Subject: Re: [PATCH V4 0/2] mm: FAULT_AROUND_ORDER patchset performance data for powerpc

From: Madhavan Srinivasan <[email-masked]>
Date: Tue, 27 May 2014 11:54:49 +0530

On Tuesday 20 May 2014 03:57 PM, Kirill A. Shutemov wrote:
> Rusty Russell wrote:
>> "Kirill A. Shutemov" <[email-masked]> writes:
>>> Andrew Morton wrote:
>>>> On Mon, 19 May 2014 16:23:07 -0700 (PDT) Hugh Dickins <[email-masked]> wrote:
>>>>
>>>>> Shouldn't FAULT_AROUND_ORDER and fault_around_order be changed to be
>>>>> the order of the fault-around size in bytes, and fault_around_pages()
>>>>> use 1UL << (fault_around_order - PAGE_SHIFT)
>>>>
>>>> Yes.  And shame on me for missing it (this time!) at review.
>>>>
>>>> There's still time to fix this.  Patches, please.
>>>
>>> Here it is. Made at 3.30 AM, build tested only.
>>
>> Prefer on top of Maddy's patch which makes it always a variable, rather
>> than CONFIG_DEBUG_FS.  It's got enough hair as it is.
> 
> Something like this?
> 
> From: "Kirill A. Shutemov" <[email-masked]>
> Date: Tue, 20 May 2014 13:02:03 +0300
> Subject: [PATCH] mm: nominate faultaround area in bytes rather then page order
> 
> There are evidences that faultaround feature is less relevant on
> architectures with page size bigger then 4k. Which makes sense since
> page fault overhead per byte of mapped area should be less there.
> 
> Let's rework the feature to specify faultaround area in bytes instead of
> page order. It's 64 kilobytes for now.
> 
> The patch effectively disables faultaround on architectures with
> page size >= 64k (like ppc64).
> 
> It's possible that some other size of faultaround area is relevant for a
> platform. We can expose `fault_around_bytes' variable to arch-specific
> code once such platforms will be found.
> 
> Signed-off-by: Kirill A. Shutemov <[email-masked]>
> ---
>  mm/memory.c | 62 +++++++++++++++++++++++--------------------------------------
>  1 file changed, 23 insertions(+), 39 deletions(-)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index 037b812a9531..252b319e8cdf 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -3402,63 +3402,47 @@ void do_set_pte(struct vm_area_struct *vma, unsigned long address,
>  	update_mmu_cache(vma, address, pte);
>  }
> 
> -#define FAULT_AROUND_ORDER 4
> +static unsigned long fault_around_bytes = 65536;
> +
> +static inline unsigned long fault_around_pages(void)
> +{
> +	return rounddown_pow_of_two(fault_around_bytes) / PAGE_SIZE;
> +}
> +
> +static inline unsigned long fault_around_mask(void)
> +{
> +	return ~(rounddown_pow_of_two(fault_around_bytes) - 1) & PAGE_MASK;
> +}
> 
> -#ifdef CONFIG_DEBUG_FS
> -static unsigned int fault_around_order = FAULT_AROUND_ORDER;
> 
> -static int fault_around_order_get(void *data, u64 *val)
> +#ifdef CONFIG_DEBUG_FS
> +static int fault_around_bytes_get(void *data, u64 *val)
>  {
> -	*val = fault_around_order;
> +	*val = fault_around_bytes;
>  	return 0;
>  }
> 
> -static int fault_around_order_set(void *data, u64 val)
> +static int fault_around_bytes_set(void *data, u64 val)
>  {

Kindly ignore the question if not relevant. Even though we need root
access to alter the value, will we be fine with
negative value?.

Regards
Maddy

> -	BUILD_BUG_ON((1UL << FAULT_AROUND_ORDER) > PTRS_PER_PTE);
> -	if (1UL << val > PTRS_PER_PTE)
> +	if (val / PAGE_SIZE > PTRS_PER_PTE)
>  		return -EINVAL;
> -	fault_around_order = val;
> +	fault_around_bytes = val;
>  	return 0;
>  }
> -DEFINE_SIMPLE_ATTRIBUTE(fault_around_order_fops,
> -		fault_around_order_get, fault_around_order_set, "%llu\n");
> +DEFINE_SIMPLE_ATTRIBUTE(fault_around_bytes_fops,
> +		fault_around_bytes_get, fault_around_bytes_set, "%llu\n");
> 
>  static int __init fault_around_debugfs(void)
>  {
>  	void *ret;
> 
> -	ret = debugfs_create_file("fault_around_order",	0644, NULL, NULL,
> -			&fault_around_order_fops);
> +	ret = debugfs_create_file("fault_around_bytes", 0644, NULL, NULL,
> +			&fault_around_bytes_fops);
>  	if (!ret)
> -		pr_warn("Failed to create fault_around_order in debugfs");
> +		pr_warn("Failed to create fault_around_bytes in debugfs");
>  	return 0;
>  }
>  late_initcall(fault_around_debugfs);
> -
> -static inline unsigned long fault_around_pages(void)
> -{
> -	return 1UL << fault_around_order;
> -}
> -
> -static inline unsigned long fault_around_mask(void)
> -{
> -	return ~((1UL << (PAGE_SHIFT + fault_around_order)) - 1);
> -}
> -#else
> -static inline unsigned long fault_around_pages(void)
> -{
> -	unsigned long nr_pages;
> -
> -	nr_pages = 1UL << FAULT_AROUND_ORDER;
> -	BUILD_BUG_ON(nr_pages > PTRS_PER_PTE);
> -	return nr_pages;
> -}
> -
> -static inline unsigned long fault_around_mask(void)
> -{
> -	return ~((1UL << (PAGE_SHIFT + FAULT_AROUND_ORDER)) - 1);
> -}
>  #endif
> 
>  static void do_fault_around(struct vm_area_struct *vma, unsigned long address,
> @@ -3515,7 +3499,7 @@ static int do_read_fault(struct mm_struct *mm, struct vm_area_struct *vma,
>  	 * if page by the offset is not ready to be mapped (cold cache or
>  	 * something).
>  	 */
> -	if (vma->vm_ops->map_pages) {
> +	if (vma->vm_ops->map_pages && fault_around_pages() > 1) {
>  		pte = pte_offset_map_lock(mm, pmd, address, &ptl);
>  		do_fault_around(vma, address, pte, pgoff, flags);
>  		if (!pte_same(*pte, orig_pte))
>