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 17:46:33 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Paolo Abeni <pabeni@...hat.com>
Cc:     netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        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 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.

> Thanks!
> 
> Paolo

Sebastian