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

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

[Gioh Kim]

Hi,

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< ----------------------------------

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

[Marek Szyprowski]

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@lge.com>
> 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@lge.com>
> Acked-by: Laura Abbott <lauraa@codeaurora.org>
> ---
>   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.

> +               invalidate_bh_lrus();
> +
>          ret = __alloc_contig_migrate_range(&cc, start, end);
>          if (ret)
>                  goto done;
> --
> 1.7.9.5
>

Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland

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

[Gioh Kim]

2014-07-18 i??i?? 4:50, Marek Szyprowski i?' e,?:
> 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@lge.com>
>> 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@lge.com>
>> Acked-by: Laura Abbott <lauraa@codeaurora.org>
>> ---
>>   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))?


>
>> +               invalidate_bh_lrus();
>> +
>>          ret = __alloc_contig_migrate_range(&cc, start, end);
>>          if (ret)
>>                  goto done;
>> --
>> 1.7.9.5
>>
>
> Best regards

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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 오후 4:50, Marek Szyprowski 쓴 글:
>> 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@lge.com>
>>> 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@lge.com>
>>> Acked-by: Laura Abbott <lauraa@codeaurora.org>
>>> ---
>>>   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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Re: [PATCH] CMA/HOTPLUG: clear buffer-head lru before page migration

[Michal Nazarewicz]

>> On 2014-07-18 08:45, Gioh Kim wrote:
>>> From: Gioh Kim <gioh.kim@lge.com>
>>> 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@lge.com>
>>> Acked-by: Laura Abbott <lauraa@codeaurora.org>

With the if removed:

Acked-by: Michal Nazarewicz <mina86@mina86.com>

>>> ---
>>>   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)

On Fri, Jul 18 2014, Gioh Kim <gioh.kim@lge.com> wrote:
> 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?

It has migratetype argument precisely because it can be CMA or MOVABLE.
If alloc_contig_range was called always with the same migrate type, the
argument would not be necessary, but because it isn't, it is.

> What do you think about removing the argument of migratetype and
> checking migratetype (if (migratetype == MIGRATE_CMA || migratetype ==
> MIGRATE_MOVABLE))?

If you remove the argument, the function would have to read migrate type
of the pageblock and that's just waste of time, since the migrate type
can be passed to the function from its caller, so the argument should
remain.

>>> +               invalidate_bh_lrus();
>>> +
>>>          ret = __alloc_contig_migrate_range(&cc, start, end);
>>>          if (ret)
>>>                  goto done;

-- 
Best regards,                                         _     _
.o. | Liege of Serenely Enlightened Majesty of      o' \,=./ `o
..o | Computer Science,  Michał “mina86” Nazarewicz    (o o)
ooo +--<mpn@google.com>--<xmpp:mina86@jabber.org>--ooO--(_)--Ooo--

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

[Laura Abbott]

On 7/17/2014 11:45 PM, Gioh Kim wrote:
> 
> Hi,
> 
> 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@lge.com>
> 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@lge.com>
> Acked-by: Laura Abbott <lauraa@codeaurora.org>\

I'd prefer if you would remove my Acked-by line until I've actually
given it :)

> ---
>  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)
> +               invalidate_bh_lrus();
> +
>         ret = __alloc_contig_migrate_range(&cc, start, end);
>         if (ret)
>                 goto done;

I agree with the others that the if (...) check doesn't actually help
anything here and should probably be removed.

Thanks,
Laura

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

[Minchan Kim]

Hi Gioh,

On Fri, Jul 18, 2014 at 03:45:36PM +0900, Gioh Kim wrote:
> 
> Hi,
> 
> 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.

Just nit:
Please write *problem* in description instead of URL link.

> 
> I have two solution to drop bhs.
> One is invalidating entire lru.

You mean? All of percpu bh_lrus so if the system has N cpu,
it invalidates N * 8?

> 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.

For whom? system or requestor of CMA?

> 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.

You mean you did Lmbench with background CMA allocation?
Could you describe in detail?

> 
> 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.

Not sure we can conclude like that.

A few weeks ago, I saw a patch which increases bh_lrus's size.
https://lkml.org/lkml/2014/7/4/107
IOW, some of workloads really affects by percpu bh_lrus so it would be
better to be careful to drain, I think.

You want to argue CMA allocation is rare so the cost is marginable.
It might but some of usecase might call it frequently with small request
(ie, 8K, 16K).

Anyway, why cannot CMA have the cost without affecting other subsystem?
I mean it's okay for CMA to consume more time to shoot out the bh
instead of simple all bh_lru invalidation because big order allocation is
kinds of slow thing in the VM and everybody already know that and even
sometime get failed so it's okay to add more code that extremly slow path.

> 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@lge.com>
> 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@lge.com>
> Acked-by: Laura Abbott <lauraa@codeaurora.org>
> ---
>  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)
> +               invalidate_bh_lrus();
> +

Q1. It's a only CMA problem? Memory-Hotplug is not a problem? Or other places?
 
I mean it would be better to handle in generic way.

Q2. Why do you call it right before calling __alloc_contig_migrate_range?

Some of pages will go bh_lrus by __alloc_contig_migrate_ranges.
In that case, it is useless without caller's retry logic.
Even you do it from caller's retrial logic, it's not a good idea because
you makes new binding alloc_contig_range and uppder layer.

So, IMHO, it would be better to handle it in migrate_pages.
Maybe we could define new API try_to_drop_buffers which calls
try_to_free_buffers and then only if the function fails due to
percpu lru count, we could drain only the bh in percpu lru list instead of
all bh draining. And places in migration path should use it rather than
try_to_relese_page.

