netdev - Re: [PATCH net-next v2] openvswitch: Introduce per-cpu upcall dispatch

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Message-ID: <CAOrHB_CUyqp-hmamZzkyEea8nOZvGpk57DRQ2EReCKzbtJ4yww@mail.gmail.com>
Date:   Thu, 15 Jul 2021 15:57:36 -0700
From:   Pravin Shelar <pravin.ovn@...il.com>
To:     Mark Gray <mark.d.gray@...hat.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        ovs dev <dev@...nvswitch.org>, dan.carpenter@...cle.com,
        Flavio Leitner <fbl@...close.org>
Subject: Re: [PATCH net-next v2] openvswitch: Introduce per-cpu upcall dispatch

On Thu, Jul 15, 2021 at 5:28 AM Mark Gray <mark.d.gray@...hat.com> wrote:
>
> The Open vSwitch kernel module uses the upcall mechanism to send
> packets from kernel space to user space when it misses in the kernel
> space flow table. The upcall sends packets via a Netlink socket.
> Currently, a Netlink socket is created for every vport. In this way,
> there is a 1:1 mapping between a vport and a Netlink socket.
> When a packet is received by a vport, if it needs to be sent to
> user space, it is sent via the corresponding Netlink socket.
>
> This mechanism, with various iterations of the corresponding user
> space code, has seen some limitations and issues:
>
> * On systems with a large number of vports, there is a correspondingly
> large number of Netlink sockets which can limit scaling.
> (https://bugzilla.redhat.com/show_bug.cgi?id=1526306)
> * Packet reordering on upcalls.
> (https://bugzilla.redhat.com/show_bug.cgi?id=1844576)
> * A thundering herd issue.
> (https://bugzilla.redhat.com/show_bug.cgi?id=1834444)
>
> This patch introduces an alternative, feature-negotiated, upcall
> mode using a per-cpu dispatch rather than a per-vport dispatch.
>
> In this mode, the Netlink socket to be used for the upcall is
> selected based on the CPU of the thread that is executing the upcall.
> In this way, it resolves the issues above as:
>
> a) The number of Netlink sockets scales with the number of CPUs
> rather than the number of vports.
> b) Ordering per-flow is maintained as packets are distributed to
> CPUs based on mechanisms such as RSS and flows are distributed
> to a single user space thread.
> c) Packets from a flow can only wake up one user space thread.
>
> The corresponding user space code can be found at:
> https://mail.openvswitch.org/pipermail/ovs-dev/2021-July/385139.html
>
> Bugzilla: https://bugzilla.redhat.com/1844576
> Signed-off-by: Mark Gray <mark.d.gray@...hat.com>
> Acked-by: Flavio Leitner <fbl@...close.org>

Acked-by: Pravin B Shelar <pshelar@....org>

Thanks,