netdev - Re: [PATCH v2 net-next 5/5] mptcp: implement delegated actions

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Message-ID: <20210121173437.1b945b01@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date:   Thu, 21 Jan 2021 17:34:37 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Paolo Abeni <pabeni@...hat.com>
Cc:     netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
        mptcp@...ts.01.org, Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH v2 net-next 5/5] mptcp: implement delegated actions

On Wed, 20 Jan 2021 15:39:14 +0100 Paolo Abeni wrote:
> On MPTCP-level ack reception, the packet scheduler
> may select a subflow other then the current one.
> 
> Prior to this commit we rely on the workqueue to trigger
> action on such subflow.
> 
> This changeset introduces an infrastructure that allows
> any MPTCP subflow to schedule actions (MPTCP xmit) on
> others subflows without resorting to (multiple) process
> reschedule.

If your work doesn't reschedule there should not be multiple 
rescheds, no?

> A dummy NAPI instance is used instead. When MPTCP needs to
> trigger action an a different subflow, it enqueues the target
> subflow on the NAPI backlog and schedule such instance as needed.
> 
> The dummy NAPI poll method walks the sockets backlog and tries
> to acquire the (BH) socket lock on each of them. If the socket
> is owned by the user space, the action will be completed by
> the sock release cb, otherwise push is started.
> 
> This change leverages the delegated action infrastructure
> to avoid invoking the MPTCP worker to spool the pending data,
> when the packet scheduler picks a subflow other then the one
> currently processing the incoming MPTCP-level ack.
> 
> Additionally we further refine the subflow selection
> invoking the packet scheduler for each chunk of data
> even inside __mptcp_subflow_push_pending().

Is there much precedence for this sort of hijacking of NAPI 
for protocol work? Do you need it because of locking?