Re: [dpdk-dev] [PATCH v2] sched: fix overflow errors in WRR weighting code

[Luca Boccassi <bluca@debian.org>]

On Thu, 2017-11-30 at 09:04 +0000, alangordondewar@gmail.com wrote:
> From: Alan Dewar <alan.dewar@att.com>
> 
> Revised patch - this version fixes an issue when a small wrr_cost is
> shifted so far right that its value becomes zero.
> 
> The WRR code calculates the lowest common denominator between the
> four
> WRR weights as a uint32_t value and divides the LCD by each of the
> WRR
> weights and casts the results as a uint8_t.  This casting can cause
> the ratios of the computed wrr costs to be wrong.  For example with
> WRR weights of 3, 5, 7 and 11, the LCD is computed to be
> 1155.  The WRR costs get computed as:
> 
>   1155/3 = 385, 1155/5 = 231, 1155/7 = 165, 1155/11 = 105.
> 
> When the value 385 is cast into an uint8_t it ends up as 129.
> Rather than casting straight into a uint8_t, this patch shifts the
> computed WRR costs right so that the largest value is only eight bits
> wide.
> 
> In grinder_schedule, the packet length is multiplied by the WRR cost
> and added to the grinder's wrr_tokens value.  The grinder's
> wrr_tokens
> field is a uint16_t, so combination of a packet length of 1500 bytes
> and a wrr cost of 44 will overflow this field on the first packet.
> 
> This patch increases the width of the grinder's wrr_tokens and
> wrr_mask fields from uint16_t to uint32_t.
> 
> In grinder_wrr_store, the remaining tokens in the grinder's
> wrr_tokens
> array are copied to the appropriate pipe's wrr_tokens array.  However
> the pipe's wrr_tokens array is only a uint8_t array so unused tokens
> were quite frequently lost which upsets the balance of traffic across
> the four WRR queues.
> 
> This patch increases the width of the pipe's wrr_tokens array from
> a uint8_t to uint32_t.
> 
> Signed-off-by: Alan Dewar <alan.dewar@att.com>
> ---
> v2 - fixed bug in the wrr_cost calculation code that could result
> in a zero wrr_cost
> 
>  lib/librte_sched/rte_sched.c        | 59
> +++++++++++++++++++++++++++++--------
>  lib/librte_sched/rte_sched_common.h | 15 ++++++++++
>  2 files changed, 61 insertions(+), 13 deletions(-)
> 
> diff --git a/lib/librte_sched/rte_sched.c
> b/lib/librte_sched/rte_sched.c
> index 7252f85..324743d 100644
> --- a/lib/librte_sched/rte_sched.c
> +++ b/lib/librte_sched/rte_sched.c
> @@ -130,7 +130,7 @@ struct rte_sched_pipe {
>  	uint32_t tc_credits[RTE_SCHED_TRAFFIC_CLASSES_PER_PIPE];
>  
>  	/* Weighted Round Robin (WRR) */
> -	uint8_t wrr_tokens[RTE_SCHED_QUEUES_PER_PIPE];
> +	uint32_t wrr_tokens[RTE_SCHED_QUEUES_PER_PIPE];
>  
>  	/* TC oversubscription */
>  	uint32_t tc_ov_credits;
> @@ -205,8 +205,8 @@ struct rte_sched_grinder {
>  	struct rte_mbuf *pkt;
>  
>  	/* WRR */
> -	uint16_t wrr_tokens[RTE_SCHED_QUEUES_PER_TRAFFIC_CLASS];
> -	uint16_t wrr_mask[RTE_SCHED_QUEUES_PER_TRAFFIC_CLASS];
> +	uint32_t wrr_tokens[RTE_SCHED_QUEUES_PER_TRAFFIC_CLASS];
> +	uint32_t wrr_mask[RTE_SCHED_QUEUES_PER_TRAFFIC_CLASS];
>  	uint8_t wrr_cost[RTE_SCHED_QUEUES_PER_TRAFFIC_CLASS];
>  };
>  
> @@ -542,6 +542,17 @@ rte_sched_time_ms_to_bytes(uint32_t time_ms,
> uint32_t rate)
>  	return time;
>  }
>  
> +static uint32_t rte_sched_reduce_to_byte(uint32_t value)
> +{
> +	uint32_t shift = 0;
> +
> +	while (value & 0xFFFFFF00) {
> +		value >>= 1;
> +		shift++;
> +	}
> +	return shift;
> +}
> +
>  static void
>  rte_sched_port_config_pipe_profile_table(struct rte_sched_port
> *port, struct rte_sched_port_params *params)
>  {
> @@ -583,6 +594,8 @@ rte_sched_port_config_pipe_profile_table(struct
> rte_sched_port *port, struct rte
>  			uint32_t
> wrr_cost[RTE_SCHED_QUEUES_PER_TRAFFIC_CLASS];
>  			uint32_t lcd, lcd1, lcd2;
>  			uint32_t qindex;
> +			uint32_t low_pos;
> +			uint32_t shift;
>  
>  			qindex = j *
