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Rusty Russell authored
The virtio_net driver is complicated by the two methods of freeing old xmit buffers (in addition to freeing old ones at the start of the xmit path). The original code used a 1/10 second timer attached to xmit_free(), reset on every xmit. Before we orphaned skbs on xmit, the transmitting userspace could block with a full socket until the timer fired, the skb destructor was called, and they were re-woken. So we added the VIRTIO_F_NOTIFY_ON_EMPTY feature: supporting devices send an interrupt (even if normally suppressed) on an empty xmit ring which makes us schedule xmit_tasklet(). This was a benchmark win. Unfortunately, VIRTIO_F_NOTIFY_ON_EMPTY makes quite a lot of work: a host which is faster than the guest will fire the interrupt every xmit packet (slowing the guest down further). Attempting mitigation in the host adds overhead of userspace timers (possibly with the additional pain of signals), and risks increasing latency anyway if you get it wrong. In practice, this effect was masked by benchmarks which take advantage of GSO (with its inherent transmit batching), but it's still there. Now we orphan xmitted skbs, the pressure is off: remove both paths and no longer request VIRTIO_F_NOTIFY_ON_EMPTY. Note that the current QEMU will notify us even if we don't negotiate this feature (legal, but suboptimal); a patch is outstanding to improve that. Move the skb_orphan/nf_reset to after we've done the send and notified the other end, for a slight optimization. Signed-off-by:
Rusty Russell <rusty@rustcorp.com.au> Cc: Mark McLoughlin <markmc@redhat.com>
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