But the problem from this approach invents new API which should be
maintained so not sure Andrew think it's worth.
Maybe we should see the code and diffstat.

Overenginnering?

>         ret = __alloc_contig_migrate_range(&cc, start, end);
>         if (ret)
>                 goto done;
> --
> 1.7.9.5
> 
> --
> To unsubscribe, send a message with 'unsubscribe linux-mm' in
> the body to majordomo@kvack.org.  For more info on Linux MM,
> see: http://www.linux-mm.org/ .
> Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>

-- 
Kind regards,
Minchan Kim

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

[Gioh Kim]

2014-07-21 i??i ? 11:50, Minchan Kim i?' e,?:
> Hi Gioh,
>
> On Fri, Jul 18, 2014 at 03:45:36PM +0900, Gioh Kim wrote:
>>
>> Hi,
>>
>> 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.
>
> Just nit:
> Please write *problem* in description instead of URL link.
>
>>
>> I have two solution to drop bhs.
>> One is invalidating entire lru.
>
> You mean? All of percpu bh_lrus so if the system has N cpu,
> it invalidates N * 8?

Yes, every bh_lru of all cpus.

>
>> 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.
>
> For whom? system or requestor of CMA?

For system 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.
>
> You mean you did Lmbench with background CMA allocation?
> Could you describe in detail?

I'm sorry not to mention the background.
I did the test without CMA allocation because I wanted to check how it affects system performance.

The first test, invalidating entire lru, is adding invalidate_bh_lrus() at alloc_contig_range().
This is not affecting system performance because alloc_contig_range() is not called
for usual file-system management.
The resulf of the first test is the *default system performance.*

The second test, dropping all bh in lru, is adding drop_buffers().
Every call of drop_buffers drops all bhs in lru of every cpu.
It can affect system performance. *But* it does not affect system performance,
because it drops only bh of migrated pages.


>
>>
>> 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.
>
> Not sure we can conclude like that.
>
> A few weeks ago, I saw a patch which increases bh_lrus's size.
> https://lkml.org/lkml/2014/7/4/107
> IOW, some of workloads really affects by percpu bh_lrus so it would be
> better to be careful to drain, I think.
>
> You want to argue CMA allocation is rare so the cost is marginable.
> It might but some of usecase might call it frequently with small request
> (ie, 8K, 16K).
>
> Anyway, why cannot CMA have the cost without affecting other subsystem?
> I mean it's okay for CMA to consume more time to shoot out the bh
> instead of simple all bh_lru invalidation because big order allocation is
> kinds of slow thing in the VM and everybody already know that and even
> sometime get failed so it's okay to add more code that extremly slow path.

There are 2 reasons to invalidate entire bh_lru.

1. I think CMA allocation is very rare so that invalidaing bh_lru affects the system little.
How do you think about it? My platform does not call CMA allocation often.
Is the CMA allocation or Memory-Hotplug called often?

2. Adding code in drop_buffers() can affect the system more that adding code in alloc_contig_range()
because the drop_buffers does not have a way to distinguish migrate type.
Even-though the lmbech results that it has almost the same performance.
But I am afraid that it can be changed.
As you said if bh_lru size can be changed it affects more than now.
SO I do not want to touch non-CMA related code.


>
>> 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@lge.com>
>> 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@lge.com>
>> Acked-by: Laura Abbott <lauraa@codeaurora.org>
>> ---
>>   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)
>> +               invalidate_bh_lrus();
>> +
>
> Q1. It's a only CMA problem? Memory-Hotplug is not a problem? Or other places?
>
> I mean it would be better to handle in generic way.

Only CMA and Memory-Hotplug needs it.
And I think invalidate_bh_lrus() is general.

>
> Q2. Why do you call it right before calling __alloc_contig_migrate_range?
>
> Some of pages will go bh_lrus by __alloc_contig_migrate_ranges.
> In that case, it is useless without caller's retry logic.
> Even you do it from caller's retrial logic, it's not a good idea because
> you makes new binding alloc_contig_range and uppder layer.
>
> So, IMHO, it would be better to handle it in migrate_pages.
> Maybe we could define new API try_to_drop_buffers which calls
> try_to_free_buffers and then only if the function fails due to
> percpu lru count, we could drain only the bh in percpu lru list instead of
> all bh draining. And places in migration path should use it rather than
> try_to_relese_page.
>
> But the problem from this approach invents new API which should be
> maintained so not sure Andrew think it's worth.
> Maybe we should see the code and diffstat.

I also consider to making new function, drop_bh_of_migrate_page in migrate_page(), just before unmap_and_move().
The migrate_page() has an argument reason that distinguish migrate-type, MR_CMA or MR_MEMORY_HOTPLUG or others.

But I DO NOT WATN TO touch non-CMA related code.
Current CMA and Memory-Hotplug code is not mature so that I am not sure it is ok to touch non-CMA related code for CMA/MemoryHotplug.


My point is:
1. CMA/Memory-hotplug is rare and invalidating bh-lru is also rare.
2. Only change CMA/Memory-hotplig related code.

