[LAD] BFS and lennart's realtimekit idea...

Isochronous scheduling. Isochronous scheduling is a unique scheduling policy designed to provide near-real-time performance to unprivileged (ie non-root) users without the ability to starve the machine indefinitely. Isochronous tasks (which means "same time") are set using, for example, the schedtool application like so: schedtool -I -e amarok This will start the audio application "amarok" as SCHED_ISO. How SCHED_ISO works is that it has a priority level between true realtime tasks and SCHED_NORMAL which would allow them to preempt all normal tasks, in a SCHED_RR fashion (ie, if multiple SCHED_ISO tasks are running, they purely round robin at rr_interval rate). However if ISO tasks run for more than a tunable finite amount of time, they are then demoted back to SCHED_NORMAL scheduling. This finite amount of time is the percentage of _total CPU_ available across the machine, configurable as a percentage in the following "resource handling" tunable (as opposed to a scheduler tunable): /proc/sys/kernel/iso_cpu and is set to 70% by default. It is calculated over a rolling 5 second average Because it is the total CPU available, it means that on a multi CPU machine, it is possible to have an ISO task running as realtime scheduling indefinitely on just one CPU, as the other CPUs will be available. Setting this to 100 is the equivalent of giving all users SCHED_RR access and setting it to 0 removes the ability to run any pseudo-realtime tasks. A feature of BFS is that it detects when an application tries to obtain a realtime policy (SCHED_RR or SCHED_FIFO) and the caller does not have the appropriate privileges to use those policies. When it detects this, it will give the task SCHED_ISO policy instead. Thus it is transparent to the user. Because some applications constantly set their policy as well as their nice level, there is potential for them to undo the override specified by the user on the command line of setting the policy to SCHED_ISO. To counter this, once a task has been set to SCHED_ISO policy, it needs superuser privileges to set it back to SCHED_NORMAL. This will ensure the task remains ISO and all child processes and threads will also inherit the ISO policy.