runtime: prevent sysmon from polling network excessivly If the network is not polled for 10ms, sysmon starts polling network on every iteration (every 20us) until another thread blocks in netpoll. Fixes issue 5922.
runtime: prevent sysmon from polling network excessivly
If the network is not polled for 10ms, sysmon starts polling network
on every iteration (every 20us) until another thread blocks in netpoll.
Fixes issue 5922.

R=golang-dev, iant
CC=golang-dev
https://codereview.appspot.com/11569043

glycerine/jeaten-go-arrayof-structof,webfd/go-zh,mhennings/marcohennings-go,sanjosh/sanjos100-tipc,d0f/go-zh,bryanxu/go-zh,rdp/rogerpack2005-golang,webfd/go-zh,bryanxu/go-zh,mhennings/marcohennings-go,webfd/go-zh,bryanxu/go-zh,glycerine/jeaten-go-arrayof-structof,rdp/rogerpack2005-golang,bryanxu/go-zh,d0f/go-zh,webfd/go-zh,mhennings/marcohennings-go,mhennings/marcohennings-go,webfd/go-zh,d0f/go-zh,mhennings/marcohennings-go,d0f/go-zh,glycerine/jeaten-go-arrayof-structof,rdp/rogerpack2005-golang,mhennings/marcohennings-go,rdp/rogerpack2005-golang,d0f/go-zh,bryanxu/go-zh,bryanxu/go-zh,d0f/go-zh,mhennings/marcohennings-go,bryanxu/go-zh,rdp/rogerpack2005-golang,glycerine/jeaten-go-arrayof-structof,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,glycerine/jeaten-go-arrayof-structof,mhennings/marcohennings-go,sanjosh/sanjos100-tipc,glycerine/jeaten-go-arrayof-structof,webfd/go-zh,rdp/rogerpack2005-golang,sanjosh/sanjos100-tipc,d0f/go-zh,sanjosh/sanjos100-tipc,d0f/go-zh,webfd/go-zh,sanjosh/sanjos100-tipc,glycerine/jeaten-go-arrayof-structof,rdp/rogerpack2005-golang,bryanxu/go-zh,webfd/go-zh,glycerine/jeaten-go-arrayof-structof,sanjosh/sanjos100-tipc,rdp/rogerpack2005-golang