On 05/08/20 14:40, pet...@infradead.org wrote: > On Mon, Aug 03, 2020 at 09:22:53PM +0200, Thomas Gleixner wrote: > >> totaltime = irqtime + tasktime >> >> Ignoring irqtime and pretending that totaltime is what the scheduler >> can control and deal with is naive at best. > > Well no, that's what we call system overhead and is assumed to be > included in the 'error margin'. > > The way things are set up is that we say that, by default, RT tasks can > consume 95% of cputime and the remaining 5% is sufficient to keep the > system alive. > > Those 5% include all system overhead, IRQs, RCU, !RT workqueues etc.. > > Obviously IRQ_TIME accounting changes the balance a bit, but that's what > it is. We can't really do anything better. >
I'm starting to think that as well. I tried some fugly hack of injecting avg_irq into sched_rt_runtime_exceeded() with something along the lines of: irq_time = (rq->avg_irq.util_avg * sched_rt_period(rt_rq)) >> SCHED_CAPACITY_SHIFT; It's pretty bad for a few reasons; one is that avg_irq already has its own period (PELT-based). Another is that it is, as Dietmar pointed out, CPU and freq invariant, so falls over on big.LITTLE. Making update_curr_rt() use rq_clock() rather than rq_clock_task() makes it "work" but goes against all the good reasons there were to introduce rq_clock_task() in the first place. > Apparently this SoC has significant IRQ time for some reason. Also, > relying on RT throttling for 'correct' behaviour is also wrong. What > needs to be done is find who is using all this RT time and why, that > isn't right. I've been tempted to say the test case is a bit bogus, but am not familiar enough with the RT throttling details to stand that ground. That said, from both looking at the execution and the stress-ng source code, it seems to unconditionally spawn 32 FIFO-50 tasks (there's even an option to make these FIFO-99!!!), which is quite a crowd on monoCPU systems.
