On Fri, Aug 23, 2024 at 04:37:17PM +0000, Michael Kelley wrote:

From: Erni Sri Satya Vennela <redacted> Sent: Thursday, August 22, 2024 8:06 AM

quoted

Change VMBus channels macro (VRSS_CHANNEL_DEFAULT) in
Linux netvsc from 8 to 16 to align with Azure Windows VM
and improve networking throughput.

For VMs having less than 16 vCPUS, the channels depend
on number of vCPUs. Between 16 to 64 vCPUs, the channels
default to VRSS_CHANNEL_DEFAULT. For greater than 64 vCPUs,
set the channels to number of physical cores / 2 returned by
netif_get_num_default_rss_queues() as a way to optimize CPU
resource utilization and scale for high-end processors with
many cores. Due to hyper-threading, the number of
physical cores = vCPUs/2.

But note that a given physical processor may or may not support
hyper-threading. For example, the physical processor used for
ARM64 VMs in Azure does not have hyper-threading. And even
if the physical processor supports hyper-threading, the VM might
not see hyper-threading as enabled. Many Azure GPU-based VM
sizes see only full cores, with no hyper-threading. It's also possible
to boot Linux with hyper-threading disabled even if the VM sees
hyper-threaded cores (the "nosmt" or "smt=1" kernel boot option).

Your code below probably isn't affected when hyper-threading
isn't present. But in the interest of accuracy, the discussion here
in the commit message should qualify the use of "vCPU/4" as
the number of channels. It might be "vCPU/2" when
hyper-threading isn't present or is disabled, and for vCPU
counts between 16 and 64, you'll get more than 16 channels.

quoted

Maximum number of channels are by default set to 64.

Based on this change the channel creation would change as follows:

-------------------------------------------------------------
| No. of vCPU	|  dev_info->num_chn	| channels created  |
-------------------------------------------------------------
|  0-16		|       16		|       vCPU        |

Nit: Presumably we won't ever have 0 vCPUs.  :-)

quoted

| >16 & <=64	|       16		|       16          |
| >64 & <=256	|       vCPU/4		|       vCPU/4      |
| >256		|       vCPU/4		|       64          |
-------------------------------------------------------------

Performance tests showed significant improvement in throughput:
- 0.54% for 16 vCPUs
- 0.83% for 32 vCPUs
- 0.86% for 48 vCPUs
- 9.72% for 64 vCPUs
- 13.57% for 96 vCPUs

Signed-off-by: Erni Sri Satya Vennela <redacted>
Reviewed-by: Haiyang Zhang <haiyangz@microsoft.com>
Reviewed-by: Shradha Gupta <redacted>
---
Changes in v3:
* Use netif_get_num_default_rss_queues() to set channels
* Change terminology for channels in commit message
---
Changes in v2:
* Set dev_info->num_chn based on vCPU count.
---
 drivers/net/hyperv/hyperv_net.h | 2 +-
 drivers/net/hyperv/netvsc_drv.c | 3 ++-
 2 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/net/hyperv/hyperv_net.h b/drivers/net/hyperv/hyperv_net.h
index 810977952f95..e690b95b1bbb 100644
--- a/drivers/net/hyperv/hyperv_net.h
+++ b/drivers/net/hyperv/hyperv_net.h

@@ -882,7 +882,7 @@ struct nvsp_message {

 #define VRSS_SEND_TAB_SIZE 16  /* must be power of 2 */
 #define VRSS_CHANNEL_MAX 64
-#define VRSS_CHANNEL_DEFAULT 8
+#define VRSS_CHANNEL_DEFAULT 16

 #define RNDIS_MAX_PKT_DEFAULT 8
 #define RNDIS_PKT_ALIGN_DEFAULT 8

diff --git a/drivers/net/hyperv/netvsc_drv.c b/drivers/net/hyperv/netvsc_drv.c
index 44142245343d..a6482afe4217 100644
--- a/drivers/net/hyperv/netvsc_drv.c
+++ b/drivers/net/hyperv/netvsc_drv.c

@@ -987,7 +987,8 @@ struct netvsc_device_info *netvsc_devinfo_get(struct netvsc_device *nvdev)
 			dev_info->bprog = prog;
 		}
 	} else {
-		dev_info->num_chn = VRSS_CHANNEL_DEFAULT;
+		dev_info->num_chn = max(VRSS_CHANNEL_DEFAULT,
+					netif_get_num_default_rss_queues());
 		dev_info->send_sections = NETVSC_DEFAULT_TX;
 		dev_info->send_section_size = NETVSC_SEND_SECTION_SIZE;
 		dev_info->recv_sections = NETVSC_DEFAULT_RX;
--

2.34.1

Thank you for the knowledge you shared. I’ll update the patch
with your suggestions in the next version.