>
> Overenginnering?
>
>>          ret = __alloc_contig_migrate_range(&cc, start, end);
>>          if (ret)
>>                  goto done;
>> --
>> 1.7.9.5
>>
>> --
>> To unsubscribe, send a message with 'unsubscribe linux-mm' in
>> the body to majordomo@kvack.org.  For more info on Linux MM,
>> see: http://www.linux-mm.org/ .
>> Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
>

--
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Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>

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

[Minchan Kim]

On Mon, Jul 21, 2014 at 03:16:10PM +0900, Gioh Kim wrote:
> 
> 
> 2014-07-21 i??i ? 11:50, Minchan Kim i?' e,?:
> >Hi Gioh,
> >
> >On Fri, Jul 18, 2014 at 03:45:36PM +0900, Gioh Kim wrote:
> >>
> >>Hi,
> >>
> >>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.
> >
> >Just nit:
> >Please write *problem* in description instead of URL link.
> >
> >>
> >>I have two solution to drop bhs.
> >>One is invalidating entire lru.
> >
> >You mean? All of percpu bh_lrus so if the system has N cpu,
> >it invalidates N * 8?
> 
> Yes, every bh_lru of all cpus.
> 
> >
> >>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.
> >
> >For whom? system or requestor of CMA?
> 
> For system 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.
> >
> >You mean you did Lmbench with background CMA allocation?
> >Could you describe in detail?
> 
> I'm sorry not to mention the background.
> I did the test without CMA allocation because I wanted to check how it affects system performance.
> 
> The first test, invalidating entire lru, is adding invalidate_bh_lrus() at alloc_contig_range().
> This is not affecting system performance because alloc_contig_range() is not called
> for usual file-system management.
> The resulf of the first test is the *default system performance.*
> 
> The second test, dropping all bh in lru, is adding drop_buffers().
> Every call of drop_buffers drops all bhs in lru of every cpu.
> It can affect system performance. *But* it does not affect system performance,
> because it drops only bh of migrated pages.
> 
> 
> >
> >>
> >>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.
> >
> >Not sure we can conclude like that.
> >
> >A few weeks ago, I saw a patch which increases bh_lrus's size.
> >https://lkml.org/lkml/2014/7/4/107
> >IOW, some of workloads really affects by percpu bh_lrus so it would be
> >better to be careful to drain, I think.
> >
> >You want to argue CMA allocation is rare so the cost is marginable.
> >It might but some of usecase might call it frequently with small request
> >(ie, 8K, 16K).
> >
> >Anyway, why cannot CMA have the cost without affecting other subsystem?
> >I mean it's okay for CMA to consume more time to shoot out the bh
> >instead of simple all bh_lru invalidation because big order allocation is
> >kinds of slow thing in the VM and everybody already know that and even
> >sometime get failed so it's okay to add more code that extremly slow path.
> 
> There are 2 reasons to invalidate entire bh_lru.
> 
> 1. I think CMA allocation is very rare so that invalidaing bh_lru affects the system little.
> How do you think about it? My platform does not call CMA allocation often.
> Is the CMA allocation or Memory-Hotplug called often?

It depends on usecase and you couldn't assume anyting because we couldn't
ask every people in the world. "Please ask to us whenever you try to use CMA".

The key point is how the patch is maintainable.
If it's too complicate to maintain, maybe we could go with simple solution
but if it's not too complicate, we can go with more smart thing to consider
other cases in future. Why not?

Another point is that how user can detect where the regression is from.
If we cannot notice the regression, it's not a good idea to go with simple
version.

> 
> 2. Adding code in drop_buffers() can affect the system more that adding code in alloc_contig_range()
> because the drop_buffers does not have a way to distinguish migrate type.
> Even-though the lmbech results that it has almost the same performance.
> But I am afraid that it can be changed.
> As you said if bh_lru size can be changed it affects more than now.
> SO I do not want to touch non-CMA related code.

I'm not saying to add hook in drop_buffers.
What I suggest is to handle failure by bh_lrus in migrate_pages
because it's not a problem only in CMA.
There is already retry logic in migrate_pages so I can think you could
handle it.

> 
> 
> >
> >>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@lge.com>
> >>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@lge.com>
> >>Acked-by: Laura Abbott <lauraa@codeaurora.org>
> >>---
> >>  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)
> >>+               invalidate_bh_lrus();
> >>+
> >
> >Q1. It's a only CMA problem? Memory-Hotplug is not a problem? Or other places?
> >
> >I mean it would be better to handle in generic way.
> 
> Only CMA and Memory-Hotplug needs it.

Memory-hotplug uses alloc_contig_range?
You are adding the logic in alloc_contig_range and it is used for
hugetlb and cma.

> And I think invalidate_bh_lrus() is general.

It couldn't handle memory-hotplug.

> 
> >
> >Q2. Why do you call it right before calling __alloc_contig_migrate_range?
> >
> >Some of pages will go bh_lrus by __alloc_contig_migrate_ranges.
> >In that case, it is useless without caller's retry logic.
> >Even you do it from caller's retrial logic, it's not a good idea because
> >you makes new binding alloc_contig_range and uppder layer.
> >
> >So, IMHO, it would be better to handle it in migrate_pages.
> >Maybe we could define new API try_to_drop_buffers which calls
> >try_to_free_buffers and then only if the function fails due to
> >percpu lru count, we could drain only the bh in percpu lru list instead of
> >all bh draining. And places in migration path should use it rather than
> >try_to_relese_page.
> >
> >But the problem from this approach invents new API which should be
> >maintained so not sure Andrew think it's worth.
> >Maybe we should see the code and diffstat.
> 
> I also consider to making new function, drop_bh_of_migrate_page in migrate_page(), just before unmap_and_move().
> The migrate_page() has an argument reason that distinguish migrate-type, MR_CMA or MR_MEMORY_HOTPLUG or others.

