Re: [mm PATCH v3 1/6] mm: Use mm_zero_struct_page from SPARC on all 64b architectures

[Pavel Tatashin <pasha.tatashin@gmail.com>]

On 10/17/18 11:07 AM, Alexander Duyck wrote:
> On 10/17/2018 1:47 AM, Michal Hocko wrote:
>> On Mon 15-10-18 13:26:56, Alexander Duyck wrote:
>>> This change makes it so that we use the same approach that was
>>> already in
>>> use on Sparc on all the archtectures that support a 64b long.
>>>
>>> This is mostly motivated by the fact that 8 to 10 store/move
>>> instructions
>>> are likely always going to be faster than having to call into a function
>>> that is not specialized for handling page init.
>>>
>>> An added advantage to doing it this way is that the compiler can get
>>> away
>>> with combining writes in the __init_single_page call. As a result the
>>> memset call will be reduced to only about 4 write operations, or at
>>> least
>>> that is what I am seeing with GCC 6.2 as the flags, LRU poitners, and
>>> count/mapcount seem to be cancelling out at least 4 of the 8
>>> assignments on
>>> my system.
>>>
>>> One change I had to make to the function was to reduce the minimum page
>>> size to 56 to support some powerpc64 configurations.
>>
>> This really begs for numbers. I do not mind the change itself with some
>> minor comments below.
>>
>> [...]
>>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>>> index bb0de406f8e7..ec6e57a0c14e 100644
>>> --- a/include/linux/mm.h
>>> +++ b/include/linux/mm.h
>>> @@ -102,8 +102,42 @@ static inline void set_max_mapnr(unsigned long
>>> limit) { }
>>>    * zeroing by defining this macro in <asm/pgtable.h>.
>>>    */
>>>   #ifndef mm_zero_struct_page
>>
>> Do we still need this ifdef? I guess we can wait for an arch which
>> doesn't like this change and then add the override. I would rather go
>> simple if possible.
> 
> We probably don't, but as soon as I remove it somebody will probably
> complain somewhere. I guess I could drop it for now and see if anybody
> screams. Adding it back should be pretty straight forward since it would
> only be 2 lines.
> 
>>> +#if BITS_PER_LONG == 64
>>> +/* This function must be updated when the size of struct page grows
>>> above 80
>>> + * or reduces below 64. The idea that compiler optimizes out switch()
>>> + * statement, and only leaves move/store instructions
>>> + */
>>> +#define    mm_zero_struct_page(pp) __mm_zero_struct_page(pp)
>>> +static inline void __mm_zero_struct_page(struct page *page)
>>> +{
>>> +    unsigned long *_pp = (void *)page;
>>> +
>>> +     /* Check that struct page is either 56, 64, 72, or 80 bytes */
>>> +    BUILD_BUG_ON(sizeof(struct page) & 7);
>>> +    BUILD_BUG_ON(sizeof(struct page) < 56);
>>> +    BUILD_BUG_ON(sizeof(struct page) > 80);
>>> +
>>> +    switch (sizeof(struct page)) {
>>> +    case 80:
>>> +        _pp[9] = 0;    /* fallthrough */
>>> +    case 72:
>>> +        _pp[8] = 0;    /* fallthrough */
>>> +    default:
>>> +        _pp[7] = 0;    /* fallthrough */
>>> +    case 56:
>>> +        _pp[6] = 0;
>>> +        _pp[5] = 0;
>>> +        _pp[4] = 0;
>>> +        _pp[3] = 0;
>>> +        _pp[2] = 0;
>>> +        _pp[1] = 0;
>>> +        _pp[0] = 0;
>>> +    }
>>
>> This just hit my eyes. I have to confess I have never seen default: to
>> be not the last one in the switch. Can we have case 64 instead or does
>> gcc
>> complain? I would be surprised with the set of BUILD_BUG_ONs.

It was me, C does not really care where default is placed, I was trying
to keep stores sequential for better cache locality, but "case 64"
should be OK, and even better for this purpose.

Pavel

> 
> I can probably just replace the "default:" with "case 64:". I think I
> have seen other switch statements in the kernel without a default so
> odds are it should be okay.
>