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 > > 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. > > 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. > >> *-------------------------------* >> | 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. Thanks, Arseniy > > Thanks, > Stefano >
