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From: Pavel Tatashin <pasha.tatashin@gmail.com>
To: Alexander Duyck <alexander.h.duyck@linux.intel.com>,
	Michal Hocko <mhocko@kernel.org>
Cc: linux-mm@kvack.org, akpm@linux-foundation.org,
	pavel.tatashin@microsoft.com, dave.jiang@intel.com,
	linux-kernel@vger.kernel.org, willy@infradead.org,
	davem@davemloft.net, yi.z.zhang@linux.intel.com,
	khalid.aziz@oracle.com, rppt@linux.vnet.ibm.com, vbabka@suse.cz,
	sparclinux@vger.kernel.org, dan.j.williams@intel.com,
	ldufour@linux.vnet.ibm.com, mgorman@techsingularity.net,
	mingo@kernel.org, kirill.shutemov@linux.intel.com
Subject: Re: [mm PATCH v3 1/6] mm: Use mm_zero_struct_page from SPARC on all 64b architectures
Date: Wed, 17 Oct 2018 11:12:04 -0400	[thread overview]
Message-ID: <8aaa0fa2-5f12-ea3c-a0ca-ded9e1a639e2@gmail.com> (raw)
In-Reply-To: <9700b00f-a8a4-e318-f6a8-71fd1e7021b3@linux.intel.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.
> 

  reply	other threads:[~2018-10-17 15:12 UTC|newest]

Thread overview: 31+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-10-15 20:26 [mm PATCH v3 0/6] Deferred page init improvements Alexander Duyck
2018-10-15 20:26 ` [mm PATCH v3 1/6] mm: Use mm_zero_struct_page from SPARC on all 64b architectures Alexander Duyck
2018-10-16 19:01   ` Pavel Tatashin
2018-10-17  7:30     ` Mike Rapoport
2018-10-17 14:52       ` Alexander Duyck
2018-10-17  8:47   ` Michal Hocko
2018-10-17 15:07     ` Alexander Duyck
2018-10-17 15:12       ` Pavel Tatashin [this message]
2018-10-17 15:40         ` David Laight
2018-10-17 16:31           ` Alexander Duyck
2018-10-17 17:08             ` Pavel Tatashin
2018-10-17 16:34       ` Michal Hocko
2018-10-15 20:27 ` [mm PATCH v3 2/6] mm: Drop meminit_pfn_in_nid as it is redundant Alexander Duyck
2018-10-16 20:33   ` Pavel Tatashin
2018-10-16 20:49     ` Alexander Duyck
2018-10-16 21:06       ` Pavel Tatashin
2018-10-17  9:04   ` Michal Hocko
2018-10-15 20:27 ` [mm PATCH v3 3/6] mm: Use memblock/zone specific iterator for handling deferred page init Alexander Duyck
2018-10-17  9:11   ` Michal Hocko
2018-10-17 15:17     ` Alexander Duyck
2018-10-17 16:42   ` Mike Rapoport
2018-10-15 20:27 ` [mm PATCH v3 4/6] mm: Move hot-plug specific memory init into separate functions and optimize Alexander Duyck
2018-10-17  9:18   ` Michal Hocko
2018-10-17 15:26     ` Alexander Duyck
2018-10-24 12:36       ` Michal Hocko
2018-10-24 15:08         ` Alexander Duyck
2018-10-24 15:27           ` Michal Hocko
2018-10-24 17:35             ` Alexander Duyck
2018-10-25 12:41               ` Michal Hocko
2018-10-15 20:27 ` [mm PATCH v3 5/6] mm: Use common iterator for deferred_init_pages and deferred_free_pages Alexander Duyck
2018-10-15 20:27 ` [mm PATCH v3 6/6] mm: Add reserved flag setting to set_page_links Alexander Duyck

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