Date:   Wed, 18 May 2022 11:04:30 +0000
From:   Arseniy Krasnov <AVKrasnov@...rdevices.ru>
To:     Stefano Garzarella <sgarzare@...hat.com>
CC:     Stefan Hajnoczi <stefanha@...hat.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        kernel <kernel@...rdevices.ru>
Subject: Re: [RFC PATCH v1 0/8] virtio/vsock: experimental zerocopy receive

Hello Stefano,

On 17.05.2022 18:14, Stefano Garzarella wrote:
> Hi Arseniy,
> 
> On Thu, May 12, 2022 at 05:04:11AM +0000, Arseniy Krasnov wrote:
>>                              INTRODUCTION
>>
>>     Hello, this is experimental implementation of virtio vsock zerocopy
>> receive. It was inspired by TCP zerocopy receive by Eric Dumazet. This API uses
>> same idea: call 'mmap()' on socket's descriptor, then every 'getsockopt()' will
>> fill provided vma area with pages of virtio RX buffers. After received data was
>> processed by user, pages must be freed by 'madvise()'  call with MADV_DONTNEED
>> flag set(if user won't call 'madvise()', next 'getsockopt()' will fail).
> 
> Sounds cool, but maybe we would need some socket/net experts here for review.

Yes, that would be great

> 
> Could we do something similar for the sending path as well?

Here are thoughts about zerocopy transmission:
  
I tried to implement this feature in the following way: user creates
some page aligned buffer, then during tx packet allocation instead of
creating data buffer with 'kmalloc()', i tried to add user's buffer
to virtio queue. But found problem: as kernel virtio API uses virtual
addresses to add new buffers, in the deep of virtio subsystem
'virt_to_phys()' is called to get physical address of buffer, so user's
virtual address won't be translated correctly to physical address(in
theory, i can perform page walk for such user's va, get physical address
and pass some "fake" virtual address to virtio API in order to make
'virt_to_phys()' return valid physical address(but i think this is ugly).


If we are talking about 'mmap()' way, i think we can do the following:
user calls 'mmap()' on socket, kernel fills newly created mapping with
allocated pages(all pages have rw permissions). Now user can use pages
of this mapping(e.g. fill it with data). Finally, to start transmission,
user calls 'getsockopt()' or some 'ioctl()' and kernel processes data of
this mapping. Also as this call will return immediately(e.g. it is
asynchronous), some completion logic must be implemented. For example
use same way as MSG_ZEROCOPY uses - poll error queue of socket to get
message that pages could be reused, or don't allow user to work with
these pages: unmap it, perform transmission and finally free pages.
To start new transmission user need to call 'mmap()' again.

                            OR

I think there is another unusual way for zerocopy tx: let's use 'vmsplice()'
/'splice()'. In this approach to transmit something, user does the following
steps:
1) Creates pipe.
2) Calls 'vmsplice(SPLICE_F_GIFT)' on this pipe, insert data pages to it.
   SPLICE_F_GIFT allows user to forget about allocated pages - kernel will
   free it.
3) Calls 'splice(SPLICE_F_MOVE)' from pipe to socket. SPLICE_F_MOVE will
   move pages from pipe to socket(e.g. in special socket callback we got
   set of pipe's pages as input argument and all pages will be inserted
   to virtio queue).

But as SPLICE_F_MOVE support is disabled, it must be repaired first.
                                                                  
