Messages in this thread | | | Date | Fri, 22 Jun 2018 08:58:53 +0100 | From | Quentin Perret <> | Subject | Re: [PATCH v6 04/11] cpufreq/schedutil: use rt utilization tracking |
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Hi Peter,
On Thursday 21 Jun 2018 at 20:45:24 (+0200), Peter Zijlstra wrote: > On Fri, Jun 08, 2018 at 02:09:47PM +0200, Vincent Guittot wrote: > > static unsigned long sugov_aggregate_util(struct sugov_cpu *sg_cpu) > > { > > struct rq *rq = cpu_rq(sg_cpu->cpu); > > + unsigned long util; > > > > if (rq->rt.rt_nr_running) > > return sg_cpu->max; > > > > + util = sg_cpu->util_dl; > > + util += sg_cpu->util_cfs; > > + util += sg_cpu->util_rt; > > + > > /* > > * Utilization required by DEADLINE must always be granted while, for > > * FAIR, we use blocked utilization of IDLE CPUs as a mechanism to > > @@ -197,7 +204,7 @@ 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)); > > + return min(sg_cpu->max, util); > > } > > So this (and the dl etc. equivalents) result in exactly the problems > complained about last time, no? > > What I proposed was something along the lines of: > > util = 1024 * sg_cpu->util_cfs; > util /= (1024 - (sg_cpu->util_rt + sg_cpu->util_dl + ...)); > > return min(sg_cpu->max, util + sg_cpu->bw_dl); > > Where we, instead of directly adding the various util signals. > > I now see an email from Quentin asking if these things are not in fact > the same, but no, they are not. The difference is that the above only > affects the CFS signal and will re-normalize the utilization of an > 'always' running task back to 1 by compensating for the stolen capacity. > > But it will not, like these here patches, affect the OPP selection of > other classes. If there is no CFS utilization (or very little), then the > renormalization will not matter, and the existing DL bandwidth > compuation will be unaffected.
Right, thinking more carefully about this re-scaling, the two things are indeed not the same, but I'm still not sure if this is what we want.
Say we have 50% of the capacity stolen by RT, and a 25% CFS task running. If we re-scale, we'll end up with a 50% request for CFS (util==512 for your code above). But if we want to see a little bit of idle time in the system, we should really request an OPP for 75%+ of capacity no ? Or am I missing something ?
And also, I think Juri had concerns when we use the util_dl (as a PELT signal) for OPP selection since that kills the benefit of DL for long running DL tasks. Or can we assume that DL tasks with very long runtime/periods are a corner case we can ignore ?
Thanks, Quentin
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