On Mon, Dec 12, 2016 at 5:24 PM, Nick Hudson <skrll%netbsd.org@localhost> wrote:
On 11/28/16 09:43, Ryota Ozaki wrote:
On Mon, Nov 28, 2016 at 4:29 PM, Nick Hudson <skrll%netbsd.org@localhost> wrote:
This is hurting me again.
FYI
https://nxr.netbsd.org/xref/src/sys/dev/usb/if_axe.c#527
I think we need NET_MPSAFE finished for netbsd-8
Further investigations show that even if we enable NET_MPSAFE by
default,
there are cases that calling driver Tx with holding softnet_lock. For
example, packet transmission via a unconnected socket and TCP syn ack
(tcp_input still needs softnet_lock).
So I have brought another idea to address the issue: Tx softint; it
defers if_start to softint, which is implemented with deferred if_start.
By doing so, driver's if_start is called without holding softnet_lock.
Note that of course the feature is only enabled for drivers that
request it.
This is a patch: http://www.netbsd.org/~ozaki-r/tx_softint.diff
The patch includes changes to if_axe for an example.
The drawback of the approach is of course performance degradation.
I tried it with vioif and saw 3% down on iperf. I haven't tested
it using USB devices, so I don't know the performance impact on USB Tx.
Nick, how do you think the approach? And can you try the patch and
benchmark if the approach satisfies you?