Re: Accounting problem of MIGRATE_ISOLATED freed page

[Minchan Kim <minchan@kernel.org>]

On 06/21/2012 05:19 AM, KOSAKI Motohiro wrote:

> (6/20/12 3:53 AM), Minchan Kim wrote:
>> On 06/20/2012 03:32 PM, KOSAKI Motohiro wrote:
>>
>>> (6/20/12 2:12 AM), Minchan Kim wrote:
>>>>
>>>> Hi Aaditya,
>>>>
>>>> I want to discuss this problem on another thread.
>>>>
>>>> On 06/19/2012 10:18 PM, Aaditya Kumar wrote:
>>>>> On Mon, Jun 18, 2012 at 6:13 AM, Minchan Kim <minchan@kernel.org> wrote:
>>>>>> On 06/17/2012 02:48 AM, Aaditya Kumar wrote:
>>>>>>
>>>>>>> On Fri, Jun 15, 2012 at 12:57 PM, Minchan Kim <minchan@kernel.org> wrote:
>>>>>>>
>>>>>>>>>
>>>>>>>>> pgdat_balanced() doesn't recognized zone. Therefore kswapd may sleep
>>>>>>>>> if node has multiple zones. Hm ok, I realized my descriptions was
>>>>>>>>> slightly misleading. priority 0 is not needed. bakance_pddat() calls
>>>>>>>>> pgdat_balanced()
>>>>>>>>> every priority. Most easy case is, movable zone has a lot of free pages and
>>>>>>>>> normal zone has no reclaimable page.
>>>>>>>>>
>>>>>>>>> btw, current pgdat_balanced() logic seems not correct. kswapd should
>>>>>>>>> sleep only if every zones have much free pages than high water mark
>>>>>>>>> _and_ 25% of present pages in node are free.
>>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>> Sorry. I can't understand your point.
>>>>>>>> Current kswapd doesn't sleep if relevant zones don't have free pages above high watermark.
>>>>>>>> It seems I am missing your point.
>>>>>>>> Please anybody correct me.
>>>>>>>
>>>>>>> Since currently direct reclaim is given up based on
>>>>>>> zone->all_unreclaimable flag,
>>>>>>> so for e.g in one of the scenarios:
>>>>>>>
>>>>>>> Lets say system has one node with two zones (NORMAL and MOVABLE) and we
>>>>>>> hot-remove the all the pages of the MOVABLE zone.
>>>>>>>
>>>>>>> While migrating pages during memory hot-unplugging, the allocation function
>>>>>>> (for new page to which the page in MOVABLE zone would be moved)  can end up
>>>>>>> looping in direct reclaim path for ever.
>>>>>>>
>>>>>>> This is so because when most of the pages in the MOVABLE zone have
>>>>>>> been migrated,
>>>>>>> the zone now contains lots of free memory (basically above low watermark)
>>>>>>> BUT all are in MIGRATE_ISOLATE list of the buddy list.
>>>>>>>
>>>>>>> So kswapd() would not balance this zone as free pages are above low watermark
>>>>>>> (but all are in isolate list). So zone->all_unreclaimable flag would
>>>>>>> never be set for this zone
>>>>>>> and allocation function would end up looping forever. (assuming the
>>>>>>> zone NORMAL is
>>>>>>> left with no reclaimable memory)
>>>>>>>
>>>>>>
>>>>>>
>>>>>> Thanks a lot, Aaditya! Scenario you mentioned makes perfect.
>>>>>> But I don't see it's a problem of kswapd.
>>>>>
>>>>> Hi Kim,
>>>>
>>>> I like called Minchan rather than Kim
>>>> Never mind. :)
>>>>
>>>>>
>>>>> Yes I agree it is not a problem of kswapd.
>>>>
>>>> Yeb.
>>>>
>>>>>
>>>>>> a5d76b54 made new migration type 'MIGRATE_ISOLATE' which is very irony type because there are many free pages in free list
>>>>>> but we can't allocate it. :(
>>>>>> It doesn't reflect right NR_FREE_PAGES while many places in the kernel use NR_FREE_PAGES to trigger some operation.
>>>>>> Kswapd is just one of them confused.
>>>>>> As right fix of this problem, we should fix hot plug code, IMHO which can fix CMA, too.
>>>>>>
>>>>>> This patch could make inconsistency between NR_FREE_PAGES and SumOf[free_area[order].nr_free]
>>>>>
>>>>>
>>>>> I assume that by the inconsistency you mention above, you mean
>>>>> temporary inconsistency.
>>>>>
>>>>> Sorry, but IMHO as for memory hot plug the main issue with this patch
>>>>> is that the inconsistency you mentioned above would NOT be a temporary
>>>>> inconsistency.
>>>>>
>>>>> Every time say 'x' number of page frames are off lined, they will
>>>>> introduce a difference of 'x' pages between
>>>>> NR_FREE_PAGES and SumOf[free_area[order].nr_free].
>>>>> (So for e.g. if we do a frequent offline/online it will make
>>>>> NR_FREE_PAGES  negative)
>>>>>
>>>>> This is so because, unset_migratetype_isolate() is called from
>>>>> offlining  code (to set the migrate type of off lined pages again back
>>>>> to MIGRATE_MOVABLE)
>>>>> after the pages have been off lined and removed from the buddy list.
>>>>> Since the pages for which unset_migratetype_isolate() is called are
>>>>> not buddy pages so move_freepages_block() does not move any page, and
