* [PATCH v2] mm/page_alloc: remove unnecessary order check in __alloc_pages_direct_compact
@ 2016-06-15 9:34 Ganesh Mahendran
2016-06-15 9:40 ` Balbir Singh
0 siblings, 1 reply; 4+ messages in thread
From: Ganesh Mahendran @ 2016-06-15 9:34 UTC (permalink / raw)
To: linux-mm, linux-kernel
Cc: akpm, vbabka, iamjoonsoo.kim, mhocko, mina86, minchan, khandual,
Ganesh Mahendran
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@gmail.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);
--
1.9.1
^ permalink raw reply related [flat|nested] 4+ messages in thread
* Re: [PATCH v2] mm/page_alloc: remove unnecessary order check in __alloc_pages_direct_compact
2016-06-15 9:34 [PATCH v2] mm/page_alloc: remove unnecessary order check in __alloc_pages_direct_compact Ganesh Mahendran
@ 2016-06-15 9:40 ` Balbir Singh
2016-06-15 9:52 ` Vlastimil Babka
2016-06-15 16:41 ` Michal Nazarewicz
0 siblings, 2 replies; 4+ messages in thread
From: Balbir Singh @ 2016-06-15 9:40 UTC (permalink / raw)
To: Ganesh Mahendran
Cc: linux-mm, linux-kernel, akpm, Vlastimil Babka, Joonsoo Kim,
mhocko, mina86, Minchan Kim, Anshuman Khandual
On Wed, Jun 15, 2016 at 7:34 PM, Ganesh Mahendran
<opensource.ganesh@gmail.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@gmail.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.
Balbir Singh
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: [PATCH v2] mm/page_alloc: remove unnecessary order check in __alloc_pages_direct_compact
2016-06-15 9:40 ` Balbir Singh
@ 2016-06-15 9:52 ` Vlastimil Babka
2016-06-15 16:41 ` Michal Nazarewicz
1 sibling, 0 replies; 4+ messages in thread
From: Vlastimil Babka @ 2016-06-15 9:52 UTC (permalink / raw)
To: Balbir Singh, Ganesh Mahendran
Cc: linux-mm, linux-kernel, akpm, Joonsoo Kim, mhocko, mina86,
Minchan Kim, Anshuman Khandual
On 06/15/2016 11:40 AM, Balbir Singh wrote:
> On Wed, Jun 15, 2016 at 7:34 PM, Ganesh Mahendran
> <opensource.ganesh@gmail.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@gmail.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.
Agree. The majority of calls should be for order == 0 where the check
avoids us from modifying current->flags and calling into compaction.c
just to return and modify the flags back. I would argue that we should
even check order before calling __alloc_pages_direct_compact() to avoid
another potential call, but the compiler might be doing the right thing
already.
So v1 was better in this aspect. But it wouldn't gain us any measurable
performance benefit anyway, so we might as well leave it.
> Balbir Singh
>
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: [PATCH v2] mm/page_alloc: remove unnecessary order check in __alloc_pages_direct_compact
2016-06-15 9:40 ` Balbir Singh
2016-06-15 9:52 ` Vlastimil Babka
@ 2016-06-15 16:41 ` Michal Nazarewicz
1 sibling, 0 replies; 4+ messages in thread
From: Michal Nazarewicz @ 2016-06-15 16:41 UTC (permalink / raw)
To: Balbir Singh, Ganesh Mahendran
Cc: linux-mm, linux-kernel, akpm, Vlastimil Babka, Joonsoo Kim,
mhocko, Minchan Kim, Anshuman Khandual
On Wed, Jun 15 2016, Balbir Singh wrote:
> On Wed, Jun 15, 2016 at 7:34 PM, Ganesh Mahendran
> <opensource.ganesh@gmail.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@gmail.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»
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2016-06-15 9:34 [PATCH v2] mm/page_alloc: remove unnecessary order check in __alloc_pages_direct_compact Ganesh Mahendran
2016-06-15 9:40 ` Balbir Singh
2016-06-15 9:52 ` Vlastimil Babka
2016-06-15 16:41 ` Michal Nazarewicz
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