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Message-ID: <23guh3txkghxpgcrcjx7h62qsoj3xgjhfzgtbmqp2slrz3rxr4@zya2z7kwt75l> 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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