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

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Date:   Fri, 16 Jul 2021 08:19:39 +0100
From:   Mark Gray <mark.d.gray@...hat.com>
To:     Pravin Shelar <pravin.ovn@...il.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 15/07/2021 23:57, Pravin Shelar wrote:
> 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,
> 

Thanks for the review Pravin.