Re: [PATCH v2] virtio-net: support setting coalescing params for multiple vqs

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Jan 15, 2024 at 09:06:02PM +0800, Heng Qi wrote:
> Currently, when each time the driver attempts to update the coalescing parameters
> for a vq, it needs to kick the device and wait for the ctrlq response to return.
> The following path is observed: 1. Driver kicks the device; 2. After the device
> receives the kick, CPU scheduling occurs and DMA multiple buffers multiple times;
> 3. The device completes processing and replies with a response.
> 
> When large-queue devices issue multiple requests and kick the device frequently,
> this often interrupt the work of the device-side CPU. In addition, each vq request
> is processed separately, causing more delays for the CPU to wait for the DMA
> request to complete.
> 
> These interruptions and overhead will strain the CPU responsible for controlling
> the path of the DPU, especially in multi-device and large-queue scenarios.
> 
> To solve the above problems, we internally tried batch request, which merges
> requests from multiple queues and sends them at once. We conservatively tested
> 8 queue commands and sent them together. The DPU processing efficiency can be
> improved by 8 times, which greatly eases the DPU's support for multi-device
> and multi-queue DIM.
> 
> Maintainers may be concerned about whether the batch command method can optimize
> the above problems: accumulate multiple request commands to kick the device once,
> and obtain the processing results of the corresponding commands asynchronously.
> The batch command method is used by us to optimize the CPU overhead of the DIM
> worker caused by the guest being busy waiting for the command response result.
> This is a different focus than batch request to solve the problem.


Hmm. Could the issue be actually that
1. you are using split vqs so processing is complex
2. Linux driver uses indirect to pass coalescing commands around
   so even more complex
?

Is packed an option for you at all? Guest should also be changed to
avoid indirect with ctrl vq. As a quick test, disable indirect maybe
that is fast enough?




