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    SubjectRe: [PATCH v7 bpf-next 00/11] Socket migration for SO_REUSEPORT.
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    On 5/21/21 8:20 PM, Kuniyuki Iwashima wrote:
    > The SO_REUSEPORT option allows sockets to listen on the same port and to
    > accept connections evenly. However, there is a defect in the current
    > implementation [1]. When a SYN packet is received, the connection is tied
    > to a listening socket. Accordingly, when the listener is closed, in-flight
    > requests during the three-way handshake and child sockets in the accept
    > queue are dropped even if other listeners on the same port could accept
    > such connections.
    >
    > This situation can happen when various server management tools restart
    > server (such as nginx) processes. For instance, when we change nginx
    > configurations and restart it, it spins up new workers that respect the new
    > configuration and closes all listeners on the old workers, resulting in the
    > in-flight ACK of 3WHS is responded by RST.
    >
    > To avoid such a situation, users have to know deeply how the kernel handles
    > SYN packets and implement connection draining by eBPF [2]:
    >
    > 1. Stop routing SYN packets to the listener by eBPF.
    > 2. Wait for all timers to expire to complete requests
    > 3. Accept connections until EAGAIN, then close the listener.
    >
    > or
    >
    > 1. Start counting SYN packets and accept syscalls using the eBPF map.
    > 2. Stop routing SYN packets.
    > 3. Accept connections up to the count, then close the listener.
    >
    > In either way, we cannot close a listener immediately. However, ideally,
    > the application need not drain the not yet accepted sockets because 3WHS
    > and tying a connection to a listener are just the kernel behaviour. The
    > root cause is within the kernel, so the issue should be addressed in kernel
    > space and should not be visible to user space. This patchset fixes it so
    > that users need not take care of kernel implementation and connection
    > draining. With this patchset, the kernel redistributes requests and
    > connections from a listener to the others in the same reuseport group
    > at/after close or shutdown syscalls.
    >
    > Although some software does connection draining, there are still merits in
    > migration. For some security reasons, such as replacing TLS certificates,
    > we may want to apply new settings as soon as possible and/or we may not be
    > able to wait for connection draining. The sockets in the accept queue have
    > not started application sessions yet. So, if we do not drain such sockets,
    > they can be handled by the newer listeners and could have a longer
    > lifetime. It is difficult to drain all connections in every case, but we
    > can decrease such aborted connections by migration. In that sense,
    > migration is always better than draining.
    >
    > Moreover, auto-migration simplifies user space logic and also works well in
    > a case where we cannot modify and build a server program to implement the
    > workaround.
    >
    > Note that the source and destination listeners MUST have the same settings
    > at the socket API level; otherwise, applications may face inconsistency and
    > cause errors. In such a case, we have to use the eBPF program to select a
    > specific listener or to cancel migration.
    >
    > Special thanks to Martin KaFai Lau for bouncing ideas and exchanging code
    > snippets along the way.
    >
    >
    > Link:
    > [1] The SO_REUSEPORT socket option
    > https://lwn.net/Articles/542629/
    >
    > [2] Re: [PATCH 1/1] net: Add SO_REUSEPORT_LISTEN_OFF socket option as drain mode
    > https://lore.kernel.org/netdev/1458828813.10868.65.camel@edumazet-glaptop3.roam.corp.google.com/

    This series needs review/ACKs from TCP maintainers. Eric/Neal/Yuchung please take
    a look again.

    Thanks,
    Daniel

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