Re: [PATCH 3/4] sched: introduce synchronized idle injection

From: Peter Zijlstra
Date: Fri Nov 20 2015 - 07:54:53 EST

On Fri, Nov 20, 2015 at 11:58:12AM +0100, Thomas Gleixner wrote:
> That's not what I meant. If you don't want to control all that from
> the scheduler than you are back to that thread which "runs" at RT
> priority and does
> if (machine_on_fire) {
> defer_timer_interrupt(5ms);
> end = now + 5ms:
> while (now < end)
> mwait();
> }
> That's what the existing code does, but the above does not longer
> claim it's idle and confuses the hell out of nohz and whatever. It's
> just a "runaway" RT task which "hogs" the CPU for 5ms and makes the
> next timer interrupt firing late.

Right; so the naive way of implementing that is by simply programing the
timer hardware 5ms in the future and leaving it at that.

The problem with that would be a device interrupt happening and mucking
with timers, this would result in the timer hardware being reprogrammed
to a 'sane' value. I see two solutions for that:

- add another check in tick_program_event(); or,

- muck about with the evtdev pointer, such that we (temporarily) neuter
its clock_event_device::set_next_*() methods.

The later is fugly, but avoids any runtime overhead.

This all makes the idle-injection muck hard depend on hres_active, but I
think that's a sane constraint anyway.

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