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

["Kinsella, Ray" <mdr@ashroe.eu>]

On 20/08/2020 15:32, Kevin Traynor wrote:
> 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.

So it looks like rte_sched_port * is an opaque pointers as it's contents are only
known to rte_sched.c and not outside. To everyone else it is an opaque data structure, 
so structural changes to it would not be an ABI breakage.

> 
>>  	/* 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;
>>
>