> 
>>
>>                                 DETAILS
>>
>>     Here is how mapping with mapped pages looks exactly: first page mapping
>> contains array of trimmed virtio vsock packet headers (in contains only length
>> of data on the corresponding page and 'flags' field):
>>
>>     struct virtio_vsock_usr_hdr {
>>         uint32_t length;
>>         uint32_t flags;
>>     };
>>
>> Field  'length' allows user to know exact size of payload within each sequence
>> of pages and 'flags' allows user to handle SOCK_SEQPACKET flags(such as message
>> bounds or record bounds). All other pages are data pages from RX queue.
>>
>>             Page 0      Page 1      Page N
>>
>>     [ hdr1 .. hdrN ][ data ] .. [ data ]
>>           |        |       ^           ^
>>           |        |       |           |
>>           |        *-------------------*
>>           |                |
>>           |                |
>>           *----------------*
>>
>>     Of course, single header could represent array of pages (when packet's
>> buffer is bigger than one page).So here is example of detailed mapping layout
>> for some set of packages. Lets consider that we have the following sequence  of
>> packages: 56 bytes, 4096 bytes and 8200 bytes. All pages: 0,1,2,3,4 and 5 will
>> be inserted to user's vma(vma is large enough).
>>
>>     Page 0: [[ hdr0 ][ hdr 1 ][ hdr 2 ][ hdr 3 ] ... ]
>>     Page 1: [ 56 ]
>>     Page 2: [ 4096 ]
>>     Page 3: [ 4096 ]
>>     Page 4: [ 4096 ]
>>     Page 5: [ 8 ]
>>
>>     Page 0 contains only array of headers:
>>     'hdr0' has 56 in length field.
>>     'hdr1' has 4096 in length field.
>>     'hdr2' has 8200 in length field.
>>     'hdr3' has 0 in length field(this is end of data marker).
>>
>>     Page 1 corresponds to 'hdr0' and has only 56 bytes of data.
>>     Page 2 corresponds to 'hdr1' and filled with data.
>>     Page 3 corresponds to 'hdr2' and filled with data.
>>     Page 4 corresponds to 'hdr2' and filled with data.
>>     Page 5 corresponds to 'hdr2' and has only 8 bytes of data.
>>
>>     This patchset also changes packets allocation way: today implementation
>> uses only 'kmalloc()' to create data buffer. Problem happens when we try to map
>> such buffers to user's vma - kernel forbids to map slab pages to user's vma(as
>> pages of "not large" 'kmalloc()' allocations are marked with PageSlab flag and
>> "not large" could be bigger than one page). So to avoid this, data buffers now
>> allocated using 'alloc_pages()' call.
>>
>>                                   TESTS
>>
>>     This patchset updates 'vsock_test' utility: two tests for new feature
>> were added. First test covers invalid cases. Second checks valid transmission
>> case.
> 
> Thanks for adding the test!
> 
>>
>>                                BENCHMARKING
>>
>>     For benchmakring I've added small utility 'rx_zerocopy'. It works in
>> client/server mode. When client connects to server, server starts sending exact
>> amount of data to client(amount is set as input argument).Client reads data and
>> waits for next portion of it. Client works in two modes: copy and zero-copy. In
>> copy mode client uses 'read()' call while in zerocopy mode sequence of 'mmap()'
>> /'getsockopt()'/'madvise()' are used. Smaller amount of time for transmission
>> is better. For server, we can set size of tx buffer and for client we can set
>> size of rx buffer or rx mapping size(in zerocopy mode). Usage of this utility
>> is quiet simple:
>>
>> For client mode:
>>
>> ./rx_zerocopy --mode client [--zerocopy] [--rx]
>>
>> For server mode:
>>
>> ./rx_zerocopy --mode server [--mb] [--tx]
>>
>> [--mb] sets number of megabytes to transfer.
>> [--rx] sets size of receive buffer/mapping in pages.
>> [--tx] sets size of transmit buffer in pages.
>>
>> I checked for transmission of 4000mb of data. Here are some results:
>>
>>                           size of rx/tx buffers in pages
>>               *---------------------------------------------------*
>>               |    8   |    32    |    64   |   256    |   512    |
>> *--------------*--------*----------*---------*----------*----------*
>> |   zerocopy   |   24   |   10.6   |  12.2   |   23.6   |    21    | secs to
>> *--------------*---------------------------------------------------- process
>> | non-zerocopy |   13   |   16.4   |  24.7   |   27.2   |   23.9   | 4000 mb
>> *--------------*----------------------------------------------------
>>
>> I think, that results are not so impressive, but at least it is not worse than
>> copy mode and there is no need to allocate memory for processing date.
> 
> Why is it twice as slow in the first column?

May be this is because memory copying for small buffers is very fast... i'll
analyze it deeply.

> 
>>
>>                                 PROBLEMS
>>
>>     Updated packet's allocation logic creates some problem: when host gets
>> data from guest(in vhost-vsock), it allocates at least one page for each packet
>> (even if packet has 1 byte payload). I think this could be resolved in several
>> ways:
> 
> Can we somehow copy the incoming packets into the payload of the already queued packet?

May be, i'll analyze it...

> 
> This reminds me that we have yet to fix a similar problem with kmalloc() as well...
> 
> https://bugzilla.kernel.org/show_bug.cgi?id=215329

Yes, but it is a little bit different case: IIUC this bug happens because 'kmalloc()'
uses memory chunks of some preallocated size.

> 
>>     1) Make zerocopy rx mode disabled by default, so if user didn't enable
>> it, current 'kmalloc()' way will be used.
> 
> That sounds reasonable to me, I guess also TCP needs a setsockopt() call to enable the feature, right?

Yes, You're right. I think i'll add this to v2.

> 
>>     2) Use 'kmalloc()' for "small" packets, else call page allocator. But
>> in this case, we have mix of packets, allocated in two different ways thus
>> during zerocopying to user(e.g. mapping pages to vma), such small packets will
>> be handled in some stupid way: we need to allocate one page for user, copy data
>> to it and then insert page to user's vma.
> 
> It seems more difficult to me, but at the same time doable. I would go more on option 1, though.
> 
>>
>> P.S: of course this is experimental RFC, so what do You think guys?
> 
> It seems cool :-)
> 
> But I would like some feedback from the net guys to have some TCP-like things.

Ok, i'll prepare v2 anyway: i need to analyze performance, may be more test coverage, rebase
over latest kernel and work on packet allocation problem(from above).

> 
> Thanks,
> Stefano
> 

Thanks

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