Patchwork [PATCHv2,net,2/2] sfc: Reduce RX scatter buffer size, and reduce alignment if appropriate

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Submitter Ben Hutchings
Date May 13, 2013, 10:01 p.m.
Message ID <1368482482.3305.50.camel@bwh-desktop.uk.solarflarecom.com>
Download mbox | patch
Permalink /patch/243545/
State Accepted
Delegated to: David Miller
Headers show

Comments

Ben Hutchings - May 13, 2013, 10:01 p.m.
efx_start_datapath() asserts that we can fit 2 RX scatter buffers plus
a software structure, each appropriately aligned, into a single page.
Where L1_CACHE_BYTES == 256 and PAGE_SIZE == 4096, which is the case
on s390, this assertion fails.

The current scatter buffer size is also not a multiple of 64 or 128,
which are more common cache line sizes.  If we can make both the start
and end of a scatter buffer cache-aligned, this will reduce the need
for read-modify-write operations on inter- processor links.

Fix the alignment by reducing EFX_RX_USR_BUF_SIZE to 2048 - 256 ==
1792.  (We could use 2048 - L1_CACHE_BYTES, but EFX_RX_USR_BUF_SIZE
also affects user-level networking where a larger amount of
housekeeping data may be needed.  Although this version of the driver
does not support user-level networking, I prefer to keep scattering
behaviour consistent with the out-of-tree version.)

This still doesn't fix the s390 build because like most architectures
it has NET_IP_ALIGN == 2.  When NET_IP_ALIGN != 0 we cannot achieve
cache line alignment at either the start or end of a scatter buffer,
so there is actually no point in padding the buffers to a multiple of
the cache line size.  All we need is 4-byte alignment of the network
header, so do that.

Adjust the assertions accordingly.

Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Reported-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
---
 drivers/net/ethernet/sfc/efx.c        |    6 ++++--
 drivers/net/ethernet/sfc/net_driver.h |   16 ++++++++++++++--
 drivers/net/ethernet/sfc/rx.c         |    2 +-
 3 files changed, 19 insertions(+), 5 deletions(-)
Geert Uytterhoeven - May 14, 2013, 7:09 a.m.
On Tue, May 14, 2013 at 12:01 AM, Ben Hutchings
<bhutchings@solarflare.com> wrote:
> efx_start_datapath() asserts that we can fit 2 RX scatter buffers plus
> a software structure, each appropriately aligned, into a single page.
> Where L1_CACHE_BYTES == 256 and PAGE_SIZE == 4096, which is the case
> on s390, this assertion fails.
>
> The current scatter buffer size is also not a multiple of 64 or 128,
> which are more common cache line sizes.  If we can make both the start
> and end of a scatter buffer cache-aligned, this will reduce the need
> for read-modify-write operations on inter- processor links.
>
> Fix the alignment by reducing EFX_RX_USR_BUF_SIZE to 2048 - 256 ==
> 1792.  (We could use 2048 - L1_CACHE_BYTES, but EFX_RX_USR_BUF_SIZE
> also affects user-level networking where a larger amount of
> housekeeping data may be needed.  Although this version of the driver
> does not support user-level networking, I prefer to keep scattering
> behaviour consistent with the out-of-tree version.)
>
> This still doesn't fix the s390 build because like most architectures
> it has NET_IP_ALIGN == 2.  When NET_IP_ALIGN != 0 we cannot achieve
> cache line alignment at either the start or end of a scatter buffer,
> so there is actually no point in padding the buffers to a multiple of
> the cache line size.  All we need is 4-byte alignment of the network
> header, so do that.
>
> Adjust the assertions accordingly.
>
> Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
> Reported-by: Heiko Carstens <heiko.carstens@de.ibm.com>
> Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>

Fixes the s390 build, so
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds
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David Miller - May 14, 2013, 6:32 p.m.
From: Ben Hutchings <bhutchings@solarflare.com>
Date: Mon, 13 May 2013 23:01:22 +0100

> efx_start_datapath() asserts that we can fit 2 RX scatter buffers plus
> a software structure, each appropriately aligned, into a single page.
> Where L1_CACHE_BYTES == 256 and PAGE_SIZE == 4096, which is the case
> on s390, this assertion fails.
> 
> The current scatter buffer size is also not a multiple of 64 or 128,
> which are more common cache line sizes.  If we can make both the start
> and end of a scatter buffer cache-aligned, this will reduce the need
> for read-modify-write operations on inter- processor links.
> 
> Fix the alignment by reducing EFX_RX_USR_BUF_SIZE to 2048 - 256 ==
> 1792.  (We could use 2048 - L1_CACHE_BYTES, but EFX_RX_USR_BUF_SIZE
> also affects user-level networking where a larger amount of
> housekeeping data may be needed.  Although this version of the driver
> does not support user-level networking, I prefer to keep scattering
> behaviour consistent with the out-of-tree version.)
> 
> This still doesn't fix the s390 build because like most architectures
> it has NET_IP_ALIGN == 2.  When NET_IP_ALIGN != 0 we cannot achieve
> cache line alignment at either the start or end of a scatter buffer,
> so there is actually no point in padding the buffers to a multiple of
> the cache line size.  All we need is 4-byte alignment of the network
> header, so do that.
> 
> Adjust the assertions accordingly.
> 
> Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
> Reported-by: Heiko Carstens <heiko.carstens@de.ibm.com>
> Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>

