Date:   Thu, 25 May 2023 18:56:42 +0300
From:   Arseniy Krasnov <avkrasnov@...rdevices.ru>
To:     Stefan Hajnoczi <stefanha@...hat.com>,
        Stefano Garzarella <sgarzare@...hat.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Bobby Eshleman <bobby.eshleman@...edance.com>
CC:     <kvm@...r.kernel.org>, <virtualization@...ts.linux-foundation.org>,
        <netdev@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <kernel@...rdevices.ru>, <oxffffaa@...il.com>
Subject: Re: [RFC PATCH v3 00/17] vsock: MSG_ZEROCOPY flag support



On 22.05.2023 10:39, Arseniy Krasnov wrote:

This patchset is unstable with SOCK_SEQPACKET. I'll fix it.

Thanks, Arseniy

> 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 and filled with pinned user pages.
> 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.
> 
> Head of this patchset is:
> https://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next.git/commit/?id=94e86ef1b801d213dfb8543633dec86abb1a457d
> 
> 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.
> 
>    ^^^
>    This is still no resolved :(
> 
> 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).
> 
>    ^^^
>    Discussed as ok during v2. Link:
>    https://lore.kernel.org/netdev/23guh3txkghxpgcrcjx7h62qsoj3xgjhfzgtbmqp2slrz3rxr4@zya2z7kwt75l/
> 
> 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 is 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
> 
> I run tests on two setups: desktop with Core i7 - I use this PC for
> development and in this case guest is nested guest, and host is normal
> guest. Another hardware is some embedded board with Atom - here I don't
> have nested virtualization - host runs on hw, and guest is normal guest.
> 
> G2H transmission (values are Gbit/s):
> 
>    Core i7 with nested guest.            Atom with normal guest.
> 
> *-------------------------------*   *-------------------------------*
> |          |         |          |   |          |         |          |
> | buf size |   copy  | zerocopy |   | buf size |   copy  | zerocopy |
> |          |         |          |   |          |         |          |
> *-------------------------------*   *-------------------------------*
> |   4KB    |    3    |    10    |   |   4KB    |   0.8   |   1.9    |
> *-------------------------------*   *-------------------------------*
> |   32KB   |   20    |    61    |   |   32KB   |   6.8   |   20.2   |
> *-------------------------------*   *-------------------------------*
> |   256KB  |   33    |   244    |   |   256KB  |   7.8   |   55     |
> *-------------------------------*   *-------------------------------*
> |    1M    |   30    |   373    |   |    1M    |   7     |   95     |
> *-------------------------------*   *-------------------------------*
> |    8M    |   22    |   475    |   |    8M    |   7     |   114    |
> *-------------------------------*   *-------------------------------*
> 
> H2G:
> 
>    Core i7 with nested guest.            Atom with normal guest.
> 
> *-------------------------------*   *-------------------------------*
> |          |         |          |   |          |         |          |
> | buf size |   copy  | zerocopy |   | buf size |   copy  | zerocopy |
> |          |         |          |   |          |         |          |
> *-------------------------------*   *-------------------------------*
> |   4KB    |   20    |    10    |   |   4KB    |   4.37  |    3     |
> *-------------------------------*   *-------------------------------*
> |   32KB   |   37    |    75    |   |   32KB   |   11    |   18     |
> *-------------------------------*   *-------------------------------*
> |   256KB  |   44    |   299    |   |   256KB  |   11    |   62     |
> *-------------------------------*   *-------------------------------*
> |    1M    |   28    |   335    |   |    1M    |   9     |   77     |
> *-------------------------------*   *-------------------------------*
> |    8M    |   27    |   417    |   |    8M    |  9.35   |  115     |
> *-------------------------------*   *-------------------------------*
> 
