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Subject: Re: [PATCH v2] virtio-net: support setting coalescing params for multiple vqs
On Wed, Jan 24, 2024 at 05:50:28AM -0500, Michael S. Tsirkin wrote: > 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. I figured it out, TX/RX are SHOULD. I will fix TX/RX and you can add the text to this one. > > > > -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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