[PATCH net-next 0/2] mvpp2: prefetch data early

From: Matteo Croce <redacted>

These two patches prefetch some data from RAM so to reduce stall
and speedup the packet processing.

Matteo Croce (2):
  mvpp2: prefetch right address
  mvpp2: prefetch page

 drivers/net/ethernet/marvell/mvpp2/mvpp2_main.c | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

-- 
2.31.1

From: Matteo Croce <redacted>

Most of the time during the RX is caused by the compound_head() call
done at the end of the RX loop:

       │     build_skb():
       [...]
       │     static inline struct page *compound_head(struct page *page)
       │     {
       │     unsigned long head = READ_ONCE(page->compound_head);
 65.23 │       ldr  x2, [x1, #8]

Prefetch the page struct as soon as possible, to speedup the RX path
noticeabily by a ~3-4% packet rate in a drop test.

       │     build_skb():
       [...]
       │     static inline struct page *compound_head(struct page *page)
       │     {
       │     unsigned long head = READ_ONCE(page->compound_head);
 17.92 │       ldr  x2, [x1, #8]

Signed-off-by: Matteo Croce <redacted>
---
 drivers/net/ethernet/marvell/mvpp2/mvpp2_main.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/drivers/net/ethernet/marvell/mvpp2/mvpp2_main.c b/drivers/net/ethernet/marvell/mvpp2/mvpp2_main.c
index 07d8f3e31b52..9bca8c8f9f8d 100644
--- a/drivers/net/ethernet/marvell/mvpp2/mvpp2_main.c
+++ b/drivers/net/ethernet/marvell/mvpp2/mvpp2_main.c

@@ -3900,15 +3900,19 @@ static int mvpp2_rx(struct mvpp2_port *port, struct napi_struct *napi,
 		phys_addr_t phys_addr;
 		u32 rx_status, timestamp;
 		int pool, rx_bytes, err, ret;
+		struct page *page;
 		void *data;
 
+		phys_addr = mvpp2_rxdesc_cookie_get(port, rx_desc);
+		data = (void *)phys_to_virt(phys_addr);
+		page = virt_to_page(data);
+		prefetch(page);
+
 		rx_done++;
 		rx_status = mvpp2_rxdesc_status_get(port, rx_desc);
 		rx_bytes = mvpp2_rxdesc_size_get(port, rx_desc);
 		rx_bytes -= MVPP2_MH_SIZE;
 		dma_addr = mvpp2_rxdesc_dma_addr_get(port, rx_desc);
-		phys_addr = mvpp2_rxdesc_cookie_get(port, rx_desc);
-		data = (void *)phys_to_virt(phys_addr);
 
 		pool = (rx_status & MVPP2_RXD_BM_POOL_ID_MASK) >>
 			MVPP2_RXD_BM_POOL_ID_OFFS;

@@ -3997,7 +4001,7 @@ static int mvpp2_rx(struct mvpp2_port *port, struct napi_struct *napi,
 		}
 
 		if (pp)
-			skb_mark_for_recycle(skb, virt_to_page(data), pp);
+			skb_mark_for_recycle(skb, page, pp);
 		else
 			dma_unmap_single_attrs(dev->dev.parent, dma_addr,
 					       bm_pool->buf_size, DMA_FROM_DEVICE,

-- 
2.31.1

From: Matteo Croce <redacted>

In the RX buffer, the received data starts after a headroom used to
align the IP header and to allow prepending headers efficiently.
The prefetch() should take this into account, and prefetch from
the very start of the received data.

We can see that ether_addr_equal_64bits(), which is the first function
to access the data, drops from the top of the perf top output.

prefetch(data):

Overhead  Shared Object     Symbol
  11.64%  [kernel]          [k] eth_type_trans

prefetch(data + MVPP2_MH_SIZE + MVPP2_SKB_HEADROOM):

Overhead  Shared Object     Symbol
  13.42%  [kernel]          [k] build_skb
  10.35%  [mvpp2]           [k] mvpp2_rx
   9.35%  [kernel]          [k] __netif_receive_skb_core
   8.24%  [kernel]          [k] kmem_cache_free
   7.97%  [kernel]          [k] dev_gro_receive
   7.68%  [kernel]          [k] page_pool_put_page
   7.32%  [kernel]          [k] kmem_cache_alloc
   7.09%  [mvpp2]           [k] mvpp2_bm_pool_put
   3.36%  [kernel]          [k] eth_type_trans

Also, move the eth_type_trans() call a bit down, to give the RAM more
time to prefetch the data.

Signed-off-by: Matteo Croce <redacted>
---
 drivers/net/ethernet/marvell/mvpp2/mvpp2_main.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/net/ethernet/marvell/mvpp2/mvpp2_main.c b/drivers/net/ethernet/marvell/mvpp2/mvpp2_main.c
index 5663c1b21870..07d8f3e31b52 100644
--- a/drivers/net/ethernet/marvell/mvpp2/mvpp2_main.c
+++ b/drivers/net/ethernet/marvell/mvpp2/mvpp2_main.c

@@ -3938,7 +3938,7 @@ static int mvpp2_rx(struct mvpp2_port *port, struct napi_struct *napi,
 			goto err_drop_frame;
 
 		/* Prefetch header */
-		prefetch(data);
+		prefetch(data + MVPP2_MH_SIZE + MVPP2_SKB_HEADROOM);
 
 		if (bm_pool->frag_size > PAGE_SIZE)
 			frag_size = 0;

@@ -4008,8 +4008,8 @@ static int mvpp2_rx(struct mvpp2_port *port, struct napi_struct *napi,
 
 		skb_reserve(skb, MVPP2_MH_SIZE + MVPP2_SKB_HEADROOM);
 		skb_put(skb, rx_bytes);
-		skb->protocol = eth_type_trans(skb, dev);
 		mvpp2_rx_csum(port, rx_status, skb);
+		skb->protocol = eth_type_trans(skb, dev);
 
 		napi_gro_receive(napi, skb);
 		continue;

-- 
2.31.1

Hi Matteo,


śr., 9 cze 2021 o 15:47 Matteo Croce [off-list ref] napisał(a):

From: Matteo Croce <redacted>

These two patches prefetch some data from RAM so to reduce stall
and speedup the packet processing.

Matteo Croce (2):
  mvpp2: prefetch right address
  mvpp2: prefetch page

 drivers/net/ethernet/marvell/mvpp2/mvpp2_main.c | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

Thank you for the patches, they seem reasonable, however I'd like to
stress it on the CN913x setup @10G - I should have some slot for that
closer to EOW.

Best regards,
Marcin

Hello:

This series was applied to netdev/net-next.git (refs/heads/master):

On Wed,  9 Jun 2021 15:47:12 +0200 you wrote:

From: Matteo Croce <redacted>

These two patches prefetch some data from RAM so to reduce stall
and speedup the packet processing.

Matteo Croce (2):
  mvpp2: prefetch right address
  mvpp2: prefetch page

[...]

Here is the summary with links:
  - [net-next,1/2] mvpp2: prefetch right address
    https://git.kernel.org/netdev/net-next/c/d8ea89fe8a49
  - [net-next,2/2] mvpp2: prefetch page
    https://git.kernel.org/netdev/net-next/c/2f128eb3308a

You are awesome, thank you!
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