net/mlx5e: RX, Defer page release in striding rq for better recycling

net/mlx5e: RX, Defer page release in striding rq for better recycling

Currently, for striding RQ, fragmented pages from the page pool can
get released in two ways:

1) In the mlx5e driver when trimming off the unused fragments AND the
   associated skb fragments have been released. This path allows
   recycling of pages to the page pool cache (allow_direct == true).

2) On the skb release path (last fragment release), which
   will always release pages to the page pool ring
   (allow_direct == false).

Whichever is releasing the last fragment will be decisive on
where the page gets released: the cache or the ring. So we
obviously want to maximize for doing the release from 1.

This patch does that by deferring the release of page fragments
right before requesting new ones from the page pool. Extra care
needs to be taken for the corner cases:

* On first call, make sure that release is not called. The
  skip_release_bitmap is used for this purpose.

* On rq shutdown, make sure that all wqes that were not
  in the linked list are released.

For a single ring, single core, default MTU (1500) TCP stream
test the number of pages allocated from the cache directly
(rx_pp_recycle_cached) increases from 31 % to 98 %:

+----------------------------------------------+
| Page Pool stats (/sec)  |  Before |   After  |
+-------------------------+---------+----------+
|rx_pp_alloc_fast         | 2137754 |  2261033 |
|rx_pp_alloc_slow         |      47 |        9 |
|rx_pp_alloc_empty        |      47 |        9 |
|rx_pp_alloc_refill       |   23230 |      819 |
|rx_pp_alloc_waive        |       0 |        0 |
|rx_pp_recycle_cached     |  672182 |  2209015 |
|rx_pp_recycle_cache_full |    1789 |        0 |
|rx_pp_recycle_ring       | 1485848 |    52259 |
|rx_pp_recycle_ring_full  |    3003 |      584 |
+----------------------------------------------+

With this patch, the performance in striding rq for the above test is
back to baseline.

Signed-off-by: Dragos Tatulea <dtatulea@nvidia.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>