Yes, that's what I suggested. If you see -EAGIN, maybe you could do it.
Even, we could enhance it with extending target bh invalidation instead of
all bhs invalidation so you could make two patches.

1. use invalidate_bh_lrus in migrate pages
2. invalidate only failed bh intead of all CPU percpu bh_blrus flushing.

So, if guys hate 2 which is rather overdesigned, we could drop 2 but 1 is
mergable still.

> 
> But I DO NOT WATN TO touch non-CMA related code.
> Current CMA and Memory-Hotplug code is not mature so that I am not sure it is ok to touch non-CMA related code for CMA/MemoryHotplug.
> 
> My point is:
> 1. CMA/Memory-hotplug is rare and invalidating bh-lru is also rare.
> 2. Only change CMA/Memory-hotplig related code.
> 
> >
> >Overenginnering?
> >
> >>         ret = __alloc_contig_migrate_range(&cc, start, end);
> >>         if (ret)
> >>                 goto done;
> >>--
> >>1.7.9.5
> >>
> >>--
> >>To unsubscribe, send a message with 'unsubscribe linux-mm' in
> >>the body to majordomo@kvack.org.  For more info on Linux MM,
> >>see: http://www.linux-mm.org/ .
> >>Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
> >
> 
> --
> To unsubscribe, send a message with 'unsubscribe linux-mm' in
> the body to majordomo@kvack.org.  For more info on Linux MM,
> see: http://www.linux-mm.org/ .
> Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>

-- 
Kind regards,
Minchan Kim

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
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Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>

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

[Minchan Kim]

On Mon, Jul 21, 2014 at 04:36:51PM +0900, Minchan Kim wrote:
> On Mon, Jul 21, 2014 at 03:16:10PM +0900, Gioh Kim wrote:
> > 
> > 
> > 2014-07-21 i??i ? 11:50, Minchan Kim i?' e,?:
> > >Hi Gioh,
> > >
> > >On Fri, Jul 18, 2014 at 03:45:36PM +0900, Gioh Kim wrote:
> > >>
> > >>Hi,
> > >>
> > >>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.
> > >
> > >Just nit:
> > >Please write *problem* in description instead of URL link.
> > >
> > >>
> > >>I have two solution to drop bhs.
> > >>One is invalidating entire lru.
> > >
> > >You mean? All of percpu bh_lrus so if the system has N cpu,
> > >it invalidates N * 8?
> > 
> > Yes, every bh_lru of all cpus.
> > 
> > >
> > >>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.
> > >
> > >For whom? system or requestor of CMA?
> > 
> > For system 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.
> > >
> > >You mean you did Lmbench with background CMA allocation?
> > >Could you describe in detail?
> > 
> > I'm sorry not to mention the background.
> > I did the test without CMA allocation because I wanted to check how it affects system performance.
> > 
> > The first test, invalidating entire lru, is adding invalidate_bh_lrus() at alloc_contig_range().
> > This is not affecting system performance because alloc_contig_range() is not called
> > for usual file-system management.
> > The resulf of the first test is the *default system performance.*
> > 
> > The second test, dropping all bh in lru, is adding drop_buffers().
> > Every call of drop_buffers drops all bhs in lru of every cpu.
> > It can affect system performance. *But* it does not affect system performance,
> > because it drops only bh of migrated pages.
> > 
> > 
> > >
> > >>
> > >>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.
> > >
> > >Not sure we can conclude like that.
> > >
> > >A few weeks ago, I saw a patch which increases bh_lrus's size.
> > >https://lkml.org/lkml/2014/7/4/107
> > >IOW, some of workloads really affects by percpu bh_lrus so it would be
> > >better to be careful to drain, I think.
> > >
> > >You want to argue CMA allocation is rare so the cost is marginable.
> > >It might but some of usecase might call it frequently with small request
> > >(ie, 8K, 16K).
> > >
> > >Anyway, why cannot CMA have the cost without affecting other subsystem?
> > >I mean it's okay for CMA to consume more time to shoot out the bh
> > >instead of simple all bh_lru invalidation because big order allocation is
> > >kinds of slow thing in the VM and everybody already know that and even
> > >sometime get failed so it's okay to add more code that extremly slow path.
> > 
> > There are 2 reasons to invalidate entire bh_lru.
> > 
> > 1. I think CMA allocation is very rare so that invalidaing bh_lru affects the system little.
> > How do you think about it? My platform does not call CMA allocation often.
> > Is the CMA allocation or Memory-Hotplug called often?
> 
> It depends on usecase and you couldn't assume anyting because we couldn't
> ask every people in the world. "Please ask to us whenever you try to use CMA".
> 
> The key point is how the patch is maintainable.
> If it's too complicate to maintain, maybe we could go with simple solution
> but if it's not too complicate, we can go with more smart thing to consider
> other cases in future. Why not?
> 
> Another point is that how user can detect where the regression is from.
> If we cannot notice the regression, it's not a good idea to go with simple
> version.
> 
> > 
> > 2. Adding code in drop_buffers() can affect the system more that adding code in alloc_contig_range()
> > because the drop_buffers does not have a way to distinguish migrate type.
> > Even-though the lmbech results that it has almost the same performance.
> > But I am afraid that it can be changed.
> > As you said if bh_lru size can be changed it affects more than now.
> > SO I do not want to touch non-CMA related code.
> 
> I'm not saying to add hook in drop_buffers.
> What I suggest is to handle failure by bh_lrus in migrate_pages
> because it's not a problem only in CMA.
> There is already retry logic in migrate_pages so I can think you could
> handle it.
> 
> > 
> > 
> > >
> > >>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@lge.com>
> > >>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@lge.com>
> > >>Acked-by: Laura Abbott <lauraa@codeaurora.org>
> > >>---
> > >>  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)
> > >>+               invalidate_bh_lrus();
> > >>+
> > >
> > >Q1. It's a only CMA problem? Memory-Hotplug is not a problem? Or other places?
> > >
> > >I mean it would be better to handle in generic way.
> > 
> > Only CMA and Memory-Hotplug needs it.
> 
> Memory-hotplug uses alloc_contig_range?
> You are adding the logic in alloc_contig_range and it is used for
> hugetlb and cma.
> 
> > And I think invalidate_bh_lrus() is general.
> 
> It couldn't handle memory-hotplug.
> 
> > 
> > >
> > >Q2. Why do you call it right before calling __alloc_contig_migrate_range?
> > >
> > >Some of pages will go bh_lrus by __alloc_contig_migrate_ranges.
> > >In that case, it is useless without caller's retry logic.
> > >Even you do it from caller's retrial logic, it's not a good idea because
> > >you makes new binding alloc_contig_range and uppder layer.
> > >
> > >So, IMHO, it would be better to handle it in migrate_pages.
> > >Maybe we could define new API try_to_drop_buffers which calls
> > >try_to_free_buffers and then only if the function fails due to
> > >percpu lru count, we could drain only the bh in percpu lru list instead of
> > >all bh draining. And places in migration path should use it rather than
> > >try_to_relese_page.
> > >
> > >But the problem from this approach invents new API which should be
> > >maintained so not sure Andrew think it's worth.
> > >Maybe we should see the code and diffstat.
> > 
> > I also consider to making new function, drop_bh_of_migrate_page in migrate_page(), just before unmap_and_move().
> > The migrate_page() has an argument reason that distinguish migrate-type, MR_CMA or MR_MEMORY_HOTPLUG or others.
> 
> Yes, that's what I suggested. If you see -EAGIN, maybe you could do it.
> Even, we could enhance it with extending target bh invalidation instead of
> all bhs invalidation so you could make two patches.
> 
> 1. use invalidate_bh_lrus in migrate pages
> 2. invalidate only failed bh intead of all CPU percpu bh_blrus flushing.

