Messages in this thread | | | Date | Thu, 31 May 2018 10:46:07 +0200 | From | Juri Lelli <> | Subject | Re: [PATCH v5 03/10] cpufreq/schedutil: add rt utilization tracking |
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On 30/05/18 17:46, Quentin Perret wrote: > Hi Vincent, > > On Friday 25 May 2018 at 15:12:24 (+0200), Vincent Guittot wrote: > > Add both cfs and rt utilization when selecting an OPP for cfs tasks as rt > > can preempt and steal cfs's running time. > > > > Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org> > > --- > > kernel/sched/cpufreq_schedutil.c | 14 +++++++++++--- > > 1 file changed, 11 insertions(+), 3 deletions(-) > > > > diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c > > index 28592b6..a84b5a5 100644 > > --- a/kernel/sched/cpufreq_schedutil.c > > +++ b/kernel/sched/cpufreq_schedutil.c > > @@ -56,6 +56,7 @@ struct sugov_cpu { > > /* The fields below are only needed when sharing a policy: */ > > unsigned long util_cfs; > > unsigned long util_dl; > > + unsigned long util_rt; > > unsigned long max; > > > > /* The field below is for single-CPU policies only: */ > > @@ -178,14 +179,21 @@ static void sugov_get_util(struct sugov_cpu *sg_cpu) > > sg_cpu->max = arch_scale_cpu_capacity(NULL, sg_cpu->cpu); > > sg_cpu->util_cfs = cpu_util_cfs(rq); > > sg_cpu->util_dl = cpu_util_dl(rq); > > + sg_cpu->util_rt = cpu_util_rt(rq); > > } > > > > 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; > > + if (rq->rt.rt_nr_running) { > > + util = sg_cpu->max; > > 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? :)
Or Patrick's utilization capping applied to RT..
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