linux-kernel - Re: [RFC PATCHC 0/3] sched/idle : find the idlest cpu with cpuidle info

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Date:	Wed, 02 Apr 2014 01:01:11 +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 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.]

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.

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?

Rafael

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