Otherwise,
  2-1. create try_to_drop_buffers and use it in migration path intead of
       try_to_release_buffers.

> 
> So, if guys hate 2 which is rather overdesigned, we could drop 2 but 1 is
> mergable still.
> 
> > 
> > But I DO NOT WATN TO touch non-CMA related code.
> > Current CMA and Memory-Hotplug code is not mature so that I am not sure it is ok to touch non-CMA related code for CMA/MemoryHotplug.
> > 
> > My point is:
> > 1. CMA/Memory-hotplug is rare and invalidating bh-lru is also rare.
> > 2. Only change CMA/Memory-hotplig related code.
> > 
> > >
> > >Overenginnering?
> > >
> > >>         ret = __alloc_contig_migrate_range(&cc, start, end);
> > >>         if (ret)
> > >>                 goto done;
> > >>--
> > >>1.7.9.5
> > >>
> > >>--
> > >>To unsubscribe, send a message with 'unsubscribe linux-mm' in
> > >>the body to majordomo@kvack.org.  For more info on Linux MM,
> > >>see: http://www.linux-mm.org/ .
> > >>Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
> > >
> > 
> > --
> > To unsubscribe, send a message with 'unsubscribe linux-mm' in
> > the body to majordomo@kvack.org.  For more info on Linux MM,
> > see: http://www.linux-mm.org/ .
> > Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
> 
> -- 
> Kind regards,
> Minchan Kim
> 
> --
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Minchan Kim

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

[Mel Gorman]

On Mon, Jul 21, 2014 at 04:36:51PM +0900, Minchan Kim wrote:

I'm not reviewing this in detail at all, didn't even look at the patch
but two things popped out at me during the discussion.

> > >Anyway, why cannot CMA have the cost without affecting other subsystem?
> > >I mean it's okay for CMA to consume more time to shoot out the bh
> > >instead of simple all bh_lru invalidation because big order allocation is
> > >kinds of slow thing in the VM and everybody already know that and even
> > >sometime get failed so it's okay to add more code that extremly slow path.
> > 
> > There are 2 reasons to invalidate entire bh_lru.
> > 
> > 1. I think CMA allocation is very rare so that invalidaing bh_lru affects the system little.
> > How do you think about it? My platform does not call CMA allocation often.
> > Is the CMA allocation or Memory-Hotplug called often?
> 
> It depends on usecase and you couldn't assume anyting because we couldn't
> ask every people in the world. "Please ask to us whenever you try to use CMA".
> 
> The key point is how the patch is maintainable.
> If it's too complicate to maintain, maybe we could go with simple solution
> but if it's not too complicate, we can go with more smart thing to consider
> other cases in future. Why not?
> 
> Another point is that how user can detect where the regression is from.
> If we cannot notice the regression, it's not a good idea to go with simple
> version.
> 

The buffer LRU avoids a lookup of a radix tree. If the LRU hit rate is
low then the performance penalty of repeated radix tree lookups is
severe but the cost of missing one hot lookup because CMA invalidate it
is not.

The real cost to be concerned with is the cost of performing the
invalidation not the fact a lookup in the LRU was missed. It's because
the cost of invalidation is high that this is being pushed to CMA because
for CMA an allocation failure can be a functional failure and not just a
performance problem.

