Messages in this thread | | | Date | Tue, 5 Jun 2018 16:13:17 +0200 | From | Juri Lelli <> | Subject | Re: [PATCH v5 00/10] track CPU utilization |
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On 05/06/18 15:01, Quentin Perret wrote: > On Tuesday 05 Jun 2018 at 15:15:18 (+0200), Juri Lelli wrote: > > On 05/06/18 14:05, Quentin Perret wrote: > > > On Tuesday 05 Jun 2018 at 14:11:53 (+0200), Juri Lelli wrote: > > > > Hi Quentin, > > > > > > > > On 05/06/18 11:57, Quentin Perret wrote: > > > > > > > > [...] > > > > > > > > > What about the diff below (just a quick hack to show the idea) applied > > > > > on tip/sched/core ? > > > > > > > > > > ---8<--- > > > > > diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c > > > > > index a8ba6d1f262a..23a4fb1c2c25 100644 > > > > > --- a/kernel/sched/cpufreq_schedutil.c > > > > > +++ b/kernel/sched/cpufreq_schedutil.c > > > > > @@ -180,9 +180,12 @@ static void sugov_get_util(struct sugov_cpu *sg_cpu) > > > > > sg_cpu->util_dl = cpu_util_dl(rq); > > > > > } > > > > > > > > > > +unsigned long scale_rt_capacity(int cpu); > > > > > static unsigned long sugov_aggregate_util(struct sugov_cpu *sg_cpu) > > > > > { > > > > > struct rq *rq = cpu_rq(sg_cpu->cpu); > > > > > + int cpu = sg_cpu->cpu; > > > > > + unsigned long util, dl_bw; > > > > > > > > > > if (rq->rt.rt_nr_running) > > > > > return sg_cpu->max; > > > > > @@ -197,7 +200,14 @@ static unsigned long sugov_aggregate_util(struct sugov_cpu *sg_cpu) > > > > > * util_cfs + util_dl as requested freq. However, cpufreq is not yet > > > > > * ready for such an interface. So, we only do the latter for now. > > > > > */ > > > > > - return min(sg_cpu->max, (sg_cpu->util_dl + sg_cpu->util_cfs)); > > > > > + util = arch_scale_cpu_capacity(NULL, cpu) * scale_rt_capacity(cpu); > > > > > > > > Sorry to be pedantinc, but this (ATM) includes DL avg contribution, so, > > > > since we use max below, we will probably have the same problem that we > > > > discussed on Vincent's approach (overestimation of DL contribution while > > > > we could use running_bw). > > > > > > Ah no, you're right, this isn't great for long running deadline tasks. > > > We should definitely account for the running_bw here, not the dl avg... > > > > > > I was trying to address the issue of RT stealing time from CFS here, but > > > the DL integration isn't quite right which this patch as-is, I agree ... > > > > > > > > > > > > + util >>= SCHED_CAPACITY_SHIFT; > > > > > + util = arch_scale_cpu_capacity(NULL, cpu) - util; > > > > > + util += sg_cpu->util_cfs; > > > > > + dl_bw = (rq->dl.this_bw * SCHED_CAPACITY_SCALE) >> BW_SHIFT; > > > > > > > > Why this_bw instead of running_bw? > > > > > > So IIUC, this_bw should basically give you the absolute reservation (== the > > > sum of runtime/deadline ratios of all DL tasks on that rq). > > > > Yep. > > > > > The reason I added this max is because I'm still not sure to understand > > > how we can safely drop the freq below that point ? If we don't guarantee > > > to always stay at least at the freq required by DL, aren't we risking to > > > start a deadline tasks stuck at a low freq because of rate limiting ? In > > > this case, if that tasks uses all of its runtime then you might start > > > missing deadlines ... > > > > We decided to avoid (software) rate limiting for DL with e97a90f7069b > > ("sched/cpufreq: Rate limits for SCHED_DEADLINE"). > > Right, I spotted that one, but yeah you could also be limited by HW ... > > > > > > My feeling is that the only safe thing to do is to guarantee to never go > > > below the freq required by DL, and to optimistically add CFS tasks > > > without raising the OPP if we have good reasons to think that DL is > > > using less than it required (which is what we should get by using > > > running_bw above I suppose). Does that make any sense ? > > > > Then we can't still avoid the hardware limits, so using running_bw is a > > trade off between safety (especially considering soft real-time > > scenarios) and energy consumption (which seems to be working in > > practice). > > Ok, I see ... Have you guys already tried something like my patch above > (keeping the freq >= this_bw) in real world use cases ? Is this costing > that much energy in practice ? If we fill the gaps left by DL (when it
IIRC, Claudio (now Cc-ed) did experiment a bit with both approaches, so he might add some numbers to my words above. I didn't (yet). But, please consider that I might be reserving (for example) 50% of bandwidth for my heavy and time sensitive task and then have that task wake up only once in a while (but I'll be keeping clock speed up for the whole time). :/
> doesn't use all the runtime) with CFS tasks that might no be so bad ... > > Thank you very much for taking the time to explain all this, I really > appreciate :-)
Sure. Thanks for participating to the discussion!
Best,
- Juri
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