> -----Original Message-----
> From: alangordonde...@gmail.com <alangordonde...@gmail.com>
> Sent: Thursday, April 16, 2020 9:48 AM
> To: Dumitrescu, Cristian <cristian.dumitre...@intel.com>
> Cc: dev@dpdk.org; Alan Dewar <alan.de...@att.com>
> Subject: [PATCH] sched: fix port time rounding error
>
> From: Alan Dewar <alan.de...@att.com>
>
> The QoS scheduler works off port time that is computed from the number
> of CPU cycles that have elapsed since the last time the port was
> polled. It divides the number of elapsed cycles to calculate how
> many bytes can be sent, however this division can generate rounding
> errors, where some fraction of a byte sent may be lost.
>
> Lose enough of these fractional bytes and the QoS scheduler
> underperforms. The problem is worse with low bandwidths.
>
> To compensate for this rounding error this fix doesn't advance the
> port's time_cpu_cycles by the number of cycles that have elapsed,
> but by multiplying the computed number of bytes that can be sent
> (which has been rounded down) by number of cycles per byte.
> This will mean that port's time_cpu_cycles will lag behind the CPU
> cycles momentarily. At the next poll, the lag will be taken into
> account.
>
> Fixes: de3cfa2c98 ("sched: initial import")
>
> Signed-off-by: Alan Dewar <alan.de...@att.com>
> ---
> lib/librte_sched/rte_sched.c | 12 ++++++++++--
> 1 file changed, 10 insertions(+), 2 deletions(-)
>
> diff --git a/lib/librte_sched/rte_sched.c b/lib/librte_sched/rte_sched.c
> index c0983ddda..c656dba2d 100644
> --- a/lib/librte_sched/rte_sched.c
> +++ b/lib/librte_sched/rte_sched.c
> @@ -222,6 +222,7 @@ struct rte_sched_port {
> uint64_t time_cpu_bytes; /* Current CPU time measured in bytes
> */
> uint64_t time; /* Current NIC TX time measured in bytes
> */
> struct rte_reciprocal inv_cycles_per_byte; /* CPU cycles per byte */
> + uint64_t cycles_per_byte;
>
> /* Grinders */
> struct rte_mbuf **pkts_out;
> @@ -852,6 +853,7 @@ rte_sched_port_config(struct
> rte_sched_port_params *params)
> cycles_per_byte = (rte_get_tsc_hz() << RTE_SCHED_TIME_SHIFT)
> / params->rate;
> port->inv_cycles_per_byte = rte_reciprocal_value(cycles_per_byte);
> + port->cycles_per_byte = cycles_per_byte;
>
> /* Grinders */
> port->pkts_out = NULL;
> @@ -2673,20 +2675,26 @@ static inline void
> rte_sched_port_time_resync(struct rte_sched_port *port)
> {
> uint64_t cycles = rte_get_tsc_cycles();
> - uint64_t cycles_diff = cycles - port->time_cpu_cycles;
> + uint64_t cycles_diff;
> uint64_t bytes_diff;
> uint32_t i;
>
> + if (cycles < port->time_cpu_cycles)
> + goto end;
> +
> + cycles_diff = cycles - port->time_cpu_cycles;
> /* Compute elapsed time in bytes */
> bytes_diff = rte_reciprocal_divide(cycles_diff <<
> RTE_SCHED_TIME_SHIFT,
> port->inv_cycles_per_byte);
>
> /* Advance port time */
> - port->time_cpu_cycles = cycles;
> + port->time_cpu_cycles +=
> + (bytes_diff * port->cycles_per_byte) >>
> RTE_SCHED_TIME_SHIFT;
> port->time_cpu_bytes += bytes_diff;
> if (port->time < port->time_cpu_bytes)
> port->time = port->time_cpu_bytes;
>
> +end:
> /* Reset pipe loop detection */
> for (i = 0; i < port->n_subports_per_port; i++)
> port->subports[i]->pipe_loop = RTE_SCHED_PIPE_INVALID;
> --
> 2.17.1
Adding Jasvinder.
