Re: [RFC PATCH v2 00/15] vsock: MSG_ZEROCOPY flag support

[Stefano Garzarella]

Hi Arseniy,
Sorry for the delay, but I have been very busy.

I can't apply this series on master or net-next, can you share with me
the base commit?

On Sun, Apr 23, 2023 at 10:26:28PM +0300, Arseniy Krasnov wrote:
> Hello,
>
>                           DESCRIPTION
>
> this is MSG_ZEROCOPY feature support for virtio/vsock. I tried to follow
> current implementation for TCP as much as possible:
>
> 1) Sender must enable SO_ZEROCOPY flag to use this feature. Without this
>   flag, data will be sent in "classic" copy manner and MSG_ZEROCOPY
>   flag will be ignored (e.g. without completion).
>
> 2) Kernel uses completions from socket's error queue. Single completion
>   for single tx syscall (or it can merge several completions to single
>   one). I used already implemented logic for MSG_ZEROCOPY support:
>   'msg_zerocopy_realloc()' etc.
>
> Difference with copy way is not significant. During packet allocation,
> non-linear skb is created, then I call 'pin_user_pages()' for each page
> from user's iov iterator and add each returned page to the skb as fragment.
> There are also some updates for vhost and guest parts of transport - in
> both cases i've added handling of non-linear skb for virtio part. vhost
> copies data from such skb to the guest's rx virtio buffers. In the guest,
> virtio transport fills tx virtio queue with pages from skb.
>
> This version has several limits/problems:
>
> 1) As this feature totally depends on transport, there is no way (or it
>   is difficult) to check whether transport is able to handle it or not
>   during SO_ZEROCOPY setting. Seems I need to call AF_VSOCK specific
>   setsockopt callback from setsockopt callback for SOL_SOCKET, but this
>   leads to lock problem, because both AF_VSOCK and SOL_SOCKET callback
>   are not considered to be called from each other. So in current version
>   SO_ZEROCOPY is set successfully to any type (e.g. transport) of
>   AF_VSOCK socket, but if transport does not support MSG_ZEROCOPY,
>   tx routine will fail with EOPNOTSUPP.

Do you plan to fix this in the next versions?

If it is too complicated, I think we can have this limitation until we
find a good solution.

> 2) When MSG_ZEROCOPY is used, for each tx system call we need to enqueue
>   one completion. In each completion there is flag which shows how tx
>   was performed: zerocopy or copy. This leads that whole message must
>   be send in zerocopy or copy way - we can't send part of message with
>   copying and rest of message with zerocopy mode (or vice versa). Now,
>   we need to account vsock credit logic, e.g. we can't send whole data
>   once - only allowed number of bytes could sent at any moment. In case
>   of copying way there is no problem as in worst case we can send single
>   bytes, but zerocopy is more complex because smallest transmission
>   unit is single page. So if there is not enough space at peer's side
>   to send integer number of pages (at least one) - we will wait, thus
>   stalling tx side. To overcome this problem i've added simple rule -
>   zerocopy is possible only when there is enough space at another side
>   for whole message (to check, that current 'msghdr' was already used
>   in previous tx iterations i use 'iov_offset' field of it's iov iter).

So, IIUC if MSG_ZEROCOPY is set, but there isn't enough space in the
destination we temporarily disable zerocopy, also if MSG_ZEROCOPY is set.
Right?

If it is the case it seems reasonable to me.

