[PATCH v2 1/2] mm/page_alloc: free order-0 pages through PCP in page_frag_free()

[Aaron Lu]

page_frag_free() calls __free_pages_ok() to free the page back to
Buddy. This is OK for high order page, but for order-0 pages, it
misses the optimization opportunity of using Per-Cpu-Pages and can
cause zone lock contention when called frequently.

PaweÅ Staszewski recently shared his result of 'how Linux kernel
handles normal traffic'[1] and from perf data, Jesper Dangaard Brouer
found the lock contention comes from page allocator:

  mlx5e_poll_tx_cq
  |
   --16.34%--napi_consume_skb
             |
             |--12.65%--__free_pages_ok
             |          |
             |           --11.86%--free_one_page
             |                     |
             |                     |--10.10%--queued_spin_lock_slowpath
             |                     |
             |                      --0.65%--_raw_spin_lock
             |
             |--1.55%--page_frag_free
             |
              --1.44%--skb_release_data

Jesper explained how it happened: mlx5 driver RX-page recycle
mechanism is not effective in this workload and pages have to go
through the page allocator. The lock contention happens during
mlx5 DMA TX completion cycle. And the page allocator cannot keep
up at these speeds.[2]

I thought that __free_pages_ok() are mostly freeing high order
pages and thought this is an lock contention for high order pages
but Jesper explained in detail that __free_pages_ok() here are
actually freeing order-0 pages because mlx5 is using order-0 pages
to satisfy its page pool allocation request.[3]

The free path as pointed out by Jesper is:
skb_free_head()
  -> skb_free_frag()
    -> page_frag_free()
And the pages being freed on this path are order-0 pages.

Fix this by doing similar things as in __page_frag_cache_drain() -
send the being freed page to PCP if it's an order-0 page, or
directly to Buddy if it is a high order page.

With this change, PaweÅ hasn't noticed lock contention yet in
his workload and Jesper has noticed a 7% performance improvement
using a micro benchmark and lock contention is gone. Ilias' test
on a 'low' speed 1Gbit interface on an cortex-a53 shows ~11%
performance boost testing with 64byte packets and __free_pages_ok()
disappeared from perf top.

[1]: https://www.spinics.net/lists/netdev/msg531362.html
[2]: https://www.spinics.net/lists/netdev/msg531421.html
[3]: https://www.spinics.net/lists/netdev/msg531556.html
Reported-by: PaweÅ Staszewski <pstaszewski@xxxxxxxxx>
Analysed-by: Jesper Dangaard Brouer <brouer@xxxxxxxxxx>
Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
Acked-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
Acked-by: Jesper Dangaard Brouer <brouer@xxxxxxxxxx>
Acked-by: Ilias Apalodimas <ilias.apalodimas@xxxxxxxxxx>
Tested-by: Ilias Apalodimas <ilias.apalodimas@xxxxxxxxxx>
Acked-by: Alexander Duyck <alexander.h.duyck@xxxxxxxxxxxxxxx>
Signed-off-by: Aaron Lu <aaron.lu@xxxxxxxxx>
---
v2: only changelog changes:
    - remove the duplicated skb_free_frag() as pointed by Jesper;
    - add Ilias' test result;
    - add people's ack/test tag.

 mm/page_alloc.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index ae31839874b8..91a9a6af41a2 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -4555,8 +4555,14 @@ void page_frag_free(void *addr)
 {
 	struct page *page = virt_to_head_page(addr);
 
-	if (unlikely(put_page_testzero(page)))
-		__free_pages_ok(page, compound_order(page));
+	if (unlikely(put_page_testzero(page))) {
+		unsigned int order = compound_order(page);
+
+		if (order == 0)
+			free_unref_page(page);
+		else
+			__free_pages_ok(page, order);
+	}
 }
 EXPORT_SYMBOL(page_frag_free);
 
-- 
2.17.2