On Fri, 28 Aug 2009, Gregory Haskins wrote: > > However, and to what I believe is your point: its not entirely clear to > > me what impact, if any, there would be w.r.t. any _other_ events that > > may be driven off of the scheduler tick (i.e. events other than > > scheduling policies, like timeslice expiration, etc). Perhaps someone > > else like Thomas, Ingo, or Peter have some input here. > > > > I guess the specific question to ask is: Does the scheduler tick code > > have any role other than timeslice policies and updating accounting > > information? Examples would include timer-expiry, for instance. I > > would think most of this logic is handled by finer grained components > > like HRT, but I am admittedly ignorant of the actual timer voodoo ;) There is not much happening in the scheduler tick: - accounting of CPU time. this can be delegated to some other CPU as long as the user space task is running and consuming 100% - timer list timers. If there is no service/device active on that CPU then there are no timers to run - rcu call backs. Same as above, but might need some tweaking. - printk tick. Not really interesting - scheduler time slicing. Not necessary in such a context - posix cpu timers. Only interesting when the application uses them So there is not much which needs the tick in such a scenario. Of course we'd need to exclude that CPU from the do_timer duty as well. > Thinking about this idea some more: I can't see why this isn't just a > trivial variation of the nohz idle code already in mainline. In both > cases (idle and FIFO tasks) the cpu is "consumed" 100% by some arbitrary > job (spinning/HLT for idle, RT thread for FIFO) while we have the > scheduler tick disabled. The only real difference is a matter of > power-management (HLT/mwait go to sleep-states, whereas spinning/rt-task > run full tilt). > > Therefore the answer may be as simple as bracketing the FIFO task with > tick_nohz_stop_sched_tick() + tick_nohz_restart_sched_tick(). The nohz > code will probably need some minor adjustments so it is not assuming > things about the state being "idle" (e.g. "isidle") for places when it > matters (idle_calls++ stat is one example). Yeah, it's similar to what we do in nohz idle already, but we'd need to split out some of the functions very carefully to reuse them. > Potential problems: > > a) disabling/renabling the tick on a per-RT task schedule() may prove to > be prohibitively expensive. For a single taks consuming 100% CPU it is a non problem. You disable it once. But yes on a standard system this needs to be investigated. > b) we will need to make sure the rt-bandwidth protection mechanism is > defeated so the task is allowed to consume 100% bandwidth. > > Perhaps these states should be in the cpuset/root-domain, and configured > when you create the partition (e.g. "tick=off", "bandwidth=off" makes it > an "offline" set). That makes sense and should not be rocket science to implement. Thanks, tglx -- To unsubscribe from this list: send the line "unsubscribe linux-rt-users" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
