net: page_pool: do not count normal frag allocation in stats

net: page_pool: do not count normal frag allocation in stats

Commit 0f6deac3a079 ("net: page_pool: add page allocation stats for
two fast page allocate path") added increments for "fast path"
allocation to page frag alloc. It mentions performance degradation
analysis but the details are unclear. Could be that the author
was simply surprised by the alloc stats not matching packet count.

In my experience the key metric for page pool is the recycling rate.
Page return stats, however, count returned _pages_ not frags.
This makes it impossible to calculate recycling rate for drivers
using the frag API. Here is example output of the page-pool
YNL sample for a driver allocating 1200B frags (4k pages)
with nearly perfect recycling:

  $ ./page-pool
    eth0[2]	page pools: 32 (zombies: 0)
		refs: 291648 bytes: 1194590208 (refs: 0 bytes: 0)
		recycling: 33.3% (alloc: 4557:2256365862 recycle: 200476245:551541893)

The recycling rate is reported as 33.3% because we give out
4096 // 1200 = 3 frags for every recycled page.

Effectively revert the aforementioned commit. This also aligns
with the stats we would see for drivers which do the fragmentation
themselves, although that's not a strong reason in itself.

On the (very unlikely) path where we can reuse the current page
let's bump the "cached" stat. The fact that we don't put the page
in the cache is just an optimization.

Acked-by: Jesper Dangaard Brouer <hawk@kernel.org>
Reviewed-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Acked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Link: https://patch.msgid.link/20241109023303.3366500-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>