> Suggested-by: Xiaoming Zhao <zxm377917@alibaba-inc.com>
> Signed-off-by: Heng Qi <hengqi@linux.alibaba.com>
> ---
> v1->v2: Updated commit log. Due to sensitivity, sorry that can not give the
>     absolute value directly. @Michael
> 
>  device-types/net/description.tex | 26 ++++++++++++++++++++------
>  1 file changed, 20 insertions(+), 6 deletions(-)
> 
> diff --git a/device-types/net/description.tex b/device-types/net/description.tex
> index aff5e08..b3766c4 100644
> --- a/device-types/net/description.tex
> +++ b/device-types/net/description.tex
> @@ -1667,8 +1667,8 @@ \subsubsection{Control Virtqueue}\label{sec:Device Types / Network Device / Devi
>  for notification coalescing.
>  
>  If the VIRTIO_NET_F_VQ_NOTF_COAL feature is negotiated, the driver can
> -send commands VIRTIO_NET_CTRL_NOTF_COAL_VQ_SET and VIRTIO_NET_CTRL_NOTF_COAL_VQ_GET
> -for virtqueue notification coalescing.
> +send commands VIRTIO_NET_CTRL_NOTF_COAL_VQ_SET, VIRTIO_NET_CTRL_NOTF_COAL_VQS_SET and
> +VIRTIO_NET_CTRL_NOTF_COAL_VQ_GET for virtqueue notification coalescing.
>  
>  \begin{lstlisting}
>  struct virtio_net_ctrl_coal {
> @@ -1682,11 +1682,17 @@ \subsubsection{Control Virtqueue}\label{sec:Device Types / Network Device / Devi
>      struct virtio_net_ctrl_coal coal;
>  };
>  
> +struct virtio_net_ctrl_mrg_coal_vq {
> +        le16 num_entries; /* indicates number of valid entries */
> +        struct virtio_net_ctrl_coal_vq entries[];
> +};
> +
>  #define VIRTIO_NET_CTRL_NOTF_COAL 6
>   #define VIRTIO_NET_CTRL_NOTF_COAL_TX_SET  0
>   #define VIRTIO_NET_CTRL_NOTF_COAL_RX_SET 1
>   #define VIRTIO_NET_CTRL_NOTF_COAL_VQ_SET 2
>   #define VIRTIO_NET_CTRL_NOTF_COAL_VQ_GET 3
> + #define VIRTIO_NET_CTRL_NOTF_COAL_VQS_SET 4
>  \end{lstlisting}
>  
>  Coalescing parameters:
> @@ -1706,6 +1712,7 @@ \subsubsection{Control Virtqueue}\label{sec:Device Types / Network Device / Devi
>  \item For the command VIRTIO_NET_CTRL_NOTF_COAL_VQ_SET, the structure virtio_net_ctrl_coal_vq is write-only for the driver.
>  \item For the command VIRTIO_NET_CTRL_NOTF_COAL_VQ_GET, \field{vq_index} and \field{reserved} are write-only
>        for the driver, and the structure virtio_net_ctrl_coal is read-only for the driver.
> +\item For the command VIRTIO_NET_CTRL_NOTF_COAL_VQS_SET, the structure virtio_net_ctrl_mrg_coal_vq is write-only for the driver.
>  \end{itemize}
>  
>  The class VIRTIO_NET_CTRL_NOTF_COAL has the following commands:
> @@ -1716,6 +1723,9 @@ \subsubsection{Control Virtqueue}\label{sec:Device Types / Network Device / Devi
>                                          for an enabled transmit/receive virtqueue whose index is \field{vq_index}.
>  \item VIRTIO_NET_CTRL_NOTF_COAL_VQ_GET: use the structure virtio_net_ctrl_coal_vq to get the \field{max_usecs} and \field{max_packets} parameters
>                                          for an enabled transmit/receive virtqueue whose index is \field{vq_index}.
> +\item VIRTIO_NET_CTRL_NOTF_COAL_VQS_SET: use the structure virtio_net_ctrl_mrg_coal_vq to set the \field{max_usecs} and \field{max_packets} parameters
> +                                         for \field{num_entries} enabled transmit/receive virtqueues. The corresponding index value
> +                                         of each configured virtqueue is \field{vq_index}.
>  \end{enumerate}
>  
>  The device may generate notifications more or less frequently than specified by set commands of the VIRTIO_NET_CTRL_NOTF_COAL class.
> @@ -1782,9 +1792,13 @@ \subsubsection{Control Virtqueue}\label{sec:Device Types / Network Device / Devi
>  
>  The driver MUST set \field{vq_index} to the virtqueue index of an enabled transmit or receive virtqueue.
>  
> +The driver MUST set \field{num_entries} to a non-zero value and MUST NOT set \field{num_entries} to
> +a value greater than the number of enabled transmit and receive virtqueues.
> +
>  The driver MUST have negotiated the VIRTIO_NET_F_NOTF_COAL feature when issuing commands VIRTIO_NET_CTRL_NOTF_COAL_TX_SET and VIRTIO_NET_CTRL_NOTF_COAL_RX_SET.
>  
> -The driver MUST have negotiated the VIRTIO_NET_F_VQ_NOTF_COAL feature when issuing commands VIRTIO_NET_CTRL_NOTF_COAL_VQ_SET and VIRTIO_NET_CTRL_NOTF_COAL_VQ_GET.
> +The driver MUST have negotiated the VIRTIO_NET_F_VQ_NOTF_COAL feature when issuing commands
> +VIRTIO_NET_CTRL_NOTF_COAL_VQ_SET, VIRTIO_NET_CTRL_NOTF_COAL_VQS_SET and VIRTIO_NET_CTRL_NOTF_COAL_VQ_GET.
>  
>  The driver MUST ignore the values of coalescing parameters received from the VIRTIO_NET_CTRL_NOTF_COAL_VQ_GET command if the device responds with VIRTIO_NET_ERR.
>  
> @@ -1794,10 +1808,10 @@ \subsubsection{Control Virtqueue}\label{sec:Device Types / Network Device / Devi
>  
>  The device SHOULD respond to VIRTIO_NET_CTRL_NOTF_COAL_TX_SET and VIRTIO_NET_CTRL_NOTF_COAL_RX_SET commands with VIRTIO_NET_ERR if it was not able to change the parameters.
>  
> -The device MUST respond to the VIRTIO_NET_CTRL_NOTF_COAL_VQ_SET command with VIRTIO_NET_ERR if it was not able to change the parameters.
> +The device MUST respond to VIRTIO_NET_CTRL_NOTF_COAL_VQ_SET and VIRTIO_NET_CTRL_NOTF_COAL_VQS_SET commands with VIRTIO_NET_ERR if it was not able to change the parameters.
>  
> -The device MUST respond to VIRTIO_NET_CTRL_NOTF_COAL_VQ_SET and VIRTIO_NET_CTRL_NOTF_COAL_VQ_GET commands with
> -VIRTIO_NET_ERR if the designated virtqueue is not an enabled transmit or receive virtqueue.
> +The device MUST respond to VIRTIO_NET_CTRL_NOTF_COAL_VQ_SET, VIRTIO_NET_CTRL_NOTF_COAL_VQS_SET and VIRTIO_NET_CTRL_NOTF_COAL_VQ_GET
> +commands with VIRTIO_NET_ERR if the designated virtqueue is not an enabled transmit or receive virtqueue.
>  
>  Upon disabling and re-enabling a transmit virtqueue, the device MUST set the coalescing parameters of the virtqueue
>  to those configured through the VIRTIO_NET_CTRL_NOTF_COAL_TX_SET command, or, if the driver did not set any TX coalescing parameters, to 0.
> -- 
> 1.8.3.1