netdev - Re: [PATCH RFC net-next 5/6] rds: support for zcopy completion notification

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Date:   Thu, 18 Jan 2018 17:46:26 -0500
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Sowmini Varadhan <sowmini.varadhan@...cle.com>
Cc:     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 5/6] rds: support for zcopy completion notification

>> This changes the semantics of these fields. Please add a new SO_EE_CODE flag,
>> even if the semantics can be derived from the packet family (for now).
>
> sounds good, I'll take care of this (and other review comments)
> for the next rev.
>
>> Even better would be if we can avoid the cookies completely. I understand
>> the issue with concurrent send threads racing on obtaining a zckey value. If
>> the sender could learn which zckey was chosen for a call, would that suffice?
>
> I'm not sure I understand the proposal- you want sendmsg to return
> the cookie ("zckey") for you? How?
>
> even if we mangled sendmsg() to return something other than
> the existing POSIX semantics, how will the application be asynchronously
> notified that send has completed (on a per-message/per-datagram) basis?

I'm purposely glossing over how the kernel returns this item for now.
Was just wondering whether we can then assume mostly in order delivery
and reuse the existing notification interface from tcp zerocopy.

>From your experiments, it sounds like this is not the case. In which case
there is little benefit to trying to force linear IDs derived from sk->sk_zckey.

>> I suspect that in even with concurrent senders, notifications arrive
>> largely in
>> order, in which case we could just maintain the existing semantics and even
>> reuse that implementation.
>
> not so. rds-stress [1] with -d8 -t8 quickly disproves this on my 10G ixgbe
> connection.

Okay. In that case, the cmsg cookie approach sounds fine. I had the same
in an early tcp zerocopy test version, as a matter of fact.

>> >                 } rdma;
>> >                 struct rm_data_op {
>> >                         unsigned int            op_active:1;
>> > -                       unsigned int            op_notify:1;
>> > +                       unsigned int            op_notify:1,
>> > +                                               op_zcopy:1,
>                                                      :
>> > +                       struct rds_znotifier    *op_mmp_znotifier;
>>
>> not necessary if op_mmp_znotifier is NULL unless set in
>> rds_message_copy_from_user
>
> To make sure I dont misunderstand, you are suggesting that we dont
> need op_zcopy, but can just check for the null-ness of
> op_mmp_znotifier (yes, true, I agree)? or something else?

Yes, that's what I meant.