> > 
> > 2. Adding code in drop_buffers() can affect the system more that adding code in alloc_contig_range()
> > because the drop_buffers does not have a way to distinguish migrate type.
> > Even-though the lmbech results that it has almost the same performance.
> > But I am afraid that it can be changed.
> > As you said if bh_lru size can be changed it affects more than now.
> > SO I do not want to touch non-CMA related code.
> 
> I'm not saying to add hook in drop_buffers.
> What I suggest is to handle failure by bh_lrus in migrate_pages
> because it's not a problem only in CMA.

No, please do not insert a global IPI to invalidate buffer heads in the
general migration case. It's too expensive for either THP allocations or
automatic NUMA migrates. The global IPI cost is justified for rare events
where it causes functional problems if it fails to migreate -- CMA, memory
hot-remove, memory poisoning etc.

-- 
Mel Gorman
SUSE Labs

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

[Minchan Kim]

Hello Mel,

On Mon, Jul 21, 2014 at 02:01:46PM +0100, Mel Gorman wrote:
> On Mon, Jul 21, 2014 at 04:36:51PM +0900, Minchan Kim wrote:
> 
> I'm not reviewing this in detail at all, didn't even look at the patch
> but two things popped out at me during the discussion.
> 
> > > >Anyway, why cannot CMA have the cost without affecting other subsystem?
> > > >I mean it's okay for CMA to consume more time to shoot out the bh
> > > >instead of simple all bh_lru invalidation because big order allocation is
> > > >kinds of slow thing in the VM and everybody already know that and even
> > > >sometime get failed so it's okay to add more code that extremly slow path.
> > > 
> > > There are 2 reasons to invalidate entire bh_lru.
> > > 
> > > 1. I think CMA allocation is very rare so that invalidaing bh_lru affects the system little.
> > > How do you think about it? My platform does not call CMA allocation often.
> > > Is the CMA allocation or Memory-Hotplug called often?
> > 
> > It depends on usecase and you couldn't assume anyting because we couldn't
> > ask every people in the world. "Please ask to us whenever you try to use CMA".
> > 
> > The key point is how the patch is maintainable.
> > If it's too complicate to maintain, maybe we could go with simple solution
> > but if it's not too complicate, we can go with more smart thing to consider
> > other cases in future. Why not?
> > 
> > Another point is that how user can detect where the regression is from.
> > If we cannot notice the regression, it's not a good idea to go with simple
> > version.
> > 
> 
> The buffer LRU avoids a lookup of a radix tree. If the LRU hit rate is
> low then the performance penalty of repeated radix tree lookups is
> severe but the cost of missing one hot lookup because CMA invalidate it
> is not.
> 
> The real cost to be concerned with is the cost of performing the
> invalidation not the fact a lookup in the LRU was missed. It's because
> the cost of invalidation is high that this is being pushed to CMA because
> for CMA an allocation failure can be a functional failure and not just a
> performance problem.
> 
> > > 
> > > 2. Adding code in drop_buffers() can affect the system more that adding code in alloc_contig_range()
> > > because the drop_buffers does not have a way to distinguish migrate type.
> > > Even-though the lmbech results that it has almost the same performance.
> > > But I am afraid that it can be changed.
> > > As you said if bh_lru size can be changed it affects more than now.
> > > SO I do not want to touch non-CMA related code.
> > 
> > I'm not saying to add hook in drop_buffers.
> > What I suggest is to handle failure by bh_lrus in migrate_pages
> > because it's not a problem only in CMA.
> 
> No, please do not insert a global IPI to invalidate buffer heads in the
> general migration case. It's too expensive for either THP allocations or
> automatic NUMA migrates. The global IPI cost is justified for rare events
> where it causes functional problems if it fails to migreate -- CMA, memory
> hot-remove, memory poisoning etc.

I didn't want to add that flushing in migrate_pages *unconditionlly*.
Please, look at this patch. It fixes only CMA although it's an issue
for others. Even, it depends on retry logic of upper layer of
alloc_contig_range but even cma_alloc(ie, upper layer of alloc_contig_range)
doesn't have retry logic. :(
That's why I suggested it in migrate_pages.

Actually, I'd like to go with making migrate_pages's user blind on pcp
draining stuff by squeezing that inside migrate_pages.
IOW, current users of migrate pages don't need to be aware of per-cpu
draining. What they should know is just they should use MIGRATE_SYNC
for best effort but costly opeartion.

For implemenation, we could use retry logic in migrate_pages.

int migrate_pages(xxx)
{
        for (pass = 0; pass < 10 && retry; pass++)
                if (retry && pass > 2 && mode == MIGRATE_SYNC)
                        flush_all_of_percpu_stuff();
}

migrate_page has migrate_mode and retry logic with 'pass', even
reason if we want ot filter out MR_CMA|MEMORY_HOTPLUG|MR_MEMORY_FAILURE.
so that we could handle all of things inside migrate_pages.

Normally, MIGRATE_SYNC would be expensive operation and mostly
it is used for CMA, memory-hotplug, memory-poisoning so THP and
automatic NUMA cannot affect so I believe adding IPI to that is not
a big problem in such trouble condition(ie, retry && pass > 2).

