netdev - Re: [PATCH RFC net-next 1/6] sock: MSG_PEEK support for sk_error

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Date:   Thu, 18 Jan 2018 17:54:46 -0500
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Sowmini Varadhan <sowmini.varadhan@...cle.com>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        Network Development <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>, rds-devel@....oracle.com,
        santosh.shilimkar@...cle.com
Subject: Re: [PATCH RFC net-next 1/6] sock: MSG_PEEK support for sk_error_queue

On Thu, Jan 18, 2018 at 12:12 PM, Sowmini Varadhan
<sowmini.varadhan@...cle.com> wrote:
> On (01/18/18 08:53), Eric Dumazet wrote:
>>
>> The thing is : MSG_PEEK 'support' will also need SO_PEEK_OFF support.
>
> sure, I'll drop the MSG_PEEK idea (which I wasnt very thrilled
> about anyway)
>
>> So lets properly design things, and not re-use legacy stuff that is
>> proven to be not multi-thread ready and too complex.
>>
>> If you want to design a new channel of communication, do it, and
>> maintain it.
>
> My instinct is to go with the fixed size ancillary data- which itself
> allows 2 options:
>
> 1. cmsg_data has a sock_extended_err preamble
>    with ee_origin = SO_EE_ORIGIN_ZEROCOPY_COOKIE (or similar),
>    and the ee_data is an array of 32 bit cookies (can pack at most 8
>    32-bit cookies, if we want to pack this into an skb->cb)
>
>    Using the sock_extended_err as preamble will allow this to be usable by
>    existing tcp zcopy applications (they can use the ee_origin to find
>    out if this a batch of cookies or the existing hi/lo values).
>
> 2. If we have the option of passing completion-notification up as ancillary
>    data on the pollin/recvmsg channel itself (instead of MSG_ERRQUEUE)

This assumes a somewhat symmetric workload, where there are enough recv
calls to reap the notification associated with the send calls.

>    we dont have to try to retain "backward compat" to the
>    SO_EE_ORIGIN_ZEROCOPY API: we can just use a completely new data
>    struct for the notification and potentially pack more cookies into
>    48 bytes (RDS could be the first guinea pig for this- doesnt even
>    have to be done across all protocol families on day-1).
>
> I think the shmem channel suggestion would be an optional optimization
> that can be added later- it may not even be necessary, since most
> applications will likely be sending *and* receiving data, so passing up
> cookies with recvmsg should be "good enough" to save syscall overhead
> for the common case.
>
> I can work #2, if there are no objections to it.

I would stay with MSG_ERRQUEUE processing. One option is to pass data
up to userspace in the data portion of the notification skb instead of
encoding it in ancillary data, like tcp_get_timestamping_opt_stats.