linux-kernel - Re: [PATCH v2] mm/page_alloc: remove unnecessary order check in __alloc_pages_direct

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Message-ID: <xa1tlh26csm8.fsf@mina86.com>
Date:	Wed, 15 Jun 2016 18:41:03 +0200
From:	Michal Nazarewicz <mina86@...a86.com>
To:	Balbir Singh <bsingharora@...il.com>,
	Ganesh Mahendran <opensource.ganesh@...il.com>
Cc:	linux-mm <linux-mm@...ck.org>,
	"linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
	"akpm\@linux-foundation.org" <akpm@...ux-foundation.org>,
	Vlastimil Babka <vbabka@...e.cz>,
	Joonsoo Kim <iamjoonsoo.kim@....com>, mhocko@...e.com,
	Minchan Kim <minchan@...nel.org>,
	Anshuman Khandual <khandual@...ux.vnet.ibm.com>
Subject: Re: [PATCH v2] mm/page_alloc: remove unnecessary order check in __alloc_pages_direct_compact

On Wed, Jun 15 2016, Balbir Singh wrote:
> On Wed, Jun 15, 2016 at 7:34 PM, Ganesh Mahendran
> <opensource.ganesh@...il.com> wrote:
>> In the callee try_to_compact_pages(), the (order == 0) is checked,
>> so remove check in __alloc_pages_direct_compact.
>>
>> Signed-off-by: Ganesh Mahendran <opensource.ganesh@...il.com>
>> ---
>> v2:
>>   remove the check in __alloc_pages_direct_compact - Anshuman Khandual
>> ---
>>  mm/page_alloc.c | 3 ---
>>  1 file changed, 3 deletions(-)
>>
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index b9ea618..2f5a82a 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -3173,9 +3173,6 @@ __alloc_pages_direct_compact(gfp_t gfp_mask, unsigned int order,
>>         struct page *page;
>>         int contended_compaction;
>>
>> -       if (!order)
>> -               return NULL;
>> -
>>         current->flags |= PF_MEMALLOC;
>>         *compact_result = try_to_compact_pages(gfp_mask, order, alloc_flags, ac,
>>                                                 mode, &contended_compaction);
>
> What is the benefit of this. Is an if check more expensive than
> calling the function and returning from it? I don't feel strongly
> about such changes, but its good to audit the overall code for reading
> and performance.

It’s a slow path so it probably doesn’t matter much.  But I also don’t
see whether this improves readability of the code.

For performance, I would rather wait for gcc to compile kernel as one
translation unit which will allow it to inline try_to_compact_pages and
notice redundant order==0 check.

-- 
Best regards
ミハウ “𝓶𝓲𝓷𝓪86” ナザレヴイツ
«If at first you don’t succeed, give up skydiving»