On Mon, 14 Mar 2022 15:01:23 +0800 Biao Huang wrote: > > Drivers are expected to stop their queues at the end of xmit routine > > if > > the ring can't accommodate another frame. It's more efficient to stop > > the queues early than have to put skbs already dequeued from the > > qdisc > > layer back into the qdiscs. > Yes, if descriptors ring is full, it's meaningful to stop the queue > at the end of xmit; > But driver seems hard to know how many descriptors the next skb will > request, e.g. 3 descriptors are available for next round send, but the > next skb may need 4 descriptors, in this case, we still need judge > whether descriptors are enough for skb transmission, then decide stop > the queue or not, at the beginning of xmit routine. > > Maybe we should judge ring is full or not at the beginning and the end > of xmit routine(seems a little redundancy). Assume the worst case scenario. You set the default ring size to 512, skb can have at most MAX_SKB_FRAGS fragments (usually 17) so the max number of descriptors should not be very high, hard to pre-compute, or problematic compared to the total ring size.
