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



å 2024/1/20 äå5:59, Parav Pandit åé:

From: Heng Qi <hengqi@linux.alibaba.com>
Sent: Wednesday, January 17, 2024 10:22 AM

å 2024/1/15 äå9:21, Parav Pandit åé:

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.

Yes, we don't have to wait.

But in general, for user commands, it is necessary to obtain the final results
synchronously.

Yes. Use initiated command can enqueue the request to cvq. Go to sleep for several micro to milliseconds.


The user command cannot return before the final result is obtained.
And wait is not the problem this patch solves.


By not holding the rtnl lock, rest of the context that needs to enqueue the request can progress such as that of netdim.


Would like to see the using of rtnl lock changed.
In addition, I have made batching and asynchronousization of the netdim command, you can refer to this patch: 
https://lore.kernel.org/all/1705410693-118895-4-git-send-email-hengqi@linux.alibaba.com/




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.

When batch user commands occur, ensuring synchronization is a concern.


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.

First we have to solve the ctrlq batch adding user (ethtool) command.
Even if processed in a batch way on device side, the number of kicks and the
number of backend DMAs has not been reduced.

Driver notifications are PCI writes so it should not hamper device side, which can ignore them when they do not bother about it.
Driver notifications need to be processed by the DPU, which interferes with the CPU on the DPU. 


Backend DMAs should be reduced by avoiding the LIFO pattern followed by the splitq driver.
Placing the descriptors contiguously like packedq reduces amount of DMA naturally.
splitq is widely used, migrating to packedq is not that easy, especially when there are many components and hardware involved. 



The second predicable DMA to avoid is having 8Bytes of data inline in the descriptor, instead of 16B indirection and extra dma.


Looking forward to working inline!
But I think this does not conflict with batch work, and combining the two will be more beneficial. 




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.

Yes, this sounds good.


This the next item to focus as soon as flow filters are stable/merged.


Greate!


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.

In practice, we do not try to accumulate multiple parameter modifications for
a specific vqn.

I meant to accumulate parameters of multiple VQs to batch in single command.
Yes, this is what is done now, practical example: https://lore.kernel.org/all/1705410693-118895-3-git-send-email-hengqi@linux.alibaba.com/ 
In our scenario, only benefits are seen, no negative effects.




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).

Do you have any good directions and attempts to remove rtnl_lock?


I think per device lock instead of rtnl is first step that we can start with.


Totally looking forward to it.

Thanks,
Heng


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.

YES. Makes sense for our DPUs.


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.
Why is it delayed by the driver? It is not delayed by the driver, the kick still
happens for every command.

Since the kick is delayed, the driver lost the amount of time and device reaction time is driven by this wait time.
The guest driver is not aware how much a shared DPU is busy.
So if it waits too much, the VQs moderation is slow.


In theory and practice, it will not affect DIM performance, but it will
significantly reduce CPU consumption caused by waiting.


Once the guest driver sleeps after enqueuing the request, it should be fine.
The DIM calls are in highly parallel worker threads, where/if the thread sleep, a new worker thread is invoked.

But for sure I agree that vq notification moderation should happen quickly enough under < 100usec or so.
May be now that we bring the aq resources, we can start migrating on it.


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
v1->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.


A new feature bit is needed for this extra functionality.

I tried to extend it to the command VIRTIO_NET_F_VQ_NOTF_COAL, is it too
late?


For existing command it is not.
But there may be a device which may not support coalesced command.
Better to have the feature bit for this coalesce command, instead of random NOSUPP failure.


Thanks!


   \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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