[dpdk-dev] Ring PMD: why are stats counters atomic?

[Bruce Richardson]

On Tue, May 10, 2016 at 11:13:08AM +0200, Mauricio Vásquez wrote:
> Hello,
> 
> Per-queue stats counters are defined as rte_atomic64_t, in the tx/rx
> functions, they are atomically increased if the rings have the multiple
> consumers/producer flag enabled.
> 
> According to the design principles, the application should not invoke those
> functions on the same queue on different cores, then I think that atomic
> increasing is not necessary.
> 
> Is there something wrong with my reasoning?, If not, I am willing to send a
> patch.
> 
> Thank you very much,
> 
Since the rte_rings, on which the ring pmd is obviously based, have multi-producer
and multi-consumer support built-in, I thought it might be useful in the ring
PMD itself to allow multiple threads to access the ring queues at the same time,
if the underlying rings are marked as MP/MC safe. When doing enqueues and dequeue
from the ring, the stats are either incremented atomically, or non-atomically,
depending on the underlying queue type.

        const uint16_t nb_rx = (uint16_t)rte_ring_dequeue_burst(r->rng,
                        ptrs, nb_bufs);
        if (r->rng->flags & RING_F_SC_DEQ)
                r->rx_pkts.cnt += nb_rx;
        else
                rte_atomic64_add(&(r->rx_pkts), nb_rx);

If people don't think this behaviour is worthwhile keeping, I'm ok with removing
it, since all other PMDs have the restriction that the queues are single-thread
only.

Regards,
/Bruce