Date: Wed, 29 May 2024 10:34:11 -0700
From: Zijian Zhang <zijianzhang@...edance.com>
To: Willem de Bruijn <willemdebruijn.kernel@...il.com>, netdev@...r.kernel.org
Cc: edumazet@...gle.com, cong.wang@...edance.com, xiaochun.lu@...edance.com
Subject: Re: [External] Re: [PATCH net-next v4 0/3] net: A lightweight
 zero-copy notification

On 5/29/24 6:59 AM, Willem de Bruijn wrote:
> zijianzhang@ wrote:
>> From: Zijian Zhang <zijianzhang@...edance.com>
>>
>> Original title is "net: socket sendmsg MSG_ZEROCOPY_UARG".
>>
>> Original notification mechanism needs poll + recvmmsg which is not
>> easy for applcations to accommodate. And, it also incurs unignorable
>> overhead including extra system calls and usage of socket optmem.
>>
>> While making maximum reuse of the existing MSG_ZEROCOPY related code,
>> this patch set introduces a new zerocopy socket notification mechanism.
>> Users of sendmsg pass a control message as a placeholder for the incoming
>> notifications. Upon returning, kernel embeds notifications directly into
>> user arguments passed in. By doing so, we can significantly reduce the
>> complexity and overhead for managing notifications. In an ideal pattern,
>> the user will keep calling sendmsg with SCM_ZC_NOTIFICATION msg_control,
>> and the notification will be delivered as soon as possible.
>>
>> Users need to pass in a user space address pointing to an array of struct
>> zc_info_elem, and the cmsg_len should be the memory size of the array
>> instead of the size of the pointer itself.
>>
>> As Willem commented,
>>
>>> The main design issue with this series is this indirection, rather
>>> than passing the array of notifications as cmsg.
>>
>>> This trick circumvents having to deal with compat issues and having to
>>> figure out copy_to_user in ____sys_sendmsg (as msg_control is an
>>> in-kernel copy).
>>
>>> This is quite hacky, from an API design PoV.
>>
>>> As is passing a pointer, but expecting msg_controllen to hold the
>>> length not of the pointer, but of the pointed to user buffer.
>>
>>> I had also hoped for more significant savings. Especially with the
>>> higher syscall overhead due to meltdown and spectre mitigations vs
>>> when MSG_ZEROCOPY was introduced and I last tried this optimization.
> 
> Thanks for quoting this.
> 
> This revision does not address either of these concerns, right?
> 

Right, this revision just fixed some code according to your comments.

>>
>> Changelog:
>>    v1 -> v2:
>>      - Reuse errormsg queue in the new notification mechanism,
>>        users can actually use these two mechanisms in hybrid way
>>        if they want to do so.
>>      - Update case SCM_ZC_NOTIFICATION in __sock_cmsg_send
>>        1. Regardless of 32-bit, 64-bit program, we will always handle
>>        u64 type user address.
>>        2. The size of data to copy_to_user is precisely calculated
>>        in case of kernel stack leak.
>>      - fix (kbuild-bot)
>>        1. Add SCM_ZC_NOTIFICATION to arch-specific header files.
>>        2. header file types.h in include/uapi/linux/socket.h
>>
>>    v2 -> v3:
>>      - 1. Users can now pass in the address of the zc_info_elem directly
>>        with appropriate cmsg_len instead of the ugly user interface. Plus,
>>        the handler is now compatible with MSG_CMSG_COMPAT and 32-bit
>>        pointer.
>>      - 2. Suggested by Willem, another strategy of getting zc info is
>>        briefly taking the lock of sk_error_queue and move to a private
>>        list, like net_rx_action. I thought sk_error_queue is protected by
>>        sock_lock, so that it's impossible for the handling of zc info and
>>        users recvmsg from the sk_error_queue at the same time.
>>        However, sk_error_queue is protected by its own lock. I am afraid
>>        that during the time it is handling the private list, users may
>>        fail to get other error messages in the queue via recvmsg. Thus,
>>        I don't implement the splice logic in this version. Any comments?
>>
>>    v3 -> v4:
>>      - 1. Change SOCK_ZC_INFO_MAX to 64 to avoid large stack frame size.
>>      - 2. Fix minor typos.
>>      - 3. Change cfg_zerocopy from int to enum in msg_zerocopy.c
>>
>> * Performance
>>
>> I extend the selftests/msg_zerocopy.c to accommodate the new mechanism,
>> test result is as follows,
>>
>> cfg_notification_limit = 1, in this case the original method approximately
>> aligns with the semantics of new one. In this case, the new flag has
>> around 13% cpu savings in TCP and 18% cpu savings in UDP.
>>
>> +---------------------+---------+---------+---------+---------+
>> | Test Type / Protocol| TCP v4  | TCP v6  | UDP v4  | UDP v6  |
>> +---------------------+---------+---------+---------+---------+
>> | ZCopy (MB)          | 5147    | 4885    | 7489    | 7854    |
>> +---------------------+---------+---------+---------+---------+
>> | New ZCopy (MB)      | 5859    | 5505    | 9053    | 9236    |
>> +---------------------+---------+---------+---------+---------+
>> | New ZCopy / ZCopy   | 113.83% | 112.69% | 120.88% | 117.59% |
>> +---------------------+---------+---------+---------+---------+
>>
>>
>> cfg_notification_limit = 32, the new mechanism performs 8% better in TCP.
>> For UDP, no obvious performance gain is observed and sometimes may lead
>> to degradation. Thus, if users don't need to retrieve the notification
>> ASAP in UDP, the original mechanism is preferred.
>>
>> +---------------------+---------+---------+---------+---------+
>> | Test Type / Protocol| TCP v4  | TCP v6  | UDP v4  | UDP v6  |
>> +---------------------+---------+---------+---------+---------+
>> | ZCopy (MB)          | 6272    | 6138    | 12138   | 10055   |
>> +---------------------+---------+---------+---------+---------+
>> | New ZCopy (MB)      | 6774    | 6620    | 11504   | 10355   |
>> +---------------------+---------+---------+---------+---------+
>> | New ZCopy / ZCopy   | 108.00% | 107.85% | 94.78%  | 102.98% |
>> +---------------------+---------+---------+---------+---------+
>>
>> Zijian Zhang (3):
>>    selftests: fix OOM problem in msg_zerocopy selftest
>>    sock: add MSG_ZEROCOPY notification mechanism based on msg_control
>>    selftests: add MSG_ZEROCOPY msg_control notification test
>>
>>   arch/alpha/include/uapi/asm/socket.h        |   2 +
>>   arch/mips/include/uapi/asm/socket.h         |   2 +
>>   arch/parisc/include/uapi/asm/socket.h       |   2 +
>>   arch/sparc/include/uapi/asm/socket.h        |   2 +
>>   include/uapi/asm-generic/socket.h           |   2 +
>>   include/uapi/linux/socket.h                 |  10 ++
>>   net/core/sock.c                             |  68 ++++++++++++
>>   tools/testing/selftests/net/msg_zerocopy.c  | 114 ++++++++++++++++++--
>>   tools/testing/selftests/net/msg_zerocopy.sh |   1 +
>>   9 files changed, 196 insertions(+), 7 deletions(-)
>>
>> -- 
>> 2.20.1
>>
> 
> 

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