netdev - Re: [net-next v7 2/2] net: sched: support hash/classid/cpuid selecting tx queue

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Date:   Wed, 26 Jan 2022 11:49:05 -0800
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Tonghao Zhang <xiangxia.m.yue@...il.com>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Jiri Pirko <jiri@...nulli.us>,
        "David S. Miller" <davem@...emloft.net>,
        Jonathan Lemon <jonathan.lemon@...il.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Alexander Lobakin <alobakin@...me>,
        Paolo Abeni <pabeni@...hat.com>,
        Talal Ahmad <talalahmad@...gle.com>,
        Kevin Hao <haokexin@...il.com>,
        Ilias Apalodimas <ilias.apalodimas@...aro.org>,
        Kees Cook <keescook@...omium.org>,
        Kumar Kartikeya Dwivedi <memxor@...il.com>,
        Antoine Tenart <atenart@...nel.org>,
        Wei Wang <weiwan@...gle.com>, Arnd Bergmann <arnd@...db.de>
Subject: Re: [net-next v7 2/2] net: sched: support hash/classid/cpuid
 selecting tx queue

On Fri, Dec 24, 2021 at 2:44 PM Jakub Kicinski <kuba@...nel.org> wrote:
>
> On Fri, 24 Dec 2021 11:28:45 -0800 Cong Wang wrote:
> > On Fri, Dec 24, 2021 at 8:49 AM <xiangxia.m.yue@...il.com> wrote:
> > > From: Tonghao Zhang <xiangxia.m.yue@...il.com>
> > >
> > > This patch allows user to select queue_mapping, range
> > > from A to B. And user can use skbhash, cgroup classid
> > > and cpuid to select Tx queues. Then we can load balance
> > > packets from A to B queue. The range is an unsigned 16bit
> > > value in decimal format.
> > >
> > > $ tc filter ... action skbedit queue_mapping skbhash A B
> > >
> > > "skbedit queue_mapping QUEUE_MAPPING" (from "man 8 tc-skbedit")
> > > is enhanced with flags:
> > > * SKBEDIT_F_TXQ_SKBHASH
> > > * SKBEDIT_F_TXQ_CLASSID
> > > * SKBEDIT_F_TXQ_CPUID
> >
> > NACK.
> >
> > These values can either obtained from user-space, or is nonsense
> > at all.
>
> Can you elaborate? What values can be obtained from user space?
> What is nonsense?

Of course.

1. SKBHASH is nonsense, because from a user's point of view, it really
has no meaning at all, skb hash itself is not visible to user and setting
an invisible value to as an skb queue mapping is pretty much like setting
a random value here.

2. CLASSID is obviously easy to obtain from user-space, as it is passed
from user-space.

3. CPUID is nonsense, because a process can be migrated easily from
one CPU to another. Putting OOO packets side, users can't figure out
which CPU the process is running *in general*. (Of course you can bind
CPU but clearly you can't bind every process)

>
> > Sorry, we don't accept enforcing such bad policies in kernel. Please
> > drop this patch.
>
> Again, unclear what your objection is. There's plenty similar
> functionality in TC. Please be more specific.

What exact TC functionality are you talking about? Please be specific.

If you mean Qdisc's, it is virtually just impossible to have a programmable
Qdisc, even my eBPF Qdisc only provides very limited programmablities.

If you mean TC filter or action, we already have eBPF filter and eBPF
action.

Thanks.