[virtio-comment] 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.
> 
> 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.

This one however is vague.
In fact the addition of VIRTIO_NET_CTRL_NOTF_COAL_VQ_SET made
VIRTIO_NET_CTRL_NOTF_COAL_TX_SET and VIRTIO_NET_CTRL_NOTF_COAL_RX_SET
also vague we just missed this.



So, it is not clear whether if the device responds with VIRTIO_NET_ERR - it can change
parameters for some VQs but not others, or does it have to either
change parameters for all VQs or none at all.

I think a better sematic is all or nothing.

So I suggest combining this statement with one of
VIRTIO_NET_CTRL_NOTF_COAL_TX_SET and VIRTIO_NET_CTRL_NOTF_COAL_RX_SET,
and adding something like
"in this case, coalescing parameters MUST remain unchanged, 
for all VQs".

This bug bothers me. Maybe we should fix it before release?

I also don't know why we repeat same text for VIRTIO_NET_CTRL_NOTF_COAL_VQ_SET,
VIRTIO_NET_CTRL_NOTF_COAL_TX_SET and VIRTIO_NET_CTRL_NOTF_COAL_RX_SET.

>  
> -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


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