Re: [RFC Get rid of shrink code - memory-hotplug]

[David Hildenbrand <david@redhat.com>]

On 10.12.18 14:53, osalvador@suse.de wrote:
> On 2018-12-07 11:35, osalvador@suse.de wrote:
>> On 2018-12-07 11:32, Michal Hocko wrote:
>>> On Fri 07-12-18 10:54:50, Vlastimil Babka wrote:
>>>> Well, __pageblock_pfn_to_page() has to be called for each pageblock 
>>>> in
>>>> compaction, when zone_contiguous is false. And that's unchanged since
>>>> the introduction of zone_contiguous, so the numbers should still 
>>>> hold.
>>>
>>> OK, this means that we have to carefully re-evaluate zone_contiguous 
>>> for
>>> each offline operation.
> 
> I started to think about this but some questions arose.
> 
> Actually, no matter what we do, if we re-evaluate zone_contiguous
> in the offline operation, zone_contiguous will be left unset.
> 
> set_zone_contiguous() calls __pageblock_pfn_to_page() in
> pageblock_nr_pages chunks to determine if the zone is contiguous,
> and checks the following:
> 
> * first/end pfn of the pageblock are valid sections
> * first pfn of the pageblock is online
> * we do not intersect different zones
> * first/end pfn belong to the same zone
> 
> Now, when dropping the shrink code and re-evaluating zone_contiguous in 
> offline
> operation, set_zone_contiguous() will return false, leaving us with 
> zone_contiguous
> unset.
> 
> I wonder if we want that, or we want to stick with the optimization that
> zone_contiguous brings us.
> 
> If we do not care, dropping everything and just calling 
> clear_zone_contiguous and
> set_zone_contiguous is the right thing to go.
> But, if we want to keep the zone_contiguous optimization, I am afraid 
> that we need
> to re-adjust zone boundaries in the offline operation whenever we remove 
> first/end
> section.
> In that case, set_zone_contiguous will still keep zone_contiguous as 
> set.
> 
> So, it seems that the only headache we still have is this 
> zone_contiguous thing.
> Ideas? Suggestions?

At least for memory hot(un)plug we always online/offline complete
sections. And they all go to the same zone. However we check on
pageblock granularity. I would assume that we only have mixed zones on
one section for some corner cases sections in our system?

I wonder if we could optimize somehow differently. E.g. mark sections as
belonging completely to a zone. If not, check on pageblock granularity.

E.g. SECTION_SINGLE_ZONE

-- 

Thanks,

David / dhildenb