[dpdk-dev] [PATCH] meter: fix excess token bucket update in srtcm implementation

[Dumitrescu, Cristian]

> -----Original Message-----
> From: Nikhil Jagtap [mailto:nikhil.jagtap at gmail.com]
> Sent: Wednesday, September 7, 2016 7:15 AM
> To: Dumitrescu, Cristian <cristian.dumitrescu at intel.com>
> Cc: dev at dpdk.org; Ramia, Kannan Babu <kannan.babu.ramia at intel.com>;
> Nikhil Jagtap <nikhil.jagtap at gmail.com>
> Subject: [PATCH] meter: fix excess token bucket update in srtcm
> implementation
> 
> As per srTCM RFC 2697, we should be updating the E bucket only after the
> C bucket overflows. This patch fixes the current DPDK implementation,
> where we are updating both the buckets simultaneously at the same rate
> (CIR) which results in token accumulation rate of (2*CIR).
> 
> Signed-off-by: Nikhil Jagtap <nikhil.jagtap at gmail.com>
> ---
>  lib/librte_meter/rte_meter.h |   26 ++++++++++++++++----------
>  1 files changed, 16 insertions(+), 10 deletions(-)
> 
> diff --git a/lib/librte_meter/rte_meter.h b/lib/librte_meter/rte_meter.h
> index 2cd8d81..0ffcb60 100644
> --- a/lib/librte_meter/rte_meter.h
> +++ b/lib/librte_meter/rte_meter.h
> @@ -232,13 +232,16 @@ rte_meter_srtcm_color_blind_check(struct
> rte_meter_srtcm *m,
>  	n_periods = time_diff / m->cir_period;
>  	m->time += n_periods * m->cir_period;
> 
> +	/* Put the tokens overflowing from tc into te bucket */
>  	tc = m->tc + n_periods * m->cir_bytes_per_period;
> -	if (tc > m->cbs)
> +	if (tc > m->cbs) {
> +		te = m->te + (tc - m->cbs);
> +		if (te > m->ebs)
> +			te = m->ebs;
>  		tc = m->cbs;
> -
> -	te = m->te + n_periods * m->cir_bytes_per_period;
> -	if (te > m->ebs)
> -		te = m->ebs;
> +	} else {
> +		te = m->te;
> +	}

Just to avoid the final else, in order to have the critical path (Tc not overflowing) as the default fall-through code path, I suggest the following small change in the code (update the Te just after Tc update, for the case of Tc overflowing), which should favour the usage of the cmov instruction:

	/* Put the tokens overflowing from tc into te bucket */
  	tc = m->tc + n_periods * m->cir_bytes_per_period;
	te = m->te;

	if (tc > m->cbs) {
		te = m->te + (tc - m->cbs);
		if (te > m->ebs)
			te = m->ebs;
  		tc = m->cbs;
	}

Are you OK with this change?

> 
>  	/* Color logic */
>  	if (tc >= pkt_len) {
> @@ -271,13 +274,16 @@ rte_meter_srtcm_color_aware_check(struct
> rte_meter_srtcm *m,
>  	n_periods = time_diff / m->cir_period;
>  	m->time += n_periods * m->cir_period;
> 
> +	/* Put the tokens overflowing from tc into te bucket */
>  	tc = m->tc + n_periods * m->cir_bytes_per_period;
> -	if (tc > m->cbs)
> +	if (tc > m->cbs) {
> +		te = m->te + (tc - m->cbs);
> +		if (te > m->ebs)
> +			te = m->ebs;
>  		tc = m->cbs;
> -
> -	te = m->te + n_periods * m->cir_bytes_per_period;
> -	if (te > m->ebs)
> -		te = m->ebs;
> +	} else {
> +		te = m->te;
> +	}

Same as above.

> 
>  	/* Color logic */
>  	if ((pkt_color == e_RTE_METER_GREEN) && (tc >= pkt_len)) {
> --
> 1.7.1