Date: Wed, 3 May 2023 15:47:51 +0200
From: Stefano Garzarella <sgarzare@...hat.com>
To: Arseniy Krasnov <avkrasnov@...rdevices.ru>
Cc: Stefan Hajnoczi <stefanha@...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>, 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 v2 00/15] vsock: MSG_ZEROCOPY flag support

On Wed, May 03, 2023 at 04:11:59PM +0300, Arseniy Krasnov wrote:
>
>
>On 03.05.2023 15:52, Stefano Garzarella wrote:
>> Hi Arseniy,
>> Sorry for the delay, but I have been very busy.
>
>Hello, no problem!
>
>>
>> I can't apply this series on master or net-next, can you share with me
>> the base commit?
>
>Here is my base:
>https://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next.git/commit/?id=b103bab0944be030954e5de23851b37980218f54
>

Thanks, it worked!

>>
>> 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.
>>
>
>I'll try to fix it again, but just didn't pay attention on it in v2.
>
>>>
>>> 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?
>
>Exactly, user still needs to get completion (because SO_ZEROCOPY is enabled and
>MSG_ZEROCOPY flag as used). But completion structure contains information that
>there was copying instead of zerocopying.

Got it.

>
>>
>> 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?
>May be, i think i need to analyze such enormous difference more. Also about
>pin/unpin: i found that there is already implemented function to fill non-linear
>skb with pages from user's iov: __zerocopy_sg_from_iter() in net/core/datagram.c.
>It uses 'get_user_pages()' instead of 'pin_user_pages()'. May be in my case it
>is also valid to user 'get_XXX()' instead of 'pin_XXX()', because it is used by
>TCP MSG_ZEROCOPY and iouring MSG_ZEROCOPY.

If we can reuse them, it will be great!

>
>>
>>> *-------------------------------*
>>> |   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?
>>
>Yes, i'll update 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.
>
>Sure, thanks! In v3 i'm also working on io_uring test, because this thing also
>supports MSG_ZEROCOPY, so we can do virtio/vsock + MSG_ZEROCOPY + io_uring.

That would be cool!

Do you want to me to review these patches or it is better to wait for 
v3?

Thanks,
Stefano

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