netdev - Re: [PATCH RFC net-next] openvswitch: Queue upcalls to userspace in per-port round-robin order

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Date:   Tue, 10 Jul 2018 11:31:19 -0700
From:   Pravin Shelar <pshelar@....org>
To:     Matteo Croce <mcroce@...hat.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        ovs dev <dev@...nvswitch.org>,
        Stefano Brivio <sbrivio@...hat.com>,
        Jiri Benc <jbenc@...hat.com>, Aaron Conole <aconole@...hat.com>
Subject: Re: [PATCH RFC net-next] openvswitch: Queue upcalls to userspace in
 per-port round-robin order

On Wed, Jul 4, 2018 at 7:23 AM, Matteo Croce <mcroce@...hat.com> wrote:
> From: Stefano Brivio <sbrivio@...hat.com>
>
> Open vSwitch sends to userspace all received packets that have
> no associated flow (thus doing an "upcall"). Then the userspace
> program creates a new flow and determines the actions to apply
> based on its configuration.
>
> When a single port generates a high rate of upcalls, it can
> prevent other ports from dispatching their own upcalls. vswitchd
> overcomes this problem by creating many netlink sockets for each
> port, but it quickly exceeds any reasonable maximum number of
> open files when dealing with huge amounts of ports.
>
> This patch queues all the upcalls into a list, ordering them in
> a per-port round-robin fashion, and schedules a deferred work to
> queue them to userspace.
>
> The algorithm to queue upcalls in a round-robin fashion,
> provided by Stefano, is based on these two rules:
>  - upcalls for a given port must be inserted after all the other
>    occurrences of upcalls for the same port already in the queue,
>    in order to avoid out-of-order upcalls for a given port
>  - insertion happens once the highest upcall count for any given
>    port (excluding the one currently at hand) is greater than the
>    count for the port we're queuing to -- if this condition is
>    never true, upcall is queued at the tail. This results in a
>    per-port round-robin order.
>
> In order to implement a fair round-robin behaviour, a variable
> queueing delay is introduced. This will be zero if the upcalls
> rate is below a given threshold, and grows linearly with the
> queue utilisation (i.e. upcalls rate) otherwise.
>
> This ensures fairness among ports under load and with few
> netlink sockets.
>
Thanks for the patch.
This patch is adding following overhead for upcall handling:
1. kmalloc.
2. global spin-lock.
3. context switch to single worker thread.
I think this could become bottle neck on most of multi core systems.
You have mentioned issue with existing fairness mechanism, Can you
elaborate on those, I think we could improve that before implementing
heavy weight fairness in upcall handling.