Re: [PATCH v5 00/10] track CPU utilization

[Juri Lelli]

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