Re: [PATCH v2 2/4] arm64: mm: Batch dsb and isb when populating pgtables

[Mark Rutland]

On Thu, Apr 04, 2024 at 03:33:06PM +0100, Ryan Roberts wrote:
> After removing uneccessary TLBIs, the next bottleneck when creating the
> page tables for the linear map is DSB and ISB, which were previously
> issued per-pte in __set_pte(). Since we are writing multiple ptes in a
> given pte table, we can elide these barriers and insert them once we
> have finished writing to the table.
> 
> 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         |   77   (0%) |  431   (0%) | 1727   (0%) |  3796   (0%)
> after          |   13 (-84%) |  162 (-62%) |  655 (-62%) |  1656 (-56%)
> 
> Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx>
> Tested-by: Itaru Kitayama <itaru.kitayama@xxxxxxxxxxx>
> Tested-by: Eric Chanudet <echanude@xxxxxxxxxx>
> ---
>  arch/arm64/include/asm/pgtable.h |  7 ++++++-
>  arch/arm64/mm/mmu.c              | 13 ++++++++++++-
>  2 files changed, 18 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> index afdd56d26ad7..105a95a8845c 100644
> --- a/arch/arm64/include/asm/pgtable.h
> +++ b/arch/arm64/include/asm/pgtable.h
> @@ -271,9 +271,14 @@ static inline pte_t pte_mkdevmap(pte_t pte)
>  	return set_pte_bit(pte, __pgprot(PTE_DEVMAP | PTE_SPECIAL));
>  }
>  
> -static inline void __set_pte(pte_t *ptep, pte_t pte)
> +static inline void __set_pte_nosync(pte_t *ptep, pte_t pte)
>  {
>  	WRITE_ONCE(*ptep, pte);
> +}
> +
> +static inline void __set_pte(pte_t *ptep, pte_t pte)
> +{
> +	__set_pte_nosync(ptep, pte);
>  
>  	/*
>  	 * Only if the new pte is valid and kernel, otherwise TLB maintenance
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index fd91b5bdb514..dc86dceb0efe 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -178,7 +178,11 @@ static pte_t *init_pte(pte_t *ptep, unsigned long addr, unsigned long end,
>  	do {
>  		pte_t old_pte = __ptep_get(ptep);
>  
> -		__set_pte(ptep, pfn_pte(__phys_to_pfn(phys), prot));
> +		/*
> +		 * Required barriers to make this visible to the table walker
> +		 * are deferred to the end of alloc_init_cont_pte().
> +		 */
> +		__set_pte_nosync(ptep, pfn_pte(__phys_to_pfn(phys), prot));
>  
>  		/*
>  		 * After the PTE entry has been populated once, we
> @@ -234,6 +238,13 @@ static void alloc_init_cont_pte(pmd_t *pmdp, unsigned long addr,
>  	} while (addr = next, addr != end);
>  
>  	pte_clear_fixmap();
> +
> +	/*
> +	 * Ensure all previous pgtable writes are visible to the table walker.
> +	 * See init_pte().
> +	 */
> +	dsb(ishst);
> +	isb();

Hmm... currently the call to pte_clear_fixmap() alone should be sufficient,
since that needs to update the PTE for the fixmap slot, then do maintenance for
that.

So we could avoid the addition of the dsb+isb here, and have a comment:

	/*
	 * Note: barriers and maintenance necessary to clear the fixmap slot
	 * ensure that all previous pgtable writes are visible to the table
	 * walker.
	 */
	pte_clear_fixmap();

.. which'd be fine as long as we keep this fixmap clearing rather than trying
to do that lazily as in patch 4.

Mark.

>  }
>  
>  static pmd_t *init_pmd(pmd_t *pmdp, unsigned long addr, unsigned long end,
> -- 
> 2.25.1
>