Re: [RFC PATCH 3/3] idle: store the idle state index in the struct rq

[Vincent Guittot]

On 30 January 2014 22:02, Nicolas Pitre <nicolas.pitre@xxxxxxxxxx> wrote:
> On Thu, 30 Jan 2014, Lorenzo Pieralisi wrote:
>
>> On Thu, Jan 30, 2014 at 05:25:27PM +0000, Daniel Lezcano wrote:
>> > On 01/30/2014 05:35 PM, Peter Zijlstra wrote:
>> > > On Thu, Jan 30, 2014 at 05:27:54PM +0100, Daniel Lezcano wrote:
>> > >> IIRC, Alex Shi sent a patchset to improve the choosing of the idlest cpu and
>> > >> the exit_latency was needed.
>> > >
>> > > Right. However if we have a 'natural' order in the state array the index
>> > > itself might often be sufficient to find the least idle state, in this
>> > > specific case the absolute exit latency doesn't matter, all we want is
>> > > the lowest one.
>> >
>> > Indeed. It could be simple as that. I feel we may need more informations
>> > in the future but comparing the indexes could be a nice simple and
>> > efficient solution.
>>
>> As long as we take into account that some states might require multiple
>> CPUs to be idle in order to be entered, fine by me. But we should
>> certainly avoid waking up a CPU in a cluster that is in eg C2 (all CPUs in
>> C2, so cluster in C2) when there are CPUs in C3 in other clusters with
>> some CPUs running in those clusters, because there C3 means "CPU in C3, not
>> cluster in C3". Overall what I am saying is that what you are doing
>> makes perfect sense but we have to take the above into account.
>>
>> Some states have CPU and cluster (or we can call it package) components,
>> and that's true on ARM and other architectures too, to the best of my
>> knowledge.
>
> The notion of cluster or package maps pretty naturally onto scheduling
> domains.  And the search for an idle CPU to wake up should avoid a
> scheduling domain with a load of zero (which is obviously a prerequisite
> for a power save mode to be applied to the cluster level) if there exist
> idle CPUs in another domain already which load is not zero (all other
> considerations being equal).  Hence your concern would be addressed
> without any particular issue even if the individual CPU idle state index
> is not exactly in sync with reality because of other hardware related
> constraints.

It's not only a problem of packing in one cluster but also to check
the cost of waking up a CPU regarding the estimated load of the task.
The main problem with only having the index is that the reality
(latency and power consumption) can be different from the targeted
c-state because the system wait that all the condition for entering
this state has been reached. So you will have the wrong values when
looking for the best core for a task.

>
> The other solution consists in making the index dynamic.  That means
> letting backend idle drivers change it i.e. when the last man in a
> cluster goes idle it could update the index for all the other CPUs in
> the cluster.  There is no locking needed as the scheduler is only
> consuming this info, and the scheduler getting it wrong on rare
> occasions is not a big deal either.  But that looks pretty ugly as at
> least 2 levels of abstractions would be breached in this case.

but it 's the only way to get an good view of the current state of a core

Vincent

>
>
> Nicolas
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