> 3) loopback transport is not supported, because it requires to implement
>   non-linear skb handling in dequeue logic (as we "send" fragged skb
>   and "receive" it from the same queue). I'm going to implement it in
>   next versions.
>
>   ^^^ fixed in v2
>
> 4) Current implementation sets max length of packet to 64KB. IIUC this
>   is due to 'kmalloc()' allocated data buffers. I think, in case of
>   MSG_ZEROCOPY this value could be increased, because 'kmalloc()' is
>   not touched for data - user space pages are used as buffers. Also
>   this limit trims every message which is > 64KB, thus such messages
>   will be send in copy mode due to 'iov_offset' check in 2).
>
>   ^^^ fixed in v2
>
>                         PATCHSET STRUCTURE
>
> Patchset has the following structure:
> 1) Handle non-linear skbuff on receive in virtio/vhost.
> 2) Handle non-linear skbuff on send in virtio/vhost.
> 3) Updates for AF_VSOCK.
> 4) Enable MSG_ZEROCOPY support on transports.
> 5) Tests/tools/docs updates.
>
>                            PERFORMANCE
>
> Performance: it is a little bit tricky to compare performance between
> copy and zerocopy transmissions. In zerocopy way we need to wait when
> user buffers will be released by kernel, so it something like synchronous
> path (wait until device driver will process it), while in copy way we
> can feed data to kernel as many as we want, don't care about device
> driver. So I compared only time which we spend in the 'send()' syscall.
> Then if this value will be combined with total number of transmitted
> bytes, we can get Gbit/s parameter. Also to avoid tx stalls due to not
> enough credit, receiver allocates same amount of space as sender needs.
>
> Sender:
> ./vsock_perf --sender <CID> --buf-size <buf size> --bytes 256M [--zc]
>
> Receiver:
> ./vsock_perf --vsk-size 256M
>
> G2H transmission (values are Gbit/s):
>
> *-------------------------------*
> |          |         |          |
> | buf size |   copy  | zerocopy |
> |          |         |          |
> *-------------------------------*
> |   4KB    |    3    |    10    |
> *-------------------------------*
> |   32KB   |    9    |    45    |
> *-------------------------------*
> |   256KB  |    24   |    195   |
> *-------------------------------*
> |    1M    |    27   |    270   |
> *-------------------------------*
> |    8M    |    22   |    277   |
> *-------------------------------*
>
> H2G:
>
> *-------------------------------*
> |          |         |          |
> | buf size |   copy  | zerocopy |
> |          |         |          |
> *-------------------------------*
> |   4KB    |    17   |    11    |

Do you know why in this case zerocopy is slower in this case?
Could be the cost of pin/unpin pages?

> *-------------------------------*
> |   32KB   |    30   |    66    |
> *-------------------------------*
> |   256KB  |    38   |    179   |
> *-------------------------------*
> |    1M    |    38   |    234   |
> *-------------------------------*
> |    8M    |    28   |    279   |
> *-------------------------------*
>
> Loopback:
>
> *-------------------------------*
> |          |         |          |
> | buf size |   copy  | zerocopy |
> |          |         |          |
> *-------------------------------*
> |   4KB    |    8    |    7     |
> *-------------------------------*
> |   32KB   |    34   |    42    |
> *-------------------------------*
> |   256KB  |    43   |    83    |
> *-------------------------------*
> |    1M    |    40   |    109   |
> *-------------------------------*
> |    8M    |    40   |    171   |
> *-------------------------------*
>
> I suppose that huge difference above between both modes has two reasons:
> 1) We don't need to copy data.
> 2) We don't need to allocate buffer for data, only for header.
>
> Zerocopy is faster than classic copy mode, but of course it requires
> specific architecture of application due to user pages pinning, buffer
> size and alignment.
>
> If host fails to send data with "Cannot allocate memory", check value
> /proc/sys/net/core/optmem_max - it is accounted during completion skb
> allocation.

What the user needs to do? Increase it?

>                            TESTING
>
> This patchset includes set of tests for MSG_ZEROCOPY feature. I tried to
> cover new code as much as possible so there are different cases for
> MSG_ZEROCOPY transmissions: with disabled SO_ZEROCOPY and several io
> vector types (different sizes, alignments, with unmapped pages). I also
> run tests with loopback transport and running vsockmon.

Thanks for the test again :-)

This cover letter is very good, with a lot of details, but please add
more details in each single patch, explaining the reason of the changes,
otherwise it is very difficult to review, because it is a very big
change.

I'll do a per-patch review in the next days.

Thanks,
Stefano