init_cont_pte(pmd_t *pmdp, unsigned long addr,
>> @@ -204,6 +201,7 @@ static void alloc_init_cont_pte(pmd_t *pmdp, unsigned long addr,
>>  {
>>  	unsigned long next;
>>  	pmd_t pmd = READ_ONCE(*pmdp);
>> +	pte_t *ptep;
>>  
>>  	BUG_ON(pmd_sect(pmd));
>>  	if (pmd_none(pmd)) {
>> @@ -219,6 +217,7 @@ static void alloc_init_cont_pte(pmd_t *pmdp, unsigned long addr,
>>  	}
>>  	BUG_ON(pmd_bad(pmd));
>>  
>> +	ptep = pte_set_fixmap_offset(pmdp, addr);
>>  	do {
>>  		pgprot_t __prot = prot;
>>  
>> @@ -229,20 +228,20 @@ static void alloc_init_cont_pte(pmd_t *pmdp, unsigned long addr,
>>  		    (flags & NO_CONT_MAPPINGS) == 0)
>>  			__prot = __pgprot(pgprot_val(prot) | PTE_CONT);
>>  
>> -		init_pte(pmdp, addr, next, phys, __prot);
>> +		ptep = init_pte(ptep, addr, next, phys, __prot);
>>  
>>  		phys += next - addr;
>>  	} while (addr = next, addr != end);
> 
> I reckon it might be better to leave init_pte() returning void, and move the
> ptep along here, e.g.
> 
> 	ptep = pte_set_fixmap_offset(pmdp, addr);
> 	do {
> 		...
> 
> 		init_pte(ptep, addr, next, phys, __prot);
> 
> 		ptep += pte_index(next) - pte_index(addr);
> 		phys += next - addr;
> 	} while (addr = next, addr != end);
> 
> 
> ... as that keeps the relationship between 'ptep' and 'phys' clear since
> they're manipulated in the same way, adjacent to one another.
> 
> Regardless this looks good, so with that change or as-is:
> 
> Acked-by: Mark Rutland <mark.rutland@xxxxxxx>
> 
> ... though I would prefer with that change. ;)

Yep, will change. And I'll do the same for pmd_init() too.

> 
> Mark.