>>>>> thus introducing a permanent inconsistency.
>>>>
>>>> Good point. Negative NR_FREE_PAGES is caused by double counting by my patch and __offline_isolated_pages.
>>>> I think at first MIGRATE_ISOLATE type freed page shouldn't account as free page.
>>>>
>>>>>
>>>>>> and it could make __zone_watermark_ok confuse so we might need to fix move_freepages_block itself to reflect
>>>>>> free_area[order].nr_free exactly.
>>>>>>
>>>>>> Any thought?
>>>>>
>>>>> As for fixing move_freepages_block(), At least for memory hot plug,
>>>>> the pages stay in MIGRATE_ISOLATE list only for duration
>>>>> offline_pages() function,
>>>>> I mean only temporarily. Since fixing move_freepages_block() for will
>>>>> introduce some overhead, So I am not very sure whether that overhead
>>>>> is justified
>>>>> for a temporary condition. What do you think?
>>>>
>>>> Yes. I don't like hurt fast path, either.
>>>> How about this? (Passed just compile test :(  )
>>>> The patch's goal is to NOT increase nr_free and NR_FREE_PAGES about freed page into MIGRATE_ISOLATED.
>>>>
>>>> This patch hurts high order page free path but I think it's not critical because higher order allocation
>>>> is rare than order-0 allocation and we already have done same thing on free_hot_cold_page on order-0 free path
>>>> which is more hot.
>>>
>>> Can't we change zone_water_mark_ok_safe() instead of page allocator? memory hotplug is really rare event.
>>
>>
>> +1 
>>
>> Firstly, I want to make zone_page_state(z, NR_FREE_PAGES) itself more accurately because it is used by
>> several places. As I looked over places, I can't find critical places except kswapd forever sleep case.
>> So it's a nice idea! 
>>
>> In that case, we need zone->lock whenever zone_watermark_ok_safe is called.
>> Most of cases, it's unnecessary and it might hurt alloc/free performance when memory pressure is high.
>> But if memory pressure is high, it may be already meaningless alloc/free performance.
>> So it does make sense, IMHO.
>>
>> Please raise your hands if anyone has a concern about this.
>>
>> barrios@bbox:~/linux-next$ git diff
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index d2a515d..82cc0a2 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -1748,16 +1748,38 @@ bool zone_watermark_ok(struct zone *z, int order, unsigned long mark,
>>                                         zone_page_state(z, NR_FREE_PAGES));
>>  }
>>  
>> -bool zone_watermark_ok_safe(struct zone *z, int order, unsigned long mark,
>> +bool zone_watermark_ok_safe(struct zone *z, int alloc_order, unsigned long mark,
>>                       int classzone_idx, int alloc_flags)
>>  {
>> +       struct free_area *area;
>> +       struct list_head *curr;
>> +       int order;
>> +       unsigned long flags;
>>         long free_pages = zone_page_state(z, NR_FREE_PAGES);
>>  
>>         if (z->percpu_drift_mark && free_pages < z->percpu_drift_mark)
>>                 free_pages = zone_page_state_snapshot(z, NR_FREE_PAGES);
>>  
>> -       return __zone_watermark_ok(z, order, mark, classzone_idx, alloc_flags,
>> -                                                               free_pages);
>> +       /*
>> +        * Memory hotplug/CMA can isolate freed page into MIGRATE_ISOLATE
>> +        * so that buddy can't allocate it although they are in free list.
>> +        */
>> +       spin_lock_irqsave(&z->lock, flags);
>> +       for (order = 0; order < MAX_ORDER; order++) {
>> +               int count = 0;
>> +               area = &(z->free_area[order]);
>> +               if (unlikely(!list_empty(&area->free_list[MIGRATE_ISOLATE]))) {
>> +                       list_for_each(curr, &area->free_list[MIGRATE_ISOLATE])
>> +                               count++;
>> +                       free_pages -= (count << order);
>> +               }
>> +       }
>> +       if (free_pages < 0)
>> +               free_pages = 0;
>> +       spin_unlock_irqrestore(&z->lock, flags);
>> +
>> +       return __zone_watermark_ok(z, alloc_order, mark,
>> +                               classzone_idx, alloc_flags, free_pages);
>>  }
> 
> number of isolate page block is almost always 0. then if we have such counter,
> we almost always can avoid zone->lock. Just idea.


Yeb. I thought about it but unfortunately we can't have a counter for MIGRATE_ISOLATE.
Because we have to tweak in page free path for pages which are going to free later after we
mark pageblock type to MIGRATE_ISOLATE.


-- 
Kind regards,
Minchan Kim

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