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 17:35:21 +0100
From:   Paolo Abeni <pabeni@...hat.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        netdev@...r.kernel.org
Cc:     "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 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.

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 ?!?

Thanks!

Paolo
> 
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> ---
>  net/core/dev.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/net/core/dev.c b/net/core/dev.c
> index 2a352e668d103..4a701cf2e2c10 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -3836,8 +3836,16 @@ static inline int __dev_xmit_skb(struct sk_buff *skb, struct Qdisc *q,
>  	 * separate lock before trying to get qdisc main lock.
>  	 * This permits qdisc->running owner to get the lock more
>  	 * often and dequeue packets faster.
> +	 * On PREEMPT_RT it is possible to preempt the qdisc owner during xmit
> +	 * and then other tasks will only enqueue packets. The packets will be
> +	 * sent after the qdisc owner is scheduled again. To prevent this
> +	 * scenario the task always serialize on the lock.
>  	 */
> -	contended = qdisc_is_running(q);
> +	if (!IS_ENABLED(CONFIG_PREEMPT_RT))
> +		contended = qdisc_is_running(q);
> +	else
> +		contended = true;
> +
>  	if (unlikely(contended))
>  		spin_lock(&q->busylock);
>