On 2023/8/16 19:30, Ilias Apalodimas wrote:

On Wed, 16 Aug 2023 at 13:04, Yunsheng Lin [off-list ref] wrote:

quoted

Currently when page_pool_create() is called with
PP_FLAG_PAGE_FRAG flag, page_pool_alloc_pages() is only
allowed to be called under the below constraints:
1. page_pool_fragment_page() need to be called to setup
   page->pp_frag_count immediately.
2. page_pool_defrag_page() often need to be called to drain
   the page->pp_frag_count when there is no more user will
   be holding on to that page.

Those constraints exist in order to support a page to be
split into multi frags.

And those constraints have some overhead because of the
cache line dirtying/bouncing and atomic update.

Those constraints are unavoidable for case when we need a
page to be split into more than one frag, but there is also
case that we want to avoid the above constraints and their
overhead when a page can't be split as it can only hold a big
frag as requested by user, depending on different use cases:
use case 1: allocate page without page splitting.
use case 2: allocate page with page splitting.
use case 3: allocate page with or without page splitting
            depending on the frag size.

Currently page pool only provide page_pool_alloc_pages() and
page_pool_alloc_frag() API to enable the 1 & 2 separately,
so we can not use a combination of 1 & 2 to enable 3, it is
not possible yet because of the per page_pool flag
PP_FLAG_PAGE_FRAG.

I really think we are inventing problems to solve here.
What would be more useful here would be an example with numbers.  Most
of what you mention are true, but what % of the packets would split a
page in a way that the remaining part cant be used is unknown.  Do you
have a usecase in the hns3 driver?  Are there any numbers that justify
the change?

There is no usecase for the hns3 driver yet.

As mentioned in the cover letter, there are usecases for veth and virtio_net
to use both frag page and non frag page in order to have the best performance
with the least memory depending on the head/tail room space for xdp_frame/shinfo
and mtu/packet size.

For virtio_net case:
I am guessing that allocating the non frag page has some advantage in some
case and allocating frag page has some advantage in other case, that
is why there is module param for the switching between those two mode.
It would be good that Liang Chen can help to confirm the gusessing with some
data.

For veth case:
It would be good that Maryam(cc) can help to do some testing to justify the
change as suggested by Jesper.


For iavf case, Alexander Lobakin mentioned some data in below disscusion:
https://patchwork.kernel.org/project/netdevbpf/patch/20230612130256.4572-5-linyunsheng@huawei.com/#25384716