Re: [PATCHv7 02/14] mm: Add support for unaccepted memory

Dave Hansen
2022-08-05T14:53:55Z

On 8/5/22 07:22, David Hildenbrand wrote:
>> A list that sits besides the existing free_area, contains only >=pageblock
>> order sizes of unaccepted pages (no migratetype distinguished) and we tap
>> into it approximately before __rmqueue_fallback()? There would be some
>> trickery around releasing zone-lock for doing accept_memory(), but should be
>> manageable.
>>
> Just curious, do we have a microbenchmark that is able to reveal the
> impact of such code changes before we start worrying?

Nope. I went looking to see if I could find any impact. I think Kirill
did too. Too bad that effort didn't make it into the changelog yet.

The merging check at least is just checking a field in a cache-hot
'struct page'. The common case is probably three instructions:

load to a register
check the bit
jump if not set

It adds a wee bit of icache pressure, but it's also the kind of thing
that should be a piece of cake for the branch predictors.

That dynamic check could easily be wrapped by a static branch. But,
that first requires more code to go dig in the nooks and crannies of the
page allocator to make sure *ALL* pages are accepted.