>  * Let's look to the first line of both tables - where copy is better
>    than zerocopy. I analyzed this case more deeply and found that
>    bottleneck is function 'vhost_work_queue()'. With 4K buffer size,
>    caller spends too much time in it with zerocopy mode (comparing to
>    copy mode). This happens only with 4K buffer size. This function just
>    calls 'wake_up_process()' and its internal logic does not depends on
>    skb, so i think potential reason (may be) is interval between two
>    calls of this function (e.g. how often it is called). Note, that
>    'vhost_work_queue()' differs from the same function at guest's side of
>    transport: 'virtio_transport_send_pkt()' uses 'queue_work()' which
>    i think is more optimized for worker purposes, than direct call to
>    'wake_up_process()'. But again - this is just my assumption.
> 
> Loopback:
> 
>    Core i7 with nested guest.            Atom with normal guest.
> 
> *-------------------------------*   *-------------------------------*
> |          |         |          |   |          |         |          |
> | buf size |   copy  | zerocopy |   | buf size |   copy  | zerocopy |
> |          |         |          |   |          |         |          |
> *-------------------------------*   *-------------------------------*
> |   4KB    |    8    |     7    |   |   4KB    |   1.8   |   1.3    |
> *-------------------------------*   *-------------------------------*
> |   32KB   |   38    |    44    |   |   32KB   |   10    |   10     |
> *-------------------------------*   *-------------------------------*
> |   256KB  |   55    |   168    |   |   256KB  |   15    |   36     |
> *-------------------------------*   *-------------------------------*
> |    1M    |   53    |   250    |   |    1M    |   12    |   45     |
> *-------------------------------*   *-------------------------------*
> |    8M    |   40    |   344    |   |    8M    |   11    |   74     |
> *-------------------------------*   *-------------------------------*
> 
> I analyzed performace difference more deeply for the following setup:
> server: ./vsock_perf --vsk-size 16M
> client: ./vsock_perf --sender 2 --bytes 16M --buf-size 16K/4K [--zc]
> 
> In other words I send 16M of data from guest to host in copy/zerocopy
> modes and with two different sizes of buffer - 4K and 64K. Let's see
> to tx path for both modes - it consists of two steps:
> 
> copy:
> 1) Allocate skb of buffer's length.
> 2) Copy data to skb from buffer.
> 
> zerocopy:
> 1) Allocate skb with header space only.
> 2) Pin pages of the buffer and insert them to skb.
> 
> I measured average number of ns (returned by 'ktime_get()') for each
> step above:
> 1) Skb allocation (for both copy and zerocopy modes).
> 2) For copy mode in 'memcpy_to_msg()' - copying.
> 3) For zerocopy mode in '__zerocopy_sg_from_iter()' - pinning.
> 
> Here are results for copy mode:
> *-------------------------------------*
> | buf | skb alloc | 'memcpy_to_msg()' |
> *-------------------------------------*
> |     |           |                   |
> | 64K |  5000ns   |      25000ns      |
> |     |           |                   |
> *-------------------------------------*
> |     |           |                   |
> | 4K  |  800ns    |      2200ns       |
> |     |           |                   |
> *-------------------------------------*
> 
> Here are results for zerocopy mode:
> *-----------------------------------------------*
> | buf | skb alloc | '__zerocopy_sg_from_iter()' |
> *-----------------------------------------------*
> |     |           |                             |
> | 64K |  250ns    |          3500ns             |
> |     |           |                             |
> *-----------------------------------------------*
> |     |           |                             |
> | 4K  |  250ns    |          3000ns             |
> |     |           |                             |
> *-----------------------------------------------*
> 
> I guess that reason of zerocopy performance is low overhead for page
> pinning: there is big difference between 4K and 64K in case of copying
