[PATCH 0/4] [RFC] virtio-net: Improve small packet performance

[Krishna Kumar <krkumar2@in.ibm.com>]

Earlier approach to improving small packet performance went
along the lines of dropping packets when the txq is full to
avoid stop/start of the txq. Though performance improved
significantly (upto 3x) for a single thread, multiple netperf
sessions showed a regression of upto -17% (starting from 4
sessions).

This patch proposes a different approach with the following
changes:

A. virtio:
	- Provide a API to get available number of slots.

B. virtio-net:
	- Remove stop/start txq's and associated callback.
	- Pre-calculate the number of slots needed to transmit
	  the skb in xmit_skb and bail out early if enough space
	  is not available. My testing shows that 2.5-3% of
	  packets are benefited by using this API.
	- Do not drop skbs but instead return TX_BUSY like other
	  drivers.
	- When returning EBUSY, set a per-txq variable to indicate
	  to dev_queue_xmit() whether to restart xmits on this txq.

C. net/sched/sch_generic.c:
	Since virtio-net now returns EBUSY, the skb is requeued to
	gso_skb. This allows adding the addional check for restart
	xmits in just the slow-path (the first re-queued packet
	case of dequeue_skb, where it checks for gso_skb) before
	deciding whether to call the driver or not.

Patch was also tested between two servers with Emulex OneConnect
10G cards to confirm there is no regression. Though the patch is
an attempt to improve only small packet performance, there was
improvement for 1K, 2K and also 16K both in BW and SD. Results
from Guest -> Remote Host (BW in Mbps) for 1K and 16K I/O sizes:

________________________________________________________
			I/O Size: 1K
#	BW1	BW2 (%)		SD1	SD2 (%)
________________________________________________________
1	1226	3313 (170.2)	6.6	1.9 (-71.2)
2	3223	7705 (139.0)	18.0	7.1 (-60.5)
4	7223	8716 (20.6)	36.5	29.7 (-18.6)
8	8689	8693 (0)	131.5	123.0 (-6.4)
16	8059	8285 (2.8)	578.3	506.2 (-12.4)
32	7758	7955 (2.5)	2281.4	2244.2 (-1.6)
64	7503	7895 (5.2)	9734.0	9424.4 (-3.1)
96	7496	7751 (3.4)	21980.9	20169.3 (-8.2)
128	7389	7741 (4.7)	40467.5	34995.5 (-13.5)
________________________________________________________
Summary:	BW: 16.2%	SD: -10.2%

________________________________________________________
			I/O Size: 16K
#	BW1	BW2 (%)		SD1	SD2 (%)
________________________________________________________
1	6684	7019 (5.0)	1.1	1.1 (0)
2	7674	7196 (-6.2)	5.0	4.8 (-4.0)
4	7358	8032 (9.1)	21.3	20.4 (-4.2)
8	7393	8015 (8.4)	82.7	82.0 (-.8)
16	7958	8366 (5.1)	283.2	310.7 (9.7)
32	7792	8113 (4.1)	1257.5	1363.0 (8.3)
64	7673	8040 (4.7)	5723.1	5812.4 (1.5)
96	7462	7883 (5.6)	12731.8	12119.8 (-4.8)
128	7338	7800 (6.2)	21331.7	21094.7 (-1.1)
________________________________________________________
Summary:	BW: 4.6%	SD: -1.5%

Signed-off-by: Krishna Kumar <krkumar2@in.ibm.com>
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