Subject: Re: [PATCH net v3] net: cbs: Fix software cbs to consider packet sending time

From: Vinicius Costa Gomes <[email-masked]>
Date: Tue, 24 Mar 2020 12:38:22 -0700

Zh-yuan Ye <[email-masked]> writes:

> Currently the software CBS does not consider the packet sending time
> when depleting the credits. It caused the throughput to be
> Idleslope[kbps] * (Port transmit rate[kbps] / |Sendslope[kbps]|) where
> Idleslope * (Port transmit rate / (Idleslope + |Sendslope|)) = Idleslope
> is expected. In order to fix the issue above, this patch takes the time
> when the packet sending completes into account by moving the anchor time
> variable "last" ahead to the send completion time upon transmission and
> adding wait when the next dequeue request comes before the send
> completion time of the previous packet.
>
> changelog:
> V2->V3:
>  - remove unnecessary whitespace cleanup
>  - add the checks if port_rate is 0 before division
>
> V1->V2:
>  - combine variable "send_completed" into "last"
>  - add the comment for estimate of the packet sending
>
> Fixes: 585d763af09c ("net/sched: Introduce Credit Based Shaper (CBS) qdisc")
> Signed-off-by: Zh-yuan Ye <[email-masked]>
> Reviewed-by: Vinicius Costa Gomes <[email-masked]>
> ---
>  net/sched/sch_cbs.c | 12 +++++++++++-
>  1 file changed, 11 insertions(+), 1 deletion(-)
>
> diff --git a/net/sched/sch_cbs.c b/net/sched/sch_cbs.c
> index b2905b03a432..2eaac2ff380f 100644
> --- a/net/sched/sch_cbs.c
> +++ b/net/sched/sch_cbs.c
> @@ -181,6 +181,11 @@ static struct sk_buff *cbs_dequeue_soft(struct Qdisc *sch)
>  	s64 credits;
>  	int len;
>  
> +	/* The previous packet is still being sent */
> +	if (now < q->last) {
> +		qdisc_watchdog_schedule_ns(&q->watchdog, q->last);
> +		return NULL;
> +	}
>  	if (q->credits < 0) {
>  		credits = timediff_to_credits(now - q->last, q->idleslope);
>  
> @@ -212,7 +217,12 @@ static struct sk_buff *cbs_dequeue_soft(struct Qdisc *sch)
>  	credits += q->credits;
>  
>  	q->credits = max_t(s64, credits, q->locredit);
> -	q->last = now;
> +	/* Estimate of the transmission of the last byte of the packet in ns */
> +	if (unlikely(atomic64_read(&q->port_rate) == 0))

Minor suggestion. I would only move 'atomic64_read()' to outside the
condition, so reading 'q->port_rate' is done only once.

It's looking good. When I saw the problems that the software mode
had with larger packets I should have thought of something like this.
Thanks for solving this.

Cheers,
-- 
Vinicius