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 16:14:05 +0530

On Tuesday 27 May 2014 03:51 PM, Kirill A. Shutemov wrote:
> Madhavan Srinivasan wrote:
>> 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?.
> ppc
> val is u64. or I miss something?
> 

My Bad. What I wanted to check was for all 0xf input and guess we are
fine. Sorry about that.

Regards
Maddy