Subject: Re: [PATCH net-next v2 1/2] net_tstamp: add SCM_TS_OPT_ID to provide OPT_ID in control message

From: Vadim Fedorenko <[email-masked]>
Date: Mon, 2 Sep 2024 22:10:21 +0100

On 02/09/2024 21:59, Willem de Bruijn wrote:
> Vadim Fedorenko wrote:
>> On 02/09/2024 16:51, Willem de Bruijn wrote:
>>> Jason Xing wrote:
>>>> On Mon, Sep 2, 2024 at 9:09 PM Vadim Fedorenko <[email-masked]> wrote:
>>>>>
>>>>> SOF_TIMESTAMPING_OPT_ID socket option flag gives a way to correlate TX
>>>>> timestamps and packets sent via socket. Unfortunately, there is no way
>>>>> to reliably predict socket timestamp ID value in case of error returned
>>>>> by sendmsg. For UDP sockets it's impossible because of lockless
>>>>> nature of UDP transmit, several threads may send packets in parallel. In
>>>>> case of RAW sockets MSG_MORE option makes things complicated. More
>>>>> details are in the conversation [1].
>>>>> This patch adds new control message type to give user-space
>>>>> software an opportunity to control the mapping between packets and
>>>>> values by providing ID with each sendmsg. This works fine for UDP
>>>>> sockets only, and explicit check is added to control message parser.
>>>>>
>>>>> [1] https://lore.kernel.org/netdev/[email-masked]/
>>>>>
>>>>> Signed-off-by: Vadim Fedorenko <[email-masked]>
>>>>> ---
>>>>>    Documentation/networking/timestamping.rst | 14 ++++++++++++++
>>>>>    arch/alpha/include/uapi/asm/socket.h      |  4 +++-
>>>>>    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/net/inet_sock.h                   |  4 +++-
>>>>>    include/net/sock.h                        |  1 +
>>>>>    include/uapi/asm-generic/socket.h         |  2 ++
>>>>>    include/uapi/linux/net_tstamp.h           |  3 ++-
>>>>>    net/core/sock.c                           | 12 ++++++++++++
>>>>>    net/ethtool/common.c                      |  1 +
>>>>>    net/ipv4/ip_output.c                      | 16 ++++++++++++----
>>>>>    net/ipv6/ip6_output.c                     | 16 ++++++++++++----
>>>>>    13 files changed, 68 insertions(+), 11 deletions(-)
>>>>>
>>>>> diff --git a/Documentation/networking/timestamping.rst b/Documentation/networking/timestamping.rst
>>>>> index 5e93cd71f99f..93b0901e4e8e 100644
>>>>> --- a/Documentation/networking/timestamping.rst
>>>>> +++ b/Documentation/networking/timestamping.rst
>>>>> @@ -193,6 +193,20 @@ SOF_TIMESTAMPING_OPT_ID:
>>>>>      among all possibly concurrently outstanding timestamp requests for
>>>>>      that socket.
>>>>>
>>>>> +  With this option enabled user-space application can provide custom
>>>>> +  ID for each message sent via UDP socket with control message with
>>>>> +  type set to SCM_TS_OPT_ID::
>>>>> +
>>>>> +    struct msghdr *msg;
>>>>> +    ...
>>>>> +    cmsg                        = CMSG_FIRSTHDR(msg);
>>>>> +    cmsg->cmsg_level            = SOL_SOCKET;
>>>>> +    cmsg->cmsg_type             = SO_TIMESTAMPING;
>>>>> +    cmsg->cmsg_len              = CMSG_LEN(sizeof(__u32));
>>>>> +    *((__u32 *) CMSG_DATA(cmsg)) = opt_id;
>>>>> +    err = sendmsg(fd, msg, 0);
>>>>> +
>>>>> +
>>>>>    SOF_TIMESTAMPING_OPT_ID_TCP:
>>>>>      Pass this modifier along with SOF_TIMESTAMPING_OPT_ID for new TCP
>>>>>      timestamping applications. SOF_TIMESTAMPING_OPT_ID defines how the
>>>>> diff --git a/arch/alpha/include/uapi/asm/socket.h b/arch/alpha/include/uapi/asm/socket.h
>>>>> index e94f621903fe..0698e6662cdf 100644
>>>>> --- a/arch/alpha/include/uapi/asm/socket.h
>>>>> +++ b/arch/alpha/include/uapi/asm/socket.h
>>>>> @@ -10,7 +10,7 @@
>>>>>     * Note: we only bother about making the SOL_SOCKET options
>>>>>     * same as OSF/1, as that's all that "normal" programs are
>>>>>     * likely to set.  We don't necessarily want to be binary
>>>>> - * compatible with _everything_.
>>>>> + * compatible with _everything_.
>>>>>     */
>>>>>    #define SOL_SOCKET     0xffff
>>>>>
>>>>> @@ -140,6 +140,8 @@
>>>>>    #define SO_PASSPIDFD           76
>>>>>    #define SO_PEERPIDFD           77
>>>>>
>>>>> +#define SCM_TS_OPT_ID          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..bb3dc8feb205 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_TS_OPT_ID          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..c3ab3b3289eb 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_TS_OPT_ID          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..9b40f0a57fbc 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_TS_OPT_ID            0x0057
