netdev - Re: [PATCH net-next] RDS: deliver zerocopy completion notification with data as an optimization

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Message-ID: <20180221230355.GH15244@oracle.com>
Date:   Wed, 21 Feb 2018 18:03:55 -0500
From:   Sowmini Varadhan <sowmini.varadhan@...cle.com>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc:     Network Development <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>, rds-devel@....oracle.com,
        Santosh Shilimkar <santosh.shilimkar@...cle.com>
Subject: Re: [PATCH net-next] RDS: deliver zerocopy completion notification
 with data as an optimization

On (02/21/18 17:50), Willem de Bruijn wrote:
> 
> In the common case no more than one notification will be outstanding,
> but with a fixed number of notifications per packet, in edge cases this
> list may be long.
   :
> Socket functions block if sk_err is non-zero. See for instance
> tcp_sendmsg_locked. It is set by most code that also calls
> sock_queue_err_skb and also on dequeue from err skb.
> 
> This is the main reason that I would consider dropping error
> queue completely if you expect all users of RDS to use the
> cmsg on regular read to get these notifications.

I see. That's a good point, and maybe it makes sense to just have
a struct sk_buff_head rs_zcookie_quese on the rds_sock, and
have rds_rm_zerocopy_callback chain cookies ot this  rs_zcookie_queue.

[discussion regarding rds_recvmsg return values elided]

> Okay. If callers must already handle 0 as a valid return code, then
> it is fine to add another case that does this.
> 
> The extra branch in the hot path is still rather unfortunately. Could
> this be integrated in the existing if (nonblock) branch below?

that's where I first started. it got even hairier because the
callers expect a retval of 0 (-EAGAIN threw rds-stress into an
infinite loop of continulally trying to recv) and the end result
was just confusing code with the same number of branches.. 
let me revisit this when I spin out V2 without the sk_error_queue..

--Sowmini