linux-kernel - Re: [RFC PATCHC 3/3] sched/fair: use the idle state info to choose the idlest cpu

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Date:	Thu, 17 Apr 2014 11:53:11 -0400 (EDT)
From:	Nicolas Pitre <nicolas.pitre@...aro.org>
To:	Daniel Lezcano <daniel.lezcano@...aro.org>
cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, peterz@...radead.org,
	rjw@...ysocki.net, linux-pm@...r.kernel.org, alex.shi@...aro.org,
	vincent.guittot@...aro.org, morten.rasmussen@....com
Subject: Re: [RFC PATCHC 3/3] sched/fair: use the idle state info to choose
 the idlest cpu

On Thu, 17 Apr 2014, Daniel Lezcano wrote:

> Ok, refreshed the patchset but before sending it out I would to discuss about
> the rational of the changes and the policy, and change the patchset
> consequently.
> 
> What order to choose if the cpu is idle ?
> 
> Let's assume all cpus are idle on a dual socket quad core.
> 
> Also, we can reasonably do the hypothesis if the cluster is in low power mode,
> the cpus belonging to the same cluster are in the same idle state (putting
> apart the auto-promote where we don't have control on).
> 
> If the policy you talk above is 'aggressive power saving', we can follow the
> rules with decreasing priority:
> 
> 1. We want to prevent to wakeup the entire cluster
> 	=> as the cpus are in the same idle state, by choosing a cpu in
> 	=> shallow 
> state, we should have the guarantee we won't wakeup a cluster (except if no
> shallowest idle cpu are found).

This is unclear to me.  Obviously, if an entire cluster is down, that 
means all the CPUs it contains have been idle for a long time. And 
therefore they shouldn't be subject to selection unless there is no 
other CPUs available.  Is that what you mean?

> 2. We want to prevent to wakeup a cpu which did not reach the target residency
> time (will need some work to unify cpuidle idle time and idle task run time)
> 	=> with the target residency and, as a first step, with the idle
> 	=> stamp, 
> we can determine if the cpu slept enough

Agreed. However, right now, the scheduler does not have any 
consideration for that.  So this should be done as a separate patch.

> 3. We want to prevent to wakeup a cpu in deep idle state
> 	=> by looking for the cpu in shallowest idle state

Obvious.

> 4. We want to prevent to wakeup a cpu where the exit latency is longer than
> the expected run time of the task (and the time to migrate the task ?)

Sure.  That would be a case for using task packing even if the policy is 
set to performance rather than powersave whereas task packing is 
normally for powersave.


Nicolas
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