> 
> -- 
> Mel Gorman
> SUSE Labs
> 
> --
> To unsubscribe, send a message with 'unsubscribe linux-mm' in
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-- 
Kind regards,
Minchan Kim

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

[Gioh Kim]

2014-07-22 i??i ? 9:15, Minchan Kim i?' e,?:
> Hello Mel,
>
> On Mon, Jul 21, 2014 at 02:01:46PM +0100, Mel Gorman wrote:
>> On Mon, Jul 21, 2014 at 04:36:51PM +0900, Minchan Kim wrote:
>>
>> I'm not reviewing this in detail at all, didn't even look at the patch
>> but two things popped out at me during the discussion.
>>
>>>>> Anyway, why cannot CMA have the cost without affecting other subsystem?
>>>>> I mean it's okay for CMA to consume more time to shoot out the bh
>>>>> instead of simple all bh_lru invalidation because big order allocation is
>>>>> kinds of slow thing in the VM and everybody already know that and even
>>>>> sometime get failed so it's okay to add more code that extremly slow path.
>>>>
>>>> There are 2 reasons to invalidate entire bh_lru.
>>>>
>>>> 1. I think CMA allocation is very rare so that invalidaing bh_lru affects the system little.
>>>> How do you think about it? My platform does not call CMA allocation often.
>>>> Is the CMA allocation or Memory-Hotplug called often?
>>>
>>> It depends on usecase and you couldn't assume anyting because we couldn't
>>> ask every people in the world. "Please ask to us whenever you try to use CMA".
>>>
>>> The key point is how the patch is maintainable.
>>> If it's too complicate to maintain, maybe we could go with simple solution
>>> but if it's not too complicate, we can go with more smart thing to consider
>>> other cases in future. Why not?
>>>
>>> Another point is that how user can detect where the regression is from.
>>> If we cannot notice the regression, it's not a good idea to go with simple
>>> version.
>>>
>>
>> The buffer LRU avoids a lookup of a radix tree. If the LRU hit rate is
>> low then the performance penalty of repeated radix tree lookups is
>> severe but the cost of missing one hot lookup because CMA invalidate it
>> is not.
>>
>> The real cost to be concerned with is the cost of performing the
>> invalidation not the fact a lookup in the LRU was missed. It's because
>> the cost of invalidation is high that this is being pushed to CMA because
>> for CMA an allocation failure can be a functional failure and not just a
>> performance problem.
>>
>>>>
>>>> 2. Adding code in drop_buffers() can affect the system more that adding code in alloc_contig_range()
>>>> because the drop_buffers does not have a way to distinguish migrate type.
>>>> Even-though the lmbech results that it has almost the same performance.
>>>> But I am afraid that it can be changed.
>>>> As you said if bh_lru size can be changed it affects more than now.
>>>> SO I do not want to touch non-CMA related code.
>>>
>>> I'm not saying to add hook in drop_buffers.
>>> What I suggest is to handle failure by bh_lrus in migrate_pages
>>> because it's not a problem only in CMA.
>>
>> No, please do not insert a global IPI to invalidate buffer heads in the
>> general migration case. It's too expensive for either THP allocations or
>> automatic NUMA migrates. The global IPI cost is justified for rare events
>> where it causes functional problems if it fails to migreate -- CMA, memory
>> hot-remove, memory poisoning etc.
>
> I didn't want to add that flushing in migrate_pages *unconditionlly*.
> Please, look at this patch. It fixes only CMA although it's an issue
> for others. Even, it depends on retry logic of upper layer of
> alloc_contig_range but even cma_alloc(ie, upper layer of alloc_contig_range)
> doesn't have retry logic. :(
> That's why I suggested it in migrate_pages.
>
> Actually, I'd like to go with making migrate_pages's user blind on pcp
> draining stuff by squeezing that inside migrate_pages.
> IOW, current users of migrate pages don't need to be aware of per-cpu
> draining. What they should know is just they should use MIGRATE_SYNC
> for best effort but costly opeartion.
>
> For implemenation, we could use retry logic in migrate_pages.
>
> int migrate_pages(xxx)
> {
>          for (pass = 0; pass < 10 && retry; pass++)
>                  if (retry && pass > 2 && mode == MIGRATE_SYNC)
>                          flush_all_of_percpu_stuff();
> }
>
> migrate_page has migrate_mode and retry logic with 'pass', even
> reason if we want ot filter out MR_CMA|MEMORY_HOTPLUG|MR_MEMORY_FAILURE.
> so that we could handle all of things inside migrate_pages.
>
> Normally, MIGRATE_SYNC would be expensive operation and mostly
> it is used for CMA, memory-hotplug, memory-poisoning so THP and
> automatic NUMA cannot affect so I believe adding IPI to that is not
> a big problem in such trouble condition(ie, retry && pass > 2).


I agree Minchan's point.
I am not sure it is ok to touch the common code such as migrate_pages().

If Mel agrees, I am going to report another patch of flush_all_of_percpu_stuff() like following:

flush_all_of_percpu_stuff()
{
	drop_only_bh_of_migrating_page();
	lru_add_drain_all();
	drain_all_pages();
}

And remove lru_add_drain_all() and drain_all_pages() in CMA/HOTPLUG codes.



