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

[Gioh Kim <gioh.kim@lge.com>]

2014-07-22 오전 10:04, Gioh Kim 쓴 글:
>
>
> 2014-07-22 오전 9:15, Minchan Kim 쓴 글:
>> 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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>>

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