linux-kernel - Re: [RFC PATCH v2 0/5] sched/cpufreq: Make schedutil energy aware

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Date:   Tue, 9 Jul 2019 11:37:50 +0100
From:   Patrick Bellasi <patrick.bellasi@....com>
To:     Douglas Raillard <douglas.raillard@....com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        mingo@...hat.com, rjw@...ysocki.net, viresh.kumar@...aro.org,
        quentin.perret@....com, dietmar.eggemann@....com
Subject: Re: [RFC PATCH v2 0/5] sched/cpufreq: Make schedutil energy aware

On 08-Jul 14:46, Douglas Raillard wrote:
> Hi Patrick,
> 
> On 7/8/19 12:09 PM, Patrick Bellasi wrote:
> > On 03-Jul 17:36, Douglas Raillard wrote:
> > > On 7/2/19 4:51 PM, Peter Zijlstra wrote:
> > > > On Thu, Jun 27, 2019 at 06:15:58PM +0100, Douglas RAILLARD wrote:

[...]

> > You are also correct in pointing out that in the steady state
> > ramp_boost will not be triggered in that steady state.
> > 
> > IMU, that's for two main reasons:
> >   a) it's very likely that enqueued <= util_avg
> >   b) even in case enqueued should turn out to be _slightly_ bigger then
> >      util_avg, the corresponding (proportional) ramp_boost would be so
> >      tiny to not have any noticeable effect on OPP selection.
> > 
> > Am I correct on point b) above?
> 
> Assuming you meant "util_avg slightly bigger than enqueued" (which is when boosting triggers),
> then yes since ramp_boost effect is proportional to "task_ue.enqueue - task_u". It makes it robust
> against that.

Right :)

> > Could you maybe come up with some experimental numbers related to that
> > case specifically?
> 
> With:
> * an rt-app task ramping up from 5% to 75% util in one big step. The
> whole cycle is 0.6s long (0.3s at 5% followed by 0.3s at 75%). This
> cycle is repeated 20 times and the average of boosting is taken.
> 
> * a hikey 960 (this impact the frequency at which the test runs at
> the beginning of 75% phase, which impacts the number of missed
> activations before the util ramped up).
> 
> * assuming an OPP exists for each util value (i.e. 1024 OPPs, so the
> effect of boost on consumption is not impacted by OPP capacities
> granularity)
> 
> Then the boosting feature would increase the average power
> consumption by 3.1%, out of which 0.12% can be considered "spurious
> boosting" due to the util taking some time to really converge to its
> steady state value.
>
> In practice, the impact of small boosts will be even lower since
> they will less likely trigger the selection of a high OPP due to OPP
> capacity granularity > 1 util unit.

That's ok for the energy side: you estimate a ~3% worst case more
energy on that specific target.

By boosting I expect the negative boost to improve.
Do you have also numbers/stats related to the negative slack?
Can you share a percentage figure for that improvement?

Best,
Patrick

-- 
#include <best/regards.h>

Patrick Bellasi