linux-kernel - RE: [PATCH v3] net: netvsc: Update default VMBus channels

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Message-ID:
 <SN6PR02MB415769AD9CC9B1C9398DCC6CD4882@SN6PR02MB4157.namprd02.prod.outlook.com>
Date: Fri, 23 Aug 2024 16:37:17 +0000
From: Michael Kelley <mhklinux@...look.com>
To: Erni Sri Satya Vennela <ernis@...ux.microsoft.com>, "kys@...rosoft.com"
	<kys@...rosoft.com>, "haiyangz@...rosoft.com" <haiyangz@...rosoft.com>,
	"wei.liu@...nel.org" <wei.liu@...nel.org>, "decui@...rosoft.com"
	<decui@...rosoft.com>, "davem@...emloft.net" <davem@...emloft.net>,
	"edumazet@...gle.com" <edumazet@...gle.com>, "kuba@...nel.org"
	<kuba@...nel.org>, "pabeni@...hat.com" <pabeni@...hat.com>,
	"linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC: "ernis@...rosoft.com" <ernis@...rosoft.com>
Subject: RE: [PATCH v3] net: netvsc: Update default VMBus channels

From: Erni Sri Satya Vennela <ernis@...ux.microsoft.com> Sent: Thursday, August 22, 2024 8:06 AM
> 
> 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.

> 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.  :-)

> | >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 <ernis@...ux.microsoft.com>
> Reviewed-by: Haiyang Zhang <haiyangz@...rosoft.com>
> Reviewed-by: Shradha Gupta <shradhagupta@...ux.microsoft.com>
> ---
> 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
>