netdev - Re: [PATCH net-next] net: dev: Always serialize on Qdisc::busylock in __dev_xmit_skb() on PREEMPT

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Date:   Fri, 10 Dec 2021 08:52:55 -0800
From:   Eric Dumazet <edumazet@...gle.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH net-next] net: dev: Always serialize on Qdisc::busylock in
 __dev_xmit_skb() on PREEMPT_RT.

On Fri, Dec 10, 2021 at 8:46 AM Sebastian Andrzej Siewior
<bigeasy@...utronix.de> wrote:
>
> On 2021-12-10 17:35:21 [+0100], Paolo Abeni wrote:
> > On Fri, 2021-12-10 at 16:41 +0100, Sebastian Andrzej Siewior wrote:
> > > The root-lock is dropped before dev_hard_start_xmit() is invoked and after
> > > setting the __QDISC___STATE_RUNNING bit. If the Qdisc owner is preempted
> > > by another sender/task with a higher priority then this new sender won't
> > > be able to submit packets to the NIC directly instead they will be
> > > enqueued into the Qdisc. The NIC will remain idle until the Qdisc owner
> > > is scheduled again and finishes the job.
> > >
> > > By serializing every task on the ->busylock then the task will be
> > > preempted by a sender only after the Qdisc has no owner.
> > >
> > > Always serialize on the busylock on PREEMPT_RT.
> >
> > Not sure how much is relevant in the RT context, but this should impact
> > the xmit tput in a relevant, negative way.
>
> Negative because everyone blocks on lock and transmits packets directly
> instead of adding it to the queue and leaving for more?
>
> > If I read correctly, you use the busylock to trigger priority ceiling
> > on each sender. I'm wondering if there are other alternative ways (no
> > contended lock, just some RT specific annotation) to mark a whole
> > section of code for priority ceiling ?!?
>
> priority ceiling as you call it, happens always with the help of a lock.
> The root_lock is dropped in sch_direct_xmit().
> qdisc_run_begin() sets only a bit with no owner association.
> If I know why the busy-lock bad than I could add another one. The
> important part is force the sende out of the section so the task with
> the higher priority can send packets instead of queueing them only.

Problem is that if you have a qdisc, qdisc_dequeue() will not know that the
packet queued by high prio thread needs to shortcut all prior packets and
be sent right away.

Because of that, it seems just better that a high prio thread returns
immediately and let the dirty work (dequeue packets and send them to devices)
be done by other threads ?

>
> > Thanks!
> >
> > Paolo
>
> Sebastian