On Thu, May 31, 2018 at 10:46:07AM +0200, Juri Lelli wrote: > On 30/05/18 17:46, Quentin Perret wrote:
> > So I understand why we want to got to max freq when a RT task is running, > > but I think there are use cases where we might want to be more conservative > > and use the util_avg of the RT rq instead. The first use case is > > battery-powered devices where going to max isn't really affordable from > > an energy standpoint. Android, for example, has been using a RT > > utilization signal to select OPPs for quite a while now, because going > > to max blindly is _very_ expensive. > > > > And the second use-case is thermal pressure. On some modern CPUs, going to > > max freq can lead to stringent thermal capping very quickly, at the > > point where your CPUs might not have enough capacity to serve your tasks > > properly. And that can ultimately hurt the very RT tasks you originally > > tried to run fast. In these systems, in the long term, you'd be better off > > not asking for more than what you really need ... > > Proposed the same at last LPC. Peter NAKed it (since RT is all about > meeting deadlines, and when using FIFO/RR we don't really know how fast > the CPU should go to meet them, so go to max is the only safe decision). > > > So what about having a sched_feature to select between going to max and > > using the RT util_avg ? Obviously the default should keep the current > > behaviour. > > Peter, would SCHED_FEAT make a difference? :) Hurmph... > Or Patrick's utilization capping applied to RT.. There might be something there, IIRC that tracks the max potential utilization for the running tasks. So at that point we can set a frequency to minimize idle time. It's not perfect, because while the clamping thing effectively sets a per-task bandwidth, the max filter is wrong. Also there's no CBS to enforce anything. With RT servers we could aggregate the group bandwidth and limit from that...
