Re: [dpdk-dev] [PATCH v2 2/2] Add in_flight_bitmask so as to use full 32 bits of tag.

[Bruce Richardson <bruce.richardson@intel.com>]

On Mon, Nov 10, 2014 at 02:52:47PM +0200, Qinglai Xiao wrote:
> With introduction of in_flight_bitmask, the whole 32 bits of tag can be
> used. Further more, this patch fixed the integer overflow when finding
> the matched tags.
> Note that currently librte_distributor supports up to 64 worker threads.
> If more workers are needed, the size of in_flight_bitmask and the
> algorithm of finding matched tag must be revised.
> 
> Signed-off-by: Qinglai Xiao <jigsaw@gmail.com>
> ---
>  lib/librte_distributor/rte_distributor.c |   45 ++++++++++++++++++++++--------
>  lib/librte_distributor/rte_distributor.h |    4 ++
>  2 files changed, 37 insertions(+), 12 deletions(-)
> 
> diff --git a/lib/librte_distributor/rte_distributor.c b/lib/librte_distributor/rte_distributor.c
> index 3dfec4a..3dfccae 100644
> --- a/lib/librte_distributor/rte_distributor.c
> +++ b/lib/librte_distributor/rte_distributor.c
> @@ -92,7 +92,13 @@ struct rte_distributor {
>  	unsigned num_workers;                 /**< Number of workers polling */
>  
>  	uint32_t in_flight_tags[RTE_MAX_LCORE];
> -		/**< Tracks the tag being processed per core, 0 == no pkt */
> +		/**< Tracks the tag being processed per core */
> +	uint64_t in_flight_bitmask;
> +		/**< on/off bits for in-flight tags.
> +		 * Note that if RTE_MAX_LCORE is larger than 64 then
> +		 * the bitmask has to expand.
> +		 */

I would suggest for this that we break the link with RTE_MAX_LCORE. Instead,
we can just enforce a hard limit on the distributor that it can only work
with 64 worker cores. That should avoid any complications.

I would suggest we do a further check in the create function something like
the below:

if (num_workers >= sizeof(d->in_flight_bitmask) * CHAR_BIT) {
	rte_errno = .....
}

> +
>  	struct rte_distributor_backlog backlog[RTE_MAX_LCORE];
>  
>  	union rte_distributor_buffer bufs[RTE_MAX_LCORE];
> @@ -189,6 +195,7 @@ static inline void
>  handle_worker_shutdown(struct rte_distributor *d, unsigned wkr)
>  {
>  	d->in_flight_tags[wkr] = 0;
> +	d->in_flight_bitmask &= ~(1UL << wkr);
>  	d->bufs[wkr].bufptr64 = 0;
>  	if (unlikely(d->backlog[wkr].count != 0)) {
>  		/* On return of a packet, we need to move the
> @@ -211,7 +218,10 @@ handle_worker_shutdown(struct rte_distributor *d, unsigned wkr)
>  			pkts[i] = (void *)((uintptr_t)(bl->pkts[idx] >>
>  					RTE_DISTRIB_FLAG_BITS));
>  		}
> -		/* recursive call */
> +		/* recursive call.
> +		 * Note that the tags were set before first level call
> +		 * to rte_distributor_process.
> +		 */
>  		rte_distributor_process(d, pkts, i);
>  		bl->count = bl->start = 0;
>  	}
> @@ -242,6 +252,7 @@ process_returns(struct rte_distributor *d)
>  			else {
>  				d->bufs[wkr].bufptr64 = RTE_DISTRIB_GET_BUF;
>  				d->in_flight_tags[wkr] = 0;
> +				d->in_flight_bitmask &= ~(1UL << wkr);
>  			}
>  			oldbuf = data >> RTE_DISTRIB_FLAG_BITS;
>  		} else if (data & RTE_DISTRIB_RETURN_BUF) {
> @@ -284,14 +295,18 @@ rte_distributor_process(struct rte_distributor *d,
>  			next_value = (((int64_t)(uintptr_t)next_mb)
>  					<< RTE_DISTRIB_FLAG_BITS);
>  			/*
> -			 * Set the low bit on the tag, so we can guarantee that
> -			 * we never store a tag value of zero. That means we can
> -			 * use the zero-value to indicate that no packet is
> -			 * being processed by a worker.
> +			 * User is advocated to set tag vaue for each
> +			 * mbuf before calling rte_distributor_process.
> +			 * User defined tags are used to identify flows,
> +			 * or sessions.
>  			 */
> -			new_tag = (next_mb->hash.usr | 1);
> +			new_tag = next_mb->hash.usr;
>  
> -			uint32_t match = 0;
> +			/*
> +			 * Note that if RTE_MAX_LCORE is larger than 64 then
> +			 * the size of match has to be expanded.
> +			 */
> +			uint64_t match = 0;
>  			unsigned i;
>  			/*
>  			 * to scan for a match use "xor" and "not" to get a 0/1
> @@ -303,9 +318,12 @@ rte_distributor_process(struct rte_distributor *d,
>  				match |= (!(d->in_flight_tags[i] ^ new_tag)
>  					<< i);
>  
> +			/* Only turned-on bits are considered as match */
> +			match &= d->in_flight_bitmask;
> +
>  			if (match) {
>  				next_mb = NULL;
> -				unsigned worker = __builtin_ctz(match);
> +				unsigned worker = __builtin_ctzl(match);
>  				if (add_to_backlog(&d->backlog[worker],
>  						next_value) < 0)
>  					next_idx--;
> @@ -322,6 +340,7 @@ rte_distributor_process(struct rte_distributor *d,
>  			else {
>  				d->bufs[wkr].bufptr64 = next_value;
>  				d->in_flight_tags[wkr] = new_tag;
> +				d->in_flight_bitmask |= (1UL << wkr);
>  				next_mb = NULL;
>  			}
>  			oldbuf = data >> RTE_DISTRIB_FLAG_BITS;
> @@ -379,11 +398,13 @@ rte_distributor_returned_pkts(struct rte_distributor *d,
>  static inline unsigned
>  total_outstanding(const struct rte_distributor *d)
>  {
> -	unsigned wkr, total_outstanding = 0;
> +	unsigned wkr, total_outstanding;
> +
> +	total_outstanding = __builtin_popcountl(d->in_flight_bitmask);
>  
>  	for (wkr = 0; wkr < d->num_workers; wkr++)
> -		total_outstanding += d->backlog[wkr].count +
> -				!!(d->in_flight_tags[wkr]);
> +		total_outstanding += d->backlog[wkr].count;
> +
>  	return total_outstanding;
>  }
>  
> diff --git a/lib/librte_distributor/rte_distributor.h b/lib/librte_distributor/rte_distributor.h
> index ec0d74a..cc1d559 100644
> --- a/lib/librte_distributor/rte_distributor.h
> +++ b/lib/librte_distributor/rte_distributor.h
> @@ -88,6 +88,10 @@ rte_distributor_create(const char *name, unsigned socket_id,
>   * packets. The distributor will ensure that no two packets that have the
>   * same flow id, or tag, in the mbuf will be procesed at the same time.
>   *
> + * The user is advocated to set tag for each mbuf before calling this function.
> + * If user doesn't set the tag, the tag value can be various values depending on
> + * driver implementation and configuration.
> + *
>   * This is not multi-thread safe and should only be called on a single lcore.
>   *
>   * @param d
> -- 
> 1.7.1
>