> (25000 vs 2200), but in pinning case - just 3000 vs 3500.
> 
> So, 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.
> 
>                              NOTES
> 
> 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. Try to update it to for example 1M and try send again:
> "echo 1048576 > /proc/sys/net/core/optmem_max" (as root).
> 
>                             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 run vsockmon. In v3 i've added
> io_uring test as separated application.
> 
> Thanks, Arseniy
> 
> Link to v1:
> https://lore.kernel.org/netdev/0e7c6fc4-b4a6-a27b-36e9-359597bba2b5@sberdevices.ru/
> Link to v2:
> https://lore.kernel.org/netdev/20230423192643.1537470-1-AVKrasnov@sberdevices.ru/
> 
> Changelog:
> v1 -> v2:
>  - Replace 'get_user_pages()' with 'pin_user_pages()'.
>  - Loopback transport support.
> 
> v2 -> v3
>  - Use 'get_user_pages()' instead of 'pin_user_pages()'. I think this
>    is right approach, because i'm using '__zerocopy_sg_from_iter()'
>    function. It is already implemented and used by io_uring zerocopy
>    tx logic to 'pin' pages of user's buffer.
> 
>  - Use 'skb_copy_datagram_iter()' to copy data from both linear and
>    non-linear skb to user's iov iter. It already has support for copying
>    data from paged part of skb (by calling 'kmap()'). In v2 i used my
>    own "from scratch" implemented function. With this and previous thing
>    I significantly reduced LOC number in kernel part.
> 
>  - Add io_uring test for AF_VSOCK. It is implemented as separated util,
>    because it depends on liburing (i think there is no need to link
>    'vsock_test' with liburing, because io_uring functionality depends
>    on environment - both in kernel and userspace).
> 
>  - Values from PERFORMANCE section are updated for all transports, but
>    I didn't found any significant difference with v2.
> 
>  - More details in commit messages.
> 
> Arseniy Krasnov (17):
>   vsock/virtio: read data from non-linear skb
>   vhost/vsock: read data from non-linear skb
>   vsock/virtio: support to send non-linear skb
>   vsock/virtio: non-linear skb handling for tap
>   vsock/virtio: MSG_ZEROCOPY flag support
>   vsock: check error queue to set EPOLLERR
>   vsock: read from socket's error queue
>   vsock: check for MSG_ZEROCOPY support
>   vsock: enable SOCK_SUPPORT_ZC bit
>   vhost/vsock: support MSG_ZEROCOPY for transport
>   vsock/virtio: support MSG_ZEROCOPY for transport
>   vsock/loopback: support MSG_ZEROCOPY for transport
>   net/sock: enable setting SO_ZEROCOPY for PF_VSOCK
>   docs: net: description of MSG_ZEROCOPY for AF_VSOCK
>   test/vsock: MSG_ZEROCOPY flag tests
>   test/vsock: MSG_ZEROCOPY support for vsock_perf
>   test/vsock: io_uring rx/tx tests
> 
>  Documentation/networking/msg_zerocopy.rst |  12 +-
>  drivers/vhost/vsock.c                     |  18 +-
>  include/linux/socket.h                    |   1 +
>  include/linux/virtio_vsock.h              |   1 +
>  include/net/af_vsock.h                    |   7 +
>  net/core/sock.c                           |   4 +-
>  net/vmw_vsock/af_vsock.c                  |  19 +-
>  net/vmw_vsock/virtio_transport.c          |  39 ++-
>  net/vmw_vsock/virtio_transport_common.c   | 352 ++++++++++++++++----
>  net/vmw_vsock/vsock_loopback.c            |   8 +
>  tools/testing/vsock/Makefile              |   9 +-
>  tools/testing/vsock/util.c                | 134 ++++++++
>  tools/testing/vsock/util.h                |  23 ++
>  tools/testing/vsock/vsock_perf.c          | 139 +++++++-
>  tools/testing/vsock/vsock_test.c          |  11 +
>  tools/testing/vsock/vsock_test_zerocopy.c | 385 ++++++++++++++++++++++
>  tools/testing/vsock/vsock_test_zerocopy.h |  12 +
>  tools/testing/vsock/vsock_uring_test.c    | 316 ++++++++++++++++++
>  18 files changed, 1396 insertions(+), 94 deletions(-)
>  create mode 100644 tools/testing/vsock/vsock_test_zerocopy.c
>  create mode 100644 tools/testing/vsock/vsock_test_zerocopy.h
>  create mode 100644 tools/testing/vsock/vsock_uring_test.c
> 

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