Applied.
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Ben Hutchings - July 23, 2013, 8:19 p.m.
On Mon, 2013-05-13 at 23:01 +0100, Ben Hutchings wrote:
[...]
> This still doesn't fix the s390 build because like most architectures
> it has NET_IP_ALIGN == 2.  When NET_IP_ALIGN != 0 we cannot achieve
> cache line alignment at either the start or end of a scatter buffer,
> so there is actually no point in padding the buffers to a multiple of
> the cache line size.  All we need is 4-byte alignment of the network
> header, so do that.
[...]

I was just thinking about this again.  Do we need to care about
alignment in a page fragment at all?  Won't all the headers be pulled
into the header area before being accessed with any instructions that
would require alignment?  (We aren't using build_skb().)

Ben.

Patch

diff --git a/drivers/net/ethernet/sfc/efx.c b/drivers/net/ethernet/sfc/efx.c
index 999289b..39e4cb3 100644
--- a/drivers/net/ethernet/sfc/efx.c
+++ b/drivers/net/ethernet/sfc/efx.c
@@ -643,9 +643,11 @@  static void efx_start_datapath(struct efx_nic *efx)
 		efx->rx_scatter = false;
 		efx->rx_buffer_order = 0;
 	} else if (efx->type->can_rx_scatter) {
+		BUILD_BUG_ON(EFX_RX_USR_BUF_SIZE % L1_CACHE_BYTES);
 		BUILD_BUG_ON(sizeof(struct efx_rx_page_state) +
-			     NET_IP_ALIGN + EFX_RX_USR_BUF_SIZE >
-			     PAGE_SIZE / 2);
+			     2 * ALIGN(NET_IP_ALIGN + EFX_RX_USR_BUF_SIZE,
+				       EFX_RX_BUF_ALIGNMENT) >
+			     PAGE_SIZE);
 		efx->rx_scatter = true;
 		efx->rx_dma_len = EFX_RX_USR_BUF_SIZE;
 		efx->rx_buffer_order = 0;
diff --git a/drivers/net/ethernet/sfc/net_driver.h b/drivers/net/ethernet/sfc/net_driver.h
index 5efddf3..39d6bd77 100644
--- a/drivers/net/ethernet/sfc/net_driver.h
+++ b/drivers/net/ethernet/sfc/net_driver.h
@@ -72,8 +72,20 @@ 
 /* Maximum possible MTU the driver supports */
 #define EFX_MAX_MTU (9 * 1024)
 
-/* Size of an RX scatter buffer.  Small enough to pack 2 into a 4K page. */
-#define EFX_RX_USR_BUF_SIZE 1824
+/* Size of an RX scatter buffer.  Small enough to pack 2 into a 4K page,
+ * and should be a multiple of the cache line size.
+ */
+#define EFX_RX_USR_BUF_SIZE	(2048 - 256)
+
+/* If possible, we should ensure cache line alignment at start and end
+ * of every buffer.  Otherwise, we just need to ensure 4-byte
+ * alignment of the network header.
+ */
+#if NET_IP_ALIGN == 0
+#define EFX_RX_BUF_ALIGNMENT	L1_CACHE_BYTES
+#else
+#define EFX_RX_BUF_ALIGNMENT	4
+#endif
 
 /* Forward declare Precision Time Protocol (PTP) support structure. */
 struct efx_ptp_data;
diff --git a/drivers/net/ethernet/sfc/rx.c b/drivers/net/ethernet/sfc/rx.c
index 99f70dd..a7dfe36 100644
--- a/drivers/net/ethernet/sfc/rx.c
+++ b/drivers/net/ethernet/sfc/rx.c
@@ -94,7 +94,7 @@  static inline void efx_sync_rx_buffer(struct efx_nic *efx,
 void efx_rx_config_page_split(struct efx_nic *efx)
 {
 	efx->rx_page_buf_step = ALIGN(efx->rx_dma_len + NET_IP_ALIGN,
-				      L1_CACHE_BYTES);
+				      EFX_RX_BUF_ALIGNMENT);
 	efx->rx_bufs_per_page = efx->rx_buffer_order ? 1 :
 		((PAGE_SIZE - sizeof(struct efx_rx_page_state)) /
 		 efx->rx_page_buf_step);