On Wed, Sep 7, 2016 at 6:32 PM, Eric Dumazet <eric.duma...@gmail.com> wrote: > On Wed, 2016-09-07 at 18:08 +0300, Saeed Mahameed wrote: >> On Wed, Sep 7, 2016 at 5:41 PM, Eric Dumazet <eric.duma...@gmail.com> wrote: >> > On Wed, 2016-09-07 at 15:42 +0300, Saeed Mahameed wrote: >> >> Previously we rang XDP SQ doorbell on every forwarded XDP packet. >> >> >> >> Here we introduce a xmit more like mechanism that will queue up more >> >> than one packet into SQ (up to RX napi budget) w/o notifying the hardware. >> >> >> >> Once RX napi budget is consumed and we exit napi RX loop, we will >> >> flush (doorbell) all XDP looped packets in case there are such. >> > >> > Why is this idea depends on XDP ? >> > >> > It looks like we could apply it to any driver having one IRQ servicing >> > one RX and one TX, without XDP being involved. >> > >> >> Yes but it is more complicated than XDP case, where the RX ring posts >> the TX descriptors and once done >> the RX ring hits the doorbell once for all the TX descriptors it >> posted, and it is the only possible place to hit a doorbell >> for XDP TX ring. >> >> For regular TX and RX ring sharing the same IRQ, there is no such >> simple connection between them, and hitting a doorbell >> from RX ring napi would race with xmit ndo function of the TX ring. >> >> How do you synchronize in such case ? >> isn't the existing xmit more mechanism sufficient enough ? > > Only if a qdisc is present and pressure is high enough. > > But in a forwarding setup, we likely receive at a lower rate than the > NIC can transmit. >
Jesper has a similar Idea to make the qdisc think it is under pressure, when the device TX ring is idle most of the time, i think his idea can come in handy here. I am not fully involved in the details, maybe he can elaborate more. But if it works, it will be transparent to napi, and xmit more will happen by design. > A simple cmpxchg could be used to synchronize the thing, if we really > cared about doorbell cost. (Ie if the cost of this cmpxchg() is way > smaller than doorbell one) >
