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Message-ID: <8e98eb7f-5263-48ca-86be-dfc43c135385@bytedance.com>
Date: Mon, 1 Jul 2024 12:58:25 -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: [PATCH net-next v6 3/4] sock: add MSG_ZEROCOPY notification
mechanism based on msg_control
On 6/30/24 7:44 AM, Willem de Bruijn wrote:
> zijianzhang@ wrote:
>> From: Zijian Zhang <zijianzhang@...edance.com>
>>
>> The MSG_ZEROCOPY flag enables copy avoidance for socket send calls.
>> However, zerocopy is not a free lunch. Apart from the management of user
>> pages, the combination of poll + recvmsg to receive notifications incurs
>> unignorable overhead in the applications. The overhead of such sometimes
>> might be more than the CPU savings from zerocopy. We try to solve this
>> problem with a new notification mechanism based on msgcontrol.
>>
>> This new mechanism aims to reduce the overhead associated with receiving
>> notifications by embedding them directly into user arguments passed with
>> each sendmsg control message. 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.
>>
>> Signed-off-by: Zijian Zhang <zijianzhang@...edance.com>
>> Signed-off-by: Xiaochun Lu <xiaochun.lu@...edance.com>
>> ---
>> 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/linux/socket.h | 2 +-
>> include/uapi/asm-generic/socket.h | 2 ++
>> include/uapi/linux/socket.h | 10 +++++++
>> net/core/sock.c | 42 +++++++++++++++++++++++++++
>> 8 files changed, 63 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/alpha/include/uapi/asm/socket.h b/arch/alpha/include/uapi/asm/socket.h
>> index e94f621903fe..7761a4e0ea2c 100644
>> --- a/arch/alpha/include/uapi/asm/socket.h
>> +++ b/arch/alpha/include/uapi/asm/socket.h
>> @@ -140,6 +140,8 @@
>> #define SO_PASSPIDFD 76
>> #define SO_PEERPIDFD 77
>>
>> +#define SCM_ZC_NOTIFICATION 78
>> +
>> #if !defined(__KERNEL__)
>>
>> #if __BITS_PER_LONG == 64
>> diff --git a/arch/mips/include/uapi/asm/socket.h b/arch/mips/include/uapi/asm/socket.h
>> index 60ebaed28a4c..89edc51380f0 100644
>> --- a/arch/mips/include/uapi/asm/socket.h
>> +++ b/arch/mips/include/uapi/asm/socket.h
>> @@ -151,6 +151,8 @@
>> #define SO_PASSPIDFD 76
>> #define SO_PEERPIDFD 77
>>
>> +#define SCM_ZC_NOTIFICATION 78
>> +
>> #if !defined(__KERNEL__)
>>
>> #if __BITS_PER_LONG == 64
>> diff --git a/arch/parisc/include/uapi/asm/socket.h b/arch/parisc/include/uapi/asm/socket.h
>> index be264c2b1a11..2911b43e6a9d 100644
>> --- a/arch/parisc/include/uapi/asm/socket.h
>> +++ b/arch/parisc/include/uapi/asm/socket.h
>> @@ -132,6 +132,8 @@
>> #define SO_PASSPIDFD 0x404A
>> #define SO_PEERPIDFD 0x404B
>>
>> +#define SCM_ZC_NOTIFICATION 0x404C
>> +
>> #if !defined(__KERNEL__)
>>
>> #if __BITS_PER_LONG == 64
>> diff --git a/arch/sparc/include/uapi/asm/socket.h b/arch/sparc/include/uapi/asm/socket.h
>> index 682da3714686..dc045e87cc8e 100644
>> --- a/arch/sparc/include/uapi/asm/socket.h
>> +++ b/arch/sparc/include/uapi/asm/socket.h
>> @@ -133,6 +133,8 @@
>> #define SO_PASSPIDFD 0x0055
>> #define SO_PEERPIDFD 0x0056
>>
>> +#define SCM_ZC_NOTIFICATION 0x0057
>> +
>> #if !defined(__KERNEL__)
>>
>>
>> diff --git a/include/linux/socket.h b/include/linux/socket.h
>> index 35adc30c9db6..f2f013166525 100644
>> --- a/include/linux/socket.h
>> +++ b/include/linux/socket.h
>> @@ -170,7 +170,7 @@ static inline struct cmsghdr * cmsg_nxthdr (struct msghdr *__msg, struct cmsghdr
>>
>> static inline bool cmsg_copy_to_user(struct cmsghdr *__cmsg)
>> {
>> - return 0;
>> + return __cmsg->cmsg_type == SCM_ZC_NOTIFICATION;
