Re: [dpdk-dev] [PATCH v2] sched: fix port time rounding error

[Kevin Traynor <ktraynor@redhat.com>]

Hi,

On 25/06/2020 10:59, alangordondewar@gmail.com wrote:
> 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.
> 
> 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.dewar@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;
>  

I was backporting this patch to 18.11. The older ABI checker complains
about this structure change.

"cycles_per_byte has been added at the middle position of this
structural type."

Isn't this an ABI break? Dropping from 18.11 for time being.

>  	/* 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;
>