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

Claudio Scordino
2018-06-06T13:53:30Z

Hi Quentin,

2018-06-06 15:20 GMT+02:00 Quentin Perret <quentin.perret@arm.com>:
>
> Hi Claudio,
>
> On Wednesday 06 Jun 2018 at 15:05:58 (+0200), Claudio Scordino wrote:
> > Hi Quentin,
> >
> > Il 05/06/2018 16:13, Juri Lelli ha scritto:
> > > 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). :/
> >
> > As far as I can remember, we never tested energy consumption of running_bw
> > vs this_bw, as at OSPM'17 we had already decided to use running_bw
> > implementing GRUB-PA.
> > The rationale is that, as Juri pointed out, the amount of spare (i.e.
> > reclaimable) bandwidth in this_bw is very user-dependent. For example,
> > the user can let this_bw be much higher than the measured bandwidth, just
> > to be sure that the deadlines are met even in corner cases.
>
> Ok I see the issue. Trusting userspace isn't necessarily the right thing
> to do, I totally agree with that.
>
> > In practice, this means that the task executes for quite a short time and
> > then blocks (with its bandwidth reclaimed, hence the CPU frequency reduced,
> > at the 0lag time).
> > Using this_bw rather than running_bw, the CPU frequency would remain at
> > the same fixed value even when the task is blocked.
> > I understand that on some cases it could even be better (i.e. no waste
> > of energy in frequency switch).
>
> +1, I'm pretty sure using this_bw is pretty much always worst than
> using running_bw from an energy standpoint,. The waste of energy in
> frequency changes should be less than the energy wasted by staying at a
> too high frequency for a long time, otherwise DVFS isn't a good idea to
> begin with :-)
>
> > However, IMHO, these are corner cases and in the average case it is better
> > to rely on running_bw and reduce the CPU frequency accordingly.
>
> My point was that accepting to go at a lower frequency than required by
> this_bw is fundamentally unsafe. If you're at a low frequency when a DL
> task starts, there are real situations where you won't be able to
> increase the frequency immediately, which can eventually lead to missing
> deadlines.

I see. Unfortunately, I'm having quite crazy days so I couldn't follow
the original discussion on LKML properly. My fault.
Anyway, to answer your question (if this time I have understood it correctly).

You're right: the tests have shown that whenever the DL task period
gets comparable with the time for switching frequency, the amount of
missed deadlines becomes not negligible.
To give you a rough idea, this already happens with periods of 10msec
on a Odroid XU4.
The reason is that the task instance starts at a too low frequency,
and the system can't switch frequency in time for meeting the
deadline.

This is a known issue, partially discussed during the RT Summit'17.
However, the community has been more in favour of reducing the energy
consumption than meeting firm deadlines.
If you need a safe system, in fact, you'd better thinking about
disabling DVFS completely and relying on a fixed CPU frequency.

A possible trade-off could be a further entry in sys to let system
designers switching from (default) running_bw to (more pessimistic)
this_bw.
However, I'm not sure the community wants a further knob on sysfs just
to make RT people happier :)

Best,

Claudio