On Wed, May 24, 2017 at 10:25:05AM +0100, Juri Lelli wrote: > Hi, > > On 23/05/17 22:23, Peter Zijlstra wrote: > > On Tue, May 23, 2017 at 09:53:43AM +0100, Juri Lelli wrote: > > > > > A point that is still very much up for discussion (more that the others > > > :) is > > > how we implement frequency/cpu scaling. SCHED_FLAG_RECLAIM tasks only need > > > grub_reclaim(), as the function already scales their reservation runtime > > > considering other reservations and maximum bandwidth a CPU has to offer. > > > However, for normal !RECLAIM tasks multiple things can be implemented > > > which > > > seem to make sense: > > > > > > - don't scale at all: normal tasks will only get a % of CPU _time_ as > > > granted > > > by AC > > > - go to max as soon as a normal task in enqueued: this because > > > dimensioning of > > > parameters is usually done at max OPP/biggest CPU and normal task > > > assume > > > that this is always the condition when they run > > > - scale runtime acconding to current frequency and max CPU capacity: > > > this is > > > what this set is currently implementing > > > > > > Opinions? > > > > > > So I'm terribly confused... > > > > By using the active bandwidth to select frequency we effectively > > reduce idle time (to 0 if we had infinite granular frequency steps and > > no margins). > > > > So !RECLAIM works as expected. They get the time they reserved, since > > that was taken into account by active bandwidth. > > > > This was my impression as well, but Luca (and please Luca correct me if > I misunderstood your point) argued (in an off-line discussion ahead of > this posting) that !reclaim tasks might not be interested in reclaiming > *at all*. Since scaling frequency down is another way of effectively > reclaiming unused bandwidth (the other being sharing unused bandwidth > among reservations while keeping frequency at max), !reclaim tasks could > not be interested in frequency scaling (my first point above) or require > frequency to be always at max (second point above). > > Does this help claryfing a bit? :)
No ;-) As you said, confusion++. A !RECLAIM task doesn't care (cannot care, should not care etc..) about any bandwidth not allocated to itself. Therefore it should/must/etc.. not have any opinion on what we do with 'spare' bandwidth.
