Re: [dpdk-dev] [PATCH] sched: fix port time rounding error

["Singh, Jasvinder" <jasvinder.singh@intel.com>]

> -----Original Message-----
> From: Thomas Monjalon <thomas@monjalon.net>
> Sent: Wednesday, June 24, 2020 11:50 PM
> To: Singh, Jasvinder <jasvinder.singh@intel.com>
> Cc: Dumitrescu, Cristian <cristian.dumitrescu@intel.com>;
> 'alangordondewar@gmail.com' <alangordondewar@gmail.com>;
> dev@dpdk.org; 'Alan Dewar' <alan.dewar@att.com>; Dewar, Alan
> <alan.dewar@intl.att.com>
> Subject: Re: [dpdk-dev] [PATCH] sched: fix port time rounding error
> 
> Jasvinder, what is the conclusion of this patch?
> 
> 21/04/2020 10:21, Dewar, Alan:
> > From: Singh, Jasvinder <jasvinder.singh@intel.com>
> > > > > From: Alan Dewar <alan.dewar@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.dewar@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.
> 

Alan, Could you please resubmit the patch with above change? Other than that, patch looks good to me.