[dpdk-dev] [PATCH v2] latencystats: fix timestamp marking and latency calculation

[Ananyev, Konstantin]

Hi Reshma,

> 
> 
> Latency calculation logic is not correct for the case where
> packets gets dropped before TX. As for the dropped packets,
> the timestamp is not cleared, and such packets still gets
> counted for latency calculation in next runs, that will result
> in inaccurate latency measurement.
> 
> So fix this issue as below,
> 
> Before setting timestamp in mbuf, check mbuf don't have
> any prior valid time stamp flag set and after marking
> the timestamp, set mbuf flags to indicate timestamp is
> valid.
> 
> Before calculating timestamp check mbuf flags are set to
> indicate timestamp is valid.
> 
> With the above logic it is guaranteed that correct timestamps
> have been used.
> 
> Fixes: 5cd3cac9ed ("latency: added new library for latency stats")
> 
> Reported-by: Bao-Long Tran <longtb5 at viettel.com.vn>
> Signed-off-by: Reshma Pattan <reshma.pattan at intel.com>
> Tested-by: Bao-Long Tran <longtb5 at viettel.com.vn>
> 
> ---
> v2: remove check for mbuf->timestamp
> ---
>  lib/librte_latencystats/rte_latencystats.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/lib/librte_latencystats/rte_latencystats.c b/lib/librte_latencystats/rte_latencystats.c
> index 1fdec68e3..0f702b722 100644
> --- a/lib/librte_latencystats/rte_latencystats.c
> +++ b/lib/librte_latencystats/rte_latencystats.c
> @@ -125,8 +125,11 @@ add_time_stamps(uint16_t pid __rte_unused,
>  	for (i = 0; i < nb_pkts; i++) {
>  		diff_tsc = now - prev_tsc;
>  		timer_tsc += diff_tsc;
> -		if (timer_tsc >= samp_intvl) {
> +
> +		if ((pkts[i]->ol_flags & PKT_RX_TIMESTAMP) == 0
> +			&& (timer_tsc >= samp_intvl)) {

As a nit - I think you need extra tab here, to follow dpdk codying style.
Apart from that:
Acked-by: Konstantin Ananyev <konstantin.ananyev at intel.com>

>  			pkts[i]->timestamp = now;
> +			pkts[i]->ol_flags |= PKT_RX_TIMESTAMP;
>  			timer_tsc = 0;
>  		}
>  		prev_tsc = now;
> @@ -156,7 +159,7 @@ calc_latency(uint16_t pid __rte_unused,
> 
>  	now = rte_rdtsc();
>  	for (i = 0; i < nb_pkts; i++) {
> -		if (pkts[i]->timestamp)
> +		if (pkts[i]->ol_flags & PKT_RX_TIMESTAMP)
>  			latency[cnt++] = now - pkts[i]->timestamp;
>  	}
> 
> --
> 2.14.4