> -----Original Message-----
> From: Thomas Monjalon <tho...@monjalon.net>
> Sent: Sunday, July 5, 2020 9:42 PM
> To: Dumitrescu, Cristian <cristian.dumitre...@intel.com>; Singh, Jasvinder
> <jasvinder.si...@intel.com>
> Cc: dev@dpdk.org; Alan Dewar <alan.de...@att.com>;
> alangordonde...@gmail.com
> Subject: Re: [dpdk-dev] [PATCH v2] sched: fix port time rounding error
>
> Cristian, Jasvinder, please review.
>
> 25/06/2020 11:59, alangordonde...@gmail.com:
> > 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.
> >
> > v2:
> > If the cycles value wraps (100 year+) reset the port's cpu cycle back
> > to zero.
> >
> > Fixes: de3cfa2c98 ("sched: initial import")
> >
> > Signed-off-by: Alan Dewar <alan.de...@att.com>
> > ---
> > lib/librte_sched/rte_sched.c | 11 +++++++++--
> > 1 file changed, 9 insertions(+), 2 deletions(-)
> >
> > diff --git a/lib/librte_sched/rte_sched.c
> > b/lib/librte_sched/rte_sched.c index c0983ddda..7c022cd61 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;
> > @@ -2673,16 +2675,21 @@ 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)
> > + port->time_cpu_cycles = 0;
> > +
> > + 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;
> >
>
Acked-by: Jasvinder Singh <jasvinder.si...@intel.com>
