Re: [PATCH net-next RFC 1/2] page_pool: add page recycling support based on elevated refcnt

[Yunsheng Lin <linyunsheng@huawei.com>]

On 2021/7/6 12:54, Ilias Apalodimas wrote:
> Hi Yunsheng,
> 
> Thanks for having a look!

Hi,

Thanks for reviewing.

> 
> On Fri, Jul 02, 2021 at 06:15:13PM +0800, Yunsheng Lin wrote:
>> On 2021/7/2 17:42, Jesper Dangaard Brouer wrote:
>>>
>>> On 30/06/2021 11.17, Yunsheng Lin wrote:
>>>> Currently page pool only support page recycling only when
>>>> refcnt of page is one, which means it can not support the
>>>> split page recycling implemented in the most ethernet driver.
>>>
>>> Cc. Alex Duyck as I consider him an expert in this area.
>>
>> Thanks.
>>
>>>
>>>
>>>> So add elevated refcnt support in page pool, and support
>>>> allocating page frag to enable multi-frames-per-page based
>>>> on the elevated refcnt support.
>>>>
>>>> As the elevated refcnt is per page, and there is no space
>>>> for that in "struct page" now, so add a dynamically allocated
>>>> "struct page_pool_info" to record page pool ptr and refcnt
>>>> corrsponding to a page for now. Later, we can recycle the
>>>> "struct page_pool_info" too, or use part of page memory to
>>>> record pp_info.
>>>
>>> I'm not happy with allocating a memory (slab) object "struct page_pool_info" per page.
>>>
>>> This also gives us an extra level of indirection.
>>
>> I'm not happy with that either, if there is better way to
>> avoid that, I will be happy to change it:)
> 
> I think what we have to answer here is, do we want and does it make sense
> for page_pool to do the housekeeping of the buffer splitting or are we
> better of having each driver do that.  IIRC your previous patch on top of
> the original recycling patchset was just 'atomic' refcnts on top of page pool.

You are right that driver was doing the the buffer splitting in previous
patch.

The reason why I abandoned that is:
1. Currently the meta-data of page in the driver is per desc, which means
   it might not be able to use first half of a page for a desc, and the
   second half of the same page for another desc, this ping-pong way of
   reusing the whole page for only one desc in the driver seems unnecessary
   and waste a lot of memory when there is already reusing in the page pool.

2. Easy use of API for the driver too, which means the driver uses
   page_pool_dev_alloc_frag() and page_pool_put_full_page() for elevated
   refcnt case, corresponding to page_pool_dev_alloc_pages() and
   page_pool_put_full_page() for non-elevated refcnt case, the driver does
   not need to worry about the meta-data of a page.

> 
> I think I'd prefer each driver having it's own meta-data of how he splits
> the page, mostly due to hardware diversity, but tbh I don't have any
> strong preference atm.

Usually how the driver split the page is fixed for a given rx configuration(
like MTU), so the driver is able to pass that info to page pool.


> 
>>
>>>
>>>
>>> You are also adding a page "frag" API inside page pool, which I'm not 100% convinced belongs inside page_pool APIs.
>>>
>>> Please notice the APIs that Alex Duyck added in mm/page_alloc.c:
>>
>> Actually, that is where the idea of using "page frag" come from.
>>
>> Aside from the performance improvement, there is memory usage
>> decrease for 64K page size kernel, which means a 64K page can
>> be used by 32 description with 2k buffer size, and that is a
>> lot of memory saving for 64 page size kernel comparing to the
>> current split page reusing implemented in the driver.
>>
> 
> Whether the driver or page_pool itself keeps the meta-data, the outcome
> here won't change.  We'll still be able to use page frags.

As above, it is the ping-pong way of reusing when the driver keeps the
meta-data, and it is page-frag way of reusing when the page pool keeps
the meta-data.

I am not sure if the page-frag way of reusing is possible when we still
keep the meta-data in the driver, which seems very complex at the initial
thinking.

> 
> 
> Cheers
> /Ilias
>>
>>>
>>>  __page_frag_cache_refill() + __page_frag_cache_drain() + page_frag_alloc_align()
>>>
>>>
>>
>> [...]
> .
>