> RTE_SCHED_QUEUES_PER_TRAFFIC_CLASS;
>  
> @@ -594,12 +607,28 @@ rte_sched_port_config_pipe_profile_table(struct
> rte_sched_port *port, struct rte
>  			lcd1 = rte_get_lcd(wrr_cost[0],
> wrr_cost[1]);
>  			lcd2 = rte_get_lcd(wrr_cost[2],
> wrr_cost[3]);
>  			lcd = rte_get_lcd(lcd1, lcd2);
> +			low_pos = rte_min_pos_4_u32(wrr_cost);
>  
>  			wrr_cost[0] = lcd / wrr_cost[0];
>  			wrr_cost[1] = lcd / wrr_cost[1];
>  			wrr_cost[2] = lcd / wrr_cost[2];
>  			wrr_cost[3] = lcd / wrr_cost[3];
>  
> +			shift =
> rte_sched_reduce_to_byte(wrr_cost[low_pos]);
> +			wrr_cost[0] >>= shift;
> +			wrr_cost[1] >>= shift;
> +			wrr_cost[2] >>= shift;
> +			wrr_cost[3] >>= shift;
> +
> +			if (wrr_cost[0] == 0)
> +				wrr_cost[0]++;
> +			if (wrr_cost[1] == 0)
> +				wrr_cost[1]++;
> +			if (wrr_cost[2] == 0)
> +				wrr_cost[2]++;
> +			if (wrr_cost[3] == 0)
> +				wrr_cost[3]++;
> +
>  			dst->wrr_cost[qindex] = (uint8_t)
> wrr_cost[0];
>  			dst->wrr_cost[qindex + 1] = (uint8_t)
> wrr_cost[1];
>  			dst->wrr_cost[qindex + 2] = (uint8_t)
> wrr_cost[2];
> @@ -1941,15 +1970,19 @@ grinder_wrr_load(struct rte_sched_port *port,
> uint32_t pos)
>  
>  	qindex = tc_index * 4;
>  
> -	grinder->wrr_tokens[0] = ((uint16_t) pipe-
> >wrr_tokens[qindex]) << RTE_SCHED_WRR_SHIFT;
> -	grinder->wrr_tokens[1] = ((uint16_t) pipe->wrr_tokens[qindex 
> + 1]) << RTE_SCHED_WRR_SHIFT;
> -	grinder->wrr_tokens[2] = ((uint16_t) pipe->wrr_tokens[qindex 
> + 2]) << RTE_SCHED_WRR_SHIFT;
> -	grinder->wrr_tokens[3] = ((uint16_t) pipe->wrr_tokens[qindex 
> + 3]) << RTE_SCHED_WRR_SHIFT;
> +	grinder->wrr_tokens[0] = pipe->wrr_tokens[qindex] <<
> +		RTE_SCHED_WRR_SHIFT;
> +	grinder->wrr_tokens[1] = pipe->wrr_tokens[qindex + 1] <<
> +		RTE_SCHED_WRR_SHIFT;
> +	grinder->wrr_tokens[2] = pipe->wrr_tokens[qindex + 2] <<
> +		RTE_SCHED_WRR_SHIFT;
> +	grinder->wrr_tokens[3] = pipe->wrr_tokens[qindex + 3] <<
> +		RTE_SCHED_WRR_SHIFT;
>  
> -	grinder->wrr_mask[0] = (qmask & 0x1) * 0xFFFF;
> -	grinder->wrr_mask[1] = ((qmask >> 1) & 0x1) * 0xFFFF;
> -	grinder->wrr_mask[2] = ((qmask >> 2) & 0x1) * 0xFFFF;
> -	grinder->wrr_mask[3] = ((qmask >> 3) & 0x1) * 0xFFFF;
> +	grinder->wrr_mask[0] = (qmask & 0x1) * 0xFFFFFFFF;
> +	grinder->wrr_mask[1] = ((qmask >> 1) & 0x1) * 0xFFFFFFFF;
> +	grinder->wrr_mask[2] = ((qmask >> 2) & 0x1) * 0xFFFFFFFF;
> +	grinder->wrr_mask[3] = ((qmask >> 3) & 0x1) * 0xFFFFFFFF;
>  
>  	grinder->wrr_cost[0] = pipe_params->wrr_cost[qindex];
>  	grinder->wrr_cost[1] = pipe_params->wrr_cost[qindex + 1];
> @@ -1981,14 +2014,14 @@ static inline void
>  grinder_wrr(struct rte_sched_port *port, uint32_t pos)
>  {
>  	struct rte_sched_grinder *grinder = port->grinder + pos;
> -	uint16_t wrr_tokens_min;
> +	uint32_t wrr_tokens_min;
>  
>  	grinder->wrr_tokens[0] |= ~grinder->wrr_mask[0];
>  	grinder->wrr_tokens[1] |= ~grinder->wrr_mask[1];
>  	grinder->wrr_tokens[2] |= ~grinder->wrr_mask[2];
>  	grinder->wrr_tokens[3] |= ~grinder->wrr_mask[3];
>  
> -	grinder->qpos = rte_min_pos_4_u16(grinder->wrr_tokens);
> +	grinder->qpos = rte_min_pos_4_u32(grinder->wrr_tokens);
>  	wrr_tokens_min = grinder->wrr_tokens[grinder->qpos];
>  
>  	grinder->wrr_tokens[0] -= wrr_tokens_min;
> diff --git a/lib/librte_sched/rte_sched_common.h
> b/lib/librte_sched/rte_sched_common.h
> index aed144b..a3c6bc2 100644
> --- a/lib/librte_sched/rte_sched_common.h
> +++ b/lib/librte_sched/rte_sched_common.h
> @@ -77,6 +77,21 @@ rte_min_pos_4_u16(uint16_t *x)
>  	return pos0;
>  }
>  
> +static inline uint32_t
> +rte_min_pos_4_u32(uint32_t *x)
> +{
> +	uint32_t pos0 = 0;
> +	uint32_t pos1 = 2;
> +
> +	if (x[1] <= x[0])
> +		pos0 = 1;
> +	if (x[3] <= x[2])
> +		pos1 = 3;
> +	if (x[pos1] <= x[pos0])
> +		pos0 = pos1;
> +
> +	return pos0;
> +}
>  #endif
>  
>  /*

Reviewed-by: Luca Boccassi <bluca@debian.org>

LGTM

-- 
Kind regards,
Luca Boccassi