Jasvinder, what is the conclusion of this patch?
21/04/2020 10:21, Dewar, Alan:
> From: Singh, Jasvinder <jasvinder.si...@intel.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.
> > > >
> > > > 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;
> >
> > Above check seems redundant as port->time_cpu_cycles will always be less
> > than the current cycles due to roundoff in previous iteration.
> >
>
> This was to catch the condition where the cycles wraps back to zero (after
> 100+ years?? depending on clock speed).
> Rather than just going to end: the conditional should at least reset
> port->time_cpu_cycles back to zero.
> So there would be a very temporary glitch in accuracy once every 100+ years.
