Date:	Fri, 04 Apr 2014 13:23:17 +0200
From:	"Rafael J. Wysocki" <rjw@...ysocki.net>
To:	Daniel Lezcano <daniel.lezcano@...aro.org>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, peterz@...radead.org,
	nicolas.pitre@...aro.org, linux-pm@...r.kernel.org,
	alex.shi@...aro.org, vincent.guittot@...aro.org,
	morten.rasmussen@....com
Subject: Re: [RFC PATCHC 0/3] sched/idle : find the idlest cpu with cpuidle info

On Wednesday, April 02, 2014 10:26:31 AM Daniel Lezcano wrote:
> On 04/02/2014 01:01 AM, Rafael J. Wysocki wrote:
> > On Friday, March 28, 2014 01:29:53 PM Daniel Lezcano wrote:
> >> The following patchset provides an interaction between cpuidle and the scheduler.
> >>
> >> The first patch encapsulate the needed information for the scheduler in a
> >> separate cpuidle structure. The second one stores the pointer to this structure
> >> when entering idle. The third one, use this information to take the decision to
> >> find the idlest cpu.
> >>
> >> After some basic testing with hackbench, it appears there is an improvement for
> >> the performances (small) and for the duration of the idle states (which provides
> >> a better power saving).
> >>
> >> The measurement has been done with the 'idlestat' tool previously posted in this
> >> mailing list.
> >>
> >> So the benefit is good for both sides performance and power saving.
> >>
> >> The select_idle_sibling could be also improved in the same way.
> >
> > Well, quite frankly, I don't really like this series.  Not the idea itself, but
> > the way it has been implemented.
> >
> > First off, if the scheduler is to access idle state data stored in struct
> > cpuidle_state, I'm not sure why we need a separate new structure for that?
> > Couldn't there be a pointer to a whole struct cpuidle_state from struct rq
> > instead?  [->exit_latency is the only field that find_idlest_cpu() in your
> > third patch seems to be using anyway.]
> 
> Hi Rafael,
> 
> thank you very much for reviewing the patchset.
> 
> I created a specific structure to encapsulate the informations needed 
> for the scheduler and to prevent to export unneeded data. This is purely 
> for code design. Also it was to separate the idle's energy 
> characteristics from the cpuidle framework data (flags, name, etc ...).
> 
> The exit_latency field is only used in this patchset but the 
> target_residency will be used also (eg. prevent to wakeup a cpu before 
> the minimum idle time target residency).

OK

It would be good to add that heuristics upfront so that we can see the full
picture.

> The power field is ... hum ... not filled by any board (except for 
> calxeda). Vendors do not like to share this information, so very likely 
> that would be changed to a normalized value, I don't know.

I'm not sure if that field is ever going to be used by everyone to be honest.

> I agree we can put a pointer to the struct cpuidle_state instead if that 
> reduce the impact of the patchset.

Yes, it will, in my opinion.

> > Second, is accessing the idle state information for all CPUs from find_idlest_cpu()
> > guaranteed to be non-racy?  I mean, what if a CPU changes its state from idle to
> > non-idle while another one is executing find_idlest_cpu()?  In other words,
> > where's the read memory barrier corresponding to the write ones in the modified
> > cpu_idle_call()?  And is the memory barrier actually sufficient?  After all,
> > you need to guarantee that the CPU is still idle after you have evaluated
> > idle_cpu() on it.
> 
> Well, as Nicolas mentioned it in another mail, we can live with races, 
> the scheduler will take a wrong decision but nothing worth than what we 

I guess you mean "worse"?  I'm not sure about that.

> have today. In any case we want to prevent any lock in the code.

Of course. :-)

> > Finally, is really the heuristics used by find_idlest_cpu() to select the "idlest"
> > CPU the best one?  What about deeper vs shallower idle states, for example?
> 
> I believe it is what is supposed to do the patchset. 1. if the cpu is 
> idle, pick the shallower, 2. if the cpu is not idle pick the less 
> loaded. But may be there is something wrong in the routine as pointed 
> Nico, I have to double check it.

Yes, that routine doesn't look entirely correct then.

Thanks!

-- 
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.
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