>>>>> +
>>>>>    #if !defined(__KERNEL__)
>>>>>
>>>>>
>>>>> diff --git a/include/net/inet_sock.h b/include/net/inet_sock.h
>>>>> index 394c3b66065e..2161d50cf0fd 100644
>>>>> --- a/include/net/inet_sock.h
>>>>> +++ b/include/net/inet_sock.h
>>>>> @@ -174,6 +174,7 @@ struct inet_cork {
>>>>>           __s16                   tos;
>>>>>           char                    priority;
>>>>>           __u16                   gso_size;
>>>>> +       u32                     ts_opt_id;
>>>>>           u64                     transmit_time;
>>>>>           u32                     mark;
>>>>>    };
>>>>> @@ -241,7 +242,8 @@ struct inet_sock {
>>>>>           struct inet_cork_full   cork;
>>>>>    };
>>>>>
>>>>> -#define IPCORK_OPT     1       /* ip-options has been held in ipcork.opt */
>>>>> +#define IPCORK_OPT             1       /* ip-options has been held in ipcork.opt */
>>>>> +#define IPCORK_TS_OPT_ID       2       /* timestmap opt id has been provided in cmsg */
>>>>>
>>>>>    enum {
>>>>>           INET_FLAGS_PKTINFO      = 0,
>>>>> diff --git a/include/net/sock.h b/include/net/sock.h
>>>>> index f51d61fab059..73e21dad5660 100644
>>>>> --- a/include/net/sock.h
>>>>> +++ b/include/net/sock.h
>>>>> @@ -1794,6 +1794,7 @@ struct sockcm_cookie {
>>>>>           u64 transmit_time;
>>>>>           u32 mark;
>>>>>           u32 tsflags;
>>>>> +       u32 ts_opt_id;
>>>>>    };
>>>>>
>>>>>    static inline void sockcm_init(struct sockcm_cookie *sockc,
>>>>> diff --git a/include/uapi/asm-generic/socket.h b/include/uapi/asm-generic/socket.h
>>>>> index 8ce8a39a1e5f..db3df3e74b01 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_TS_OPT_ID          78
>>>>> +
>>>>>    #if !defined(__KERNEL__)
>>>>>
>>>>>    #if __BITS_PER_LONG == 64 || (defined(__x86_64__) && defined(__ILP32__))
>>>>> diff --git a/include/uapi/linux/net_tstamp.h b/include/uapi/linux/net_tstamp.h
>>>>> index a2c66b3d7f0f..e2f145e3f3a1 100644
>>>>> --- a/include/uapi/linux/net_tstamp.h
>>>>> +++ b/include/uapi/linux/net_tstamp.h
>>>>> @@ -32,8 +32,9 @@ enum {
>>>>>           SOF_TIMESTAMPING_OPT_TX_SWHW = (1<<14),
>>>>>           SOF_TIMESTAMPING_BIND_PHC = (1 << 15),
>>>>>           SOF_TIMESTAMPING_OPT_ID_TCP = (1 << 16),
>>>>> +       SOF_TIMESTAMPING_OPT_ID_CMSG = (1 << 17),
>>>>
>>>> I'm not sure if the new flag needs to be documented as well? After
>>>> this patch, people may search the key word in the documentation file
>>>> and then find nothing.
>>>>
>>>> If we have this flag here, normally it means we can pass it through
>>>> setsockopt, so is it expected? If it's an exception, I reckon that we
>>>> can forbid passing/setting this option in sock_set_timestamping() and
>>>> document this rule?
>>>
>>> Good point, thanks.
>>>
>>> It must definitely not be part of SOF_TIMESTAMPING_MASK. My bad for
>>> suggesting without giving it much thought.
>>>
>>> The bit is kernel-internal. No need to even mention it in user-facing
>>> documentation. But anyone reading net_tstamp.h might wonder what it
>>> does.
>>>
>>> It should not even be in a UAPI header, but in an internal one.
>>> Probably include/net/sock.h, near SK_FLAGS_TIMESTAMP.
>>>
>>> Maybe we can reserve bit 31 in u32 sk_tsflags. And if we ever have
>>> to double that flag size, it can move up to 63, as it is not UAPI in
>>> any way. This is a workaround to having a separate flags field in
>>> sockcm_cookie.
>>>
>>> And have a BUILD_BUG_ON if SOF_TIMESTAMPING_LAST reaches this reserved
>>> region.
>>
>> Yeah, I was also thinking of it not being UAPI, that's why I tried to
>> avoid it in my RFC using 0 as a reserved value. Do you think
>> SK_FLAGS_CMSG_TS_OPT_ID is good naming for it?
> 
> It's relevant only to sockcm_cookie, so maybe SOCKCM_FLAG_TS_OPT_ID?
> 
> One day we'll need another sockcm_cookie flag, we'll grow it to add a
> real flags field and can get rid of this hack.
> 
> The struct is used embedded in ipcm_cookie and ipcm6_cookie, which
> would grow as a result.
> 
> It is also simply stack allocated in cases like performance sensitive
> tcp_sendmsg_locked. Here, the main cost is a slightly more expensive
> zeroing in sockcm_init.
> 
> For now, I think we should just stick with using the highest bit in
> sockcm.tsflags.

I totally agree with using the highest bit in sockcm.tsflags for now.
I was just wondering about the name as naming is always the hardest
part. SOCKCM_FLAG_TS_OPT_ID looks good and reasonable, I'll use it.

Thanks,
Vadim