Subject: [PATCH v2] net: netvsc: Update default VMBus channels

From: Erni Sri Satya Vennela <[email-masked]>
Date: Wed, 14 Aug 2024 09:59:13 -0700

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 32 vCPUs, the channels
default to VRSS_CHANNEL_DEFAULT. For greater than 32 vCPUs,
set the channels to number of physical cores / 2 as a way
to optimize CPU resource utilization and scale for high-end
processors with many cores.
Maximum number of channels are by default set to 64.

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

-------------------------------------------------------------
|No. of vCPU	|dev_info->num_chn	|subchannel created |
-------------------------------------------------------------
|  0-16		|	16		|	vCPU	    |
| >16 & <=32	|	16		|	16          |
| >32 & <=128	|	vCPU/2		|	vCPU/2      |
| >128		|	vCPU/2		|	64          |
-------------------------------------------------------------

Performance tests showed significant improvement in throughput:
- 0.54% for 16 vCPUs
- 0.83% for 32 vCPUs
- 1.76% for 48 vCPUs
- 10.35% for 64 vCPUs
- 13.47% for 96 vCPUs

Signed-off-by: Erni Sri Satya Vennela <[email-masked]>
---
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 | 5 ++++-
 2 files changed, 5 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..e32eb2997bf7 100644
--- a/drivers/net/hyperv/netvsc_drv.c
+++ b/drivers/net/hyperv/netvsc_drv.c
@@ -27,6 +27,7 @@
 #include <linux/rtnetlink.h>
 #include <linux/netpoll.h>
 #include <linux/bpf.h>
+#include <linux/cpumask.h>
 
 #include <net/arp.h>
 #include <net/route.h>
@@ -987,7 +988,9 @@ struct netvsc_device_info *netvsc_devinfo_get(struct netvsc_device *nvdev)
 			dev_info->bprog = prog;
 		}
 	} else {
-		dev_info->num_chn = VRSS_CHANNEL_DEFAULT;
+		int count = num_online_cpus();
+
+		dev_info->num_chn = (count < 32) ? VRSS_CHANNEL_DEFAULT : DIV_ROUND_UP(count, 2);
 		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