>> }
>>
>> static inline size_t msg_data_left(struct msghdr *msg)
>> diff --git a/include/uapi/asm-generic/socket.h b/include/uapi/asm-generic/socket.h
>> index 8ce8a39a1e5f..7474c8a244bc 100644
>> --- a/include/uapi/asm-generic/socket.h
>> +++ b/include/uapi/asm-generic/socket.h
>> @@ -135,6 +135,8 @@
>> #define SO_PASSPIDFD 76
>> #define SO_PEERPIDFD 77
>>
>> +#define SCM_ZC_NOTIFICATION 78
>> +
>> #if !defined(__KERNEL__)
>>
>> #if __BITS_PER_LONG == 64 || (defined(__x86_64__) && defined(__ILP32__))
>> diff --git a/include/uapi/linux/socket.h b/include/uapi/linux/socket.h
>> index d3fcd3b5ec53..26bee6291c6c 100644
>> --- a/include/uapi/linux/socket.h
>> +++ b/include/uapi/linux/socket.h
>> @@ -2,6 +2,8 @@
>> #ifndef _UAPI_LINUX_SOCKET_H
>> #define _UAPI_LINUX_SOCKET_H
>>
>> +#include <linux/types.h>
>> +
>> /*
>> * Desired design of maximum size and alignment (see RFC2553)
>> */
>> @@ -35,4 +37,12 @@ struct __kernel_sockaddr_storage {
>> #define SOCK_TXREHASH_DISABLED 0
>> #define SOCK_TXREHASH_ENABLED 1
>>
>> +#define SOCK_ZC_INFO_MAX 16
>> +
>> +struct zc_info_elem {
>> + __u32 lo;
>> + __u32 hi;
>> + __u8 zerocopy;
>> +};
>> +
>> #endif /* _UAPI_LINUX_SOCKET_H */
>> diff --git a/net/core/sock.c b/net/core/sock.c
>> index 4a766a91ff5c..1b2ce72e1338 100644
>> --- a/net/core/sock.c
>> +++ b/net/core/sock.c
>> @@ -2863,6 +2863,48 @@ int __sock_cmsg_send(struct sock *sk, struct cmsghdr *cmsg,
>> case SCM_RIGHTS:
>> case SCM_CREDENTIALS:
>> break;
>> + case SCM_ZC_NOTIFICATION: {
>> + struct zc_info_elem *zc_info_kern = CMSG_DATA(cmsg);
>> + int cmsg_data_len, zc_info_elem_num;
>> + struct sock_exterr_skb *serr;
>> + struct sk_buff_head *q;
>> + unsigned long flags;
>> + struct sk_buff *skb;
>> + int i = 0;
>> +
>> + if (!sock_flag(sk, SOCK_ZEROCOPY) || sk->sk_family == PF_RDS)
>> + return -EINVAL;
>> +
>> + cmsg_data_len = cmsg->cmsg_len - sizeof(struct cmsghdr);
>> + if (cmsg_data_len % sizeof(struct zc_info_elem))
>> + return -EINVAL;
>> +
>> + zc_info_elem_num = cmsg_data_len / sizeof(struct zc_info_elem);
>> + if (!zc_info_elem_num || zc_info_elem_num > SOCK_ZC_INFO_MAX)
>> + return -EINVAL;
>> +
>> + q = &sk->sk_error_queue;
>> + spin_lock_irqsave(&q->lock, flags);
>> + skb = skb_peek(q);
>> + while (skb && i < zc_info_elem_num) {
>> + struct sk_buff *skb_next = skb_peek_next(skb, q);
>> +
>> + serr = SKB_EXT_ERR(skb);
>> + if (serr->ee.ee_errno == 0 &&
>> + serr->ee.ee_origin == SO_EE_ORIGIN_ZEROCOPY) {
>> + zc_info_kern[i].hi = serr->ee.ee_data;
>> + zc_info_kern[i].lo = serr->ee.ee_info;
>> + zc_info_kern[i].zerocopy = !(serr->ee.ee_code
>> + & SO_EE_CODE_ZEROCOPY_COPIED);
>> + __skb_unlink(skb, q);
>> + consume_skb(skb);
>> + }
>> + skb = skb_next;
>> + i++;
>> + }
>
> How will userspace know the number of entries written? Since the
> cmsg_len is not updated, is the expectation that the CMSG_DATA is zero
> initialized by the user and the list will be zero-element terminated?
>
Since zero-element is a valid value(the first sendmsg with non-zerocopy)
in our case, zero-element terminated might not be feasible.
In current selftest, the user initializes zerocopy field to a invalid
value, and upon returning iterate the array until the invalid value. It
is very dependent to the user, I think it's not good enough.
I agree that updating cmsg_len is better.
>> + spin_unlock_irqrestore(&q->lock, flags);
>> + break;
>> + }
>> default:
>> return -EINVAL;
>> }
>> --
>> 2.20.1
>>
>
>
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