net: lls select poll support

David,

Here is a rework of the select/poll patch.

When select or poll are used on a lot of sockets the sysctl value needs to be set higher than 50. For 300 sockets a setting of 100 works for me. For 1000 sockets a setting of 200 works well but the gain is very small, probably not worth it.

I should mention that unlike the version we had in v9, with this version of the patch, LLS always performs better than no LLS.

In this version I split the sysctl entries into two. sysctl.net.core.low_latency_read for blocking socket reads. sysctl.net.core.low_latency_poll for select/poll.

The main reason we need two sysctl entries is that the values are different.

This also allows poll to busy poll only on some sockets. You set sysctl.net.core.low_latency_poll, but leave the read value 0. Then you use the socket option to enable SO_LL only on the sockets you would like to buys poll on.

Maybe what we really want is a cgroup that would allow settings per-process group.

-Eliezer

Change log:
v4
- cleaned up formatting

v3
- split sysctl value into two separate ones, one for read and one for poll.
- updated Documentation/sysctl/net.txt
- fixed possible overflow (again) reported by Eric Dumazet.
- poll/select loop optimizations by Eric Dumazet and Willem de Bruijn.

v2
- added POLL_LL flag, used to signal between the syscalls and sock_poll().
- add a separate ll_end_time() function to be used from poll.
- slight reorder of sk_poll_ll so the timing primitives are not used when called from sock_poll().
- select/poll stop busy polling as soon as there is something to return to the user.

Change log from the original LLS patch series:
v9
- better mask testing in sock_poll(), reported by Eric Dumazet.

v8
- split out udp and select/poll into separate patches. what used to be patch 2/5 is now three patches.

v5
- added simple poll/select support