>
>>
>> --
>> Mel Gorman
>> SUSE Labs
>>
>> --
>> To unsubscribe, send a message with 'unsubscribe linux-mm' in
>> the body to majordomo@kvack.org.  For more info on Linux MM,
>> see: http://www.linux-mm.org/ .
>> Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
>

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

[Gioh Kim]

2014-07-22 i??i ? 10:04, Gioh Kim i?' e,?:
>
>
> 2014-07-22 i??i ? 9:15, Minchan Kim i?' e,?:
>> Hello Mel,
>>
>> On Mon, Jul 21, 2014 at 02:01:46PM +0100, Mel Gorman wrote:
>>> On Mon, Jul 21, 2014 at 04:36:51PM +0900, Minchan Kim wrote:
>>>
>>> I'm not reviewing this in detail at all, didn't even look at the patch
>>> but two things popped out at me during the discussion.
>>>
>>>>>> Anyway, why cannot CMA have the cost without affecting other subsystem?
>>>>>> I mean it's okay for CMA to consume more time to shoot out the bh
>>>>>> instead of simple all bh_lru invalidation because big order allocation is
>>>>>> kinds of slow thing in the VM and everybody already know that and even
>>>>>> sometime get failed so it's okay to add more code that extremly slow path.
>>>>>
>>>>> There are 2 reasons to invalidate entire bh_lru.
>>>>>
>>>>> 1. I think CMA allocation is very rare so that invalidaing bh_lru affects the system little.
>>>>> How do you think about it? My platform does not call CMA allocation often.
>>>>> Is the CMA allocation or Memory-Hotplug called often?
>>>>
>>>> It depends on usecase and you couldn't assume anyting because we couldn't
>>>> ask every people in the world. "Please ask to us whenever you try to use CMA".
>>>>
>>>> The key point is how the patch is maintainable.
>>>> If it's too complicate to maintain, maybe we could go with simple solution
>>>> but if it's not too complicate, we can go with more smart thing to consider
>>>> other cases in future. Why not?
>>>>
>>>> Another point is that how user can detect where the regression is from.
>>>> If we cannot notice the regression, it's not a good idea to go with simple
>>>> version.
>>>>
>>>
>>> The buffer LRU avoids a lookup of a radix tree. If the LRU hit rate is
>>> low then the performance penalty of repeated radix tree lookups is
>>> severe but the cost of missing one hot lookup because CMA invalidate it
>>> is not.
>>>
>>> The real cost to be concerned with is the cost of performing the
>>> invalidation not the fact a lookup in the LRU was missed. It's because
>>> the cost of invalidation is high that this is being pushed to CMA because
>>> for CMA an allocation failure can be a functional failure and not just a
>>> performance problem.
>>>
>>>>>
>>>>> 2. Adding code in drop_buffers() can affect the system more that adding code in alloc_contig_range()
>>>>> because the drop_buffers does not have a way to distinguish migrate type.
>>>>> Even-though the lmbech results that it has almost the same performance.
>>>>> But I am afraid that it can be changed.
>>>>> As you said if bh_lru size can be changed it affects more than now.
>>>>> SO I do not want to touch non-CMA related code.
>>>>
>>>> I'm not saying to add hook in drop_buffers.
>>>> What I suggest is to handle failure by bh_lrus in migrate_pages
>>>> because it's not a problem only in CMA.
>>>
>>> No, please do not insert a global IPI to invalidate buffer heads in the
>>> general migration case. It's too expensive for either THP allocations or
>>> automatic NUMA migrates. The global IPI cost is justified for rare events
>>> where it causes functional problems if it fails to migreate -- CMA, memory
>>> hot-remove, memory poisoning etc.
>>
>> I didn't want to add that flushing in migrate_pages *unconditionlly*.
>> Please, look at this patch. It fixes only CMA although it's an issue
>> for others. Even, it depends on retry logic of upper layer of
>> alloc_contig_range but even cma_alloc(ie, upper layer of alloc_contig_range)
>> doesn't have retry logic. :(
>> That's why I suggested it in migrate_pages.
>>
>> Actually, I'd like to go with making migrate_pages's user blind on pcp
>> draining stuff by squeezing that inside migrate_pages.
>> IOW, current users of migrate pages don't need to be aware of per-cpu
>> draining. What they should know is just they should use MIGRATE_SYNC
>> for best effort but costly opeartion.
>>
>> For implemenation, we could use retry logic in migrate_pages.
>>
>> int migrate_pages(xxx)
>> {
>>          for (pass = 0; pass < 10 && retry; pass++)
>>                  if (retry && pass > 2 && mode == MIGRATE_SYNC)
>>                          flush_all_of_percpu_stuff();
>> }
>>
>> migrate_page has migrate_mode and retry logic with 'pass', even
>> reason if we want ot filter out MR_CMA|MEMORY_HOTPLUG|MR_MEMORY_FAILURE.
>> so that we could handle all of things inside migrate_pages.
>>
>> Normally, MIGRATE_SYNC would be expensive operation and mostly
>> it is used for CMA, memory-hotplug, memory-poisoning so THP and
>> automatic NUMA cannot affect so I believe adding IPI to that is not
>> a big problem in such trouble condition(ie, retry && pass > 2).
>
>
> I agree Minchan's point.
> I am not sure it is ok to touch the common code such as migrate_pages().
>
> If Mel agrees, I am going to report another patch of flush_all_of_percpu_stuff() like following:
>
> flush_all_of_percpu_stuff()
> {
>      drop_only_bh_of_migrating_page();
>      lru_add_drain_all();
>      drain_all_pages();
> }
>
> And remove lru_add_drain_all() and drain_all_pages() in CMA/HOTPLUG codes.


First things first.

I think the first step is making CMA/HOTPLUG work.
I'm going to make v2 patch that inserts invalidate_bh_lrus() in both of CMA and HOTPLUG.

Minchan's idea can be applied later.



>
>
>
>>
>>>
>>> --
>>> Mel Gorman
>>> SUSE Labs
>>>
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