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Subject: RE: [virtio-comment] [PATCH v2] virtio-net: support setting coalescing params for multiple vqs
> From: virtio-comment@lists.oasis-open.org <virtio-comment@lists.oasis-
> open.org> On Behalf Of Heng Qi
> Sent: Monday, January 15, 2024 6:36 PM
>
> 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.
It does not need to wait. This is some driver limitation that does not use the queue as "queue".
Such driver limitation should be removed in the driver. It does not qualify as limitation.
This will enable driver to enqueue multiple cvq commands without waiting for previous one.
This will also enable device to find natural coalescing done on multiple commands.
> 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.
When there is large devices and multiple driver notifications by a cpu that is N times faster than the device side cpu,
the device may find natural coalescing on the commands of a given cvq.
For multiple DMAs, we need to way to send 8 bytes of data without 16 bytes of indirection via a descriptor.
This is what we discussed a while back to do in txq and Stefan suggested to generalize for more queues, which is also a good idea.
> 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
The batching done may end up modifying the given VQ's parameters multiple times.
The right test on Linux to do without rtnl lock which is anyway ugly and wrong semantic to use blocking the whole netdev stack.
(in case if you used that).
> 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.
>
This is good.
> 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.
This is unlikely to improve, rather it will have negative impact as it only means that moderation parameters are just delayed by the driver.
> 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.
In that case fixing the guest driver which is not yet written is the right fix.
> 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
> v1->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.
>
A new feature bit is needed for this extra functionality.
> \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
>
>
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