Messages in this thread |  | | Date | Fri, 14 Sep 2018 14:57:12 +0100 | From | Patrick Bellasi <> | Subject | Re: [PATCH v4 06/16] sched/cpufreq: uclamp: add utilization clamping for FAIR tasks |
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On 14-Sep 15:36, Peter Zijlstra wrote: > On Fri, Sep 14, 2018 at 02:19:19PM +0100, Patrick Bellasi wrote: > > On 14-Sep 11:32, Peter Zijlstra wrote: > > > > Should that not be: > > > > > > util = clamp_util(rq, cpu_util_cfs(rq)); > > > > > > Because if !util might we not still want to enforce the min clamp? > > > > If !util CFS tasks should have been gone since a long time > > (proportional to their estimated utilization) and thus it probably > > makes sense to not affect further energy efficiency for tasks of other > > classes. > > I don't remember what we do for util for new tasks; but weren't we > talking about setting that to 0 recently? IIRC the problem was that if > we start at 1 with util we'll always run new tasks on big cores, or > something along those lines.
Mmm.. could have been in a recent discussion with Quentin, but I think I've missed it. I know we have something similar on Android for similar reasons.
> So new tasks would still trigger this case until they'd accrued enough > history.
Well, yes and no. New tasks will be clamped which means that if they are generated from a capped parent (or within a cgroups with a suitable util_max) they can still live in a smaller capacity CPU despite their utilization being 1024. Thus, to a certain extend, UtilClamp could be a fix for the above misbehavior whenever needed.
NOTE: this series does not include tasks biasing bits.
> Either way around, I don't much care at this point except I think it > would be good to have a comment to record the assumptions.
Sure, will add a comment on that and a warning about possible side effects on tasks placement
> > > Would that not be more readable as: > > > > > > static inline unsigned int uclamp_value(struct rq *rq, int clamp_id) > > > { > > > unsigned int val = rq->uclamp.value[clamp_id]; > > > > > > if (unlikely(val == UCLAMP_NOT_VALID)) > > > val = uclamp_none(clamp_id); > > > > > > return val; > > > } > > > > I'm trying to keep consistency in variable names usages by always > > accessing the rq's clamps via a *uc_cpu to make it easy grepping the > > code. Does this argument make sense ? > > > > On the other side, what you propose above is more easy to read > > by looking just at that function.... so, if you prefer it better, I'll > > update it on v5. > > I prefer my version, also because it has a single load of the value (yes > I know about CSE passes). I figure one can always grep for uclamp or > something.
+1
> > > And how come NOT_VALID is possible? I thought the idea was to always > > > have all things a valid value. > > > > When we update the CPU's clamp for a "newly idle" CPU, there are not > > tasks refcounting clamps and thus we end up with UCLAMP_NOT_VALID for > > that CPU. That's how uclamp_cpu_update() is currently encoded. > > > > Perhaps we can set the value to uclamp_none(clamp_id) from that > > function, but I was thinking that perhaps it could be useful to track > > explicitly that the CPU is now idle. > > IIRC you added an explicit flag to track idle somewhere.. to keep the > last max clamp in effect or something.
Right... that patch was after this one on v3, but know that I've moved it before we can probably simplify this path.
> I think, but haven't overly thought about this, that if you always > ensure these things are valid you can avoid a bunch of NOT_VALID > conditions. And less conditions is always good, right? :-)
Right, will check better all the usages and remove them when not strictly required.
Cheers, Patrick
-- #include <best/regards.h>
Patrick Bellasi
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