Re: [PATCH] CMA/HOTPLUG: clear buffer-head lru before page migration

[Zhang Yanfei]

Hello,

On 07/18/2014 04:23 PM, Gioh Kim wrote:
> 
> 
> 2014-07-18 ìí 4:50, Marek Szyprowski ì ê:
>> Hello,
>>
>> On 2014-07-18 08:45, Gioh Kim wrote:
>>> For page migration of CMA, buffer-heads of lru should be dropped.
>>> Please refer to https://lkml.org/lkml/2014/7/4/101 for the history.
>>>
>>> I have two solution to drop bhs.
>>> One is invalidating entire lru.
>>> Another is searching the lru and dropping only one bh that Laura proposed
>>> at https://lkml.org/lkml/2012/8/31/313.
>>>
>>> I'm not sure which has better performance.
>>> So I did performance test on my cortex-a7 platform with Lmbench
>>> that has "File & VM system latencies" test.
>>> I am attaching the results.
>>> The first line is of invalidating entire lru and the second is dropping selected bh.
>>>
>>> File & VM system latencies in microseconds - smaller is better
>>> -------------------------------------------------------------------------------
>>> Host                 OS   0K File      10K File     Mmap    Prot   Page   100fd
>>>                          Create Delete Create Delete Latency Fault  Fault  selct
>>> --------- ------------- ------ ------ ------ ------ ------- ----- ------- -----
>>> 10.178.33 Linux 3.10.19   25.1   19.6   32.6   19.7  5098.0 0.666 3.45880 6.506
>>> 10.178.33 Linux 3.10.19   24.9   19.5   32.3   19.4  5059.0 0.563 3.46380 6.521
>>>
>>>
>>> I tried several times but the result tells that they are the same under 1% gap
>>> except Protection Fault.
>>> But the latency of Protection Fault is very small and I think it has little effect.
>>>
>>> Therefore we can choose anything but I choose invalidating entire lru.
>>> The try_to_free_buffers() which is calling drop_buffers() is called by many filesystem code.
>>> So I think inserting codes in drop_buffers() can affect the system.
>>> And also we cannot distinguish migration type in drop_buffers().
>>>
>>> In alloc_contig_range() we can distinguish migration type and invalidate lru if it needs.
>>> I think alloc_contig_range() is proper to deal with bh like following patch.
>>>
>>> Laura, can I have you name on Acked-by line?
>>> Please let me represent my thanks.
>>>
>>> Thanks for any feedback.
>>>
>>> ------------------------------- 8< ----------------------------------
>>>
>>> >From 33c894b1bab9bc26486716f0c62c452d3a04d35d Mon Sep 17 00:00:00 2001
>>> From: Gioh Kim <gioh.kim@xxxxxxx>
>>> Date: Fri, 18 Jul 2014 13:40:01 +0900
>>> Subject: [PATCH] CMA/HOTPLUG: clear buffer-head lru before page migration
>>>
>>> The bh must be free to migrate a page at which bh is mapped.
>>> The reference count of bh is increased when it is installed
>>> into lru so that the bh of lru must be freed before migrating the page.
>>>
>>> This frees every bh of lru. We could free only bh of migrating page.
>>> But searching lru costs more than invalidating entire lru.
>>>
>>> Signed-off-by: Gioh Kim <gioh.kim@xxxxxxx>
>>> Acked-by: Laura Abbott <lauraa@xxxxxxxxxxxxxx>
>>> ---
>>>   mm/page_alloc.c |    3 +++
>>>   1 file changed, 3 insertions(+)
>>>
>>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>>> index b99643d4..3b474e0 100644
>>> --- a/mm/page_alloc.c
>>> +++ b/mm/page_alloc.c
>>> @@ -6369,6 +6369,9 @@ int alloc_contig_range(unsigned long start, unsigned long end,
>>>          if (ret)
>>>                  return ret;
>>>
>>> +       if (migratetype == MIGRATE_CMA || migratetype == MIGRATE_MOVABLE)
>>
>> I'm not sure if it really makes sense to check the migratetype here. This check
>> doesn't add any new information to the code and make false impression that this
>> function can be called for other migratetypes than CMA or MOVABLE. Even if so,
>> then invalidating bh_lrus unconditionally will make more sense, IMHO.
> 
> I agree. I cannot understand why alloc_contig_range has an argument of migratetype.
> Can the alloc_contig_range is called for other migrate type than CMA/MOVABLE?
> 
> What do you think about removing the argument of migratetype and
> checking migratetype (if (migratetype == MIGRATE_CMA || migratetype == MIGRATE_MOVABLE))?
> 

Remove the checking only. Because gigantic page allocation used for hugetlb is
using alloc_contig_range(...... MIGRATE_MOVABLE).

Thanks.

-- 
Thanks.
Zhang Yanfei
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