Re: [PATCH -mm v2] mm: have order > 0 compaction start off where it left

[Minchan Kim <minchan@kernel.org>]

Hi Rik,

On 07/03/2012 11:59 PM, Rik van Riel wrote:

> On 06/28/2012 07:27 PM, Minchan Kim wrote:
> 
>>> index 7ea259d..2668b77 100644
>>> --- a/mm/compaction.c
>>> +++ b/mm/compaction.c
>>> @@ -422,6 +422,17 @@ static void isolate_freepages(struct zone *zone,
>>>                       pfn -= pageblock_nr_pages) {
>>>           unsigned long isolated;
>>>
>>> +        /*
>>> +         * Skip ahead if another thread is compacting in the area
>>> +         * simultaneously. If we wrapped around, we can only skip
>>> +         * ahead if zone->compact_cached_free_pfn also wrapped to
>>> +         * above our starting point.
>>> +         */
>>> +        if (cc->order>  0&&  (!cc->wrapped ||
>>
>>
>> So if (partial_compaction(cc)&&  ... ) or if (!full_compaction(cc)&&  
>> ...
> 
> I am not sure that we want to abstract away what is happening
> here.  We also are quite explicit with the meaning of cc->order
> in compact_finished and other places in the compaction code.
> 
>>> +                      zone->compact_cached_free_pfn>
>>> +                      cc->start_free_pfn))
>>> +            pfn = min(pfn, zone->compact_cached_free_pfn);
>>
>>
>> The pfn can be where migrate_pfn below?
>> I mean we need this?
>>
>> if (pfn<= low_pfn)
>>     goto out;
> 
> That is a good point. I guess there is a small possibility that
> another compaction thread is below us with cc->free_pfn and
> cc->migrate_pfn, and we just inherited its cc->free_pfn via
> zone->compact_cached_free_pfn, bringing us to below our own
> cc->migrate_pfn.
> 
> Given that this was already possible with parallel compaction
> in the past, I am not sure how important it is. It could result
> in wasting a little bit of CPU, but your fix for it looks easy
> enough.


In the past, it was impossible since we have per-compaction context free_pfn.
 

> 
> Mel, any downside to compaction bailing (well, wrapping around)
> a little earlier, like Minchan suggested?


I can't speak for Mel. But IMHO, if we meet such case, we can ignore compact_cached_free_pfn
, then go with just pfn instead of early bailing.

> 
>>> @@ -463,6 +474,8 @@ static void isolate_freepages(struct zone *zone,
>>>            */
>>>           if (isolated)
>>>               high_pfn = max(high_pfn, pfn);
>>> +        if (cc->order>  0)
>>> +            zone->compact_cached_free_pfn = high_pfn;
>>
>>
>> Why do we cache high_pfn instead of pfn?
> 
> Reading the code, because we may not have isolated every
> possible free page from this memory block.  The same reason
> cc->free_pfn is set to high_pfn right before the function
> exits.

> 

>> If we can't isolate any page, compact_cached_free_pfn would become
>> low_pfn.
>> I expect it's not what you want.
> 
> I guess we should only cache the value of high_pfn if
> we isolated some pages?  In other words, this:
> 
>     if (isolated) {
>         high_pfn = max(high_pfn, pfn);
>         if (cc->order > 0)
>             zone->compact_cached_free_pfn = high_pfn;
>     }
> 
> 


I agree.

-- 
Kind regards,
Minchan Kim

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