(Nearly) full tickless operation in 3.10

AFAICT the scheduler doesn't switch tasks at every timer tick, even when there is contention for a CPU -- it has its own concept of timeslice length that changes with load. So why does a contended CPU need to run the timer tick if it's not going to switch tasks?

And grandparent wrote something that seemed to match that line of thinking:

> After a little RTFC, I found that a HR-timer was used to calculate the next preemption point. I.e. instead of preempting on 100 Hz clock, it preempts exactly when the timeslot of the current process ends.

The hrtick is used to denote exact time slices for the CFS scheduler to create more fairness. It really doesn't do much more than that. But this does not replace the scheduler_tick, which does among other things, keeps track of the SCHED_RR time slices, manages load balancing, and updates task timings.

But I'm sure in the future the hrtick may be used more to get rid of the periodic tick.