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

[Daniel Lezcano]

On 01/31/2014 09:45 AM, Preeti Murthy wrote:
> Hi,
>
> On Thu, Jan 30, 2014 at 10:55 PM, Daniel Lezcano
> <daniel.lezcano@xxxxxxxxxx> wrote:
> > On 01/30/2014 05:35 PM, Peter Zijlstra wrote:
> > > On Thu, Jan 30, 2014 at 05:27:54PM +0100, Daniel Lezcano wrote:
> > > > struct cpuidle_state *state = &drv->states[rq->index];
> > > >
> > > > And from the state, we have the following informations:
> > > >
> > > > struct cpuidle_state {
> > > >
> > > >          [ ... ]
> > > >
> > > >           unsigned int    exit_latency; /* in US */
> > > >           int             power_usage; /* in mW */
> > > >           unsigned int    target_residency; /* in US */
> > > >           bool            disabled; /* disabled on all CPUs */
> > > >
> > > >          [ ... ]
> > > > };
> > >
> > >
> > > Right, but can we say that a higher index will save more power and have
> > > a higher exit latency? Or is a driver free to have a random mapping from
> > > idle_index to state?
> >
> >
> > If the driver does its own random mapping that will break the governor
> > logic. So yes, the states are ordered, the higher the index is, the more you
> > save power and the higher the exit latency is.
>
> The above point holds true for only the ladder governor which sees the idle
> states indexed in the increasing order of target_residency/exit_latency.

The cpuidle framework has been modified for both governor, see commit 
8aef33a7.

The power field was initially used to do the selection, but no power 
value was ever used to filled this field by any hardware. So the field 
was arbitrarily filled with a decreasing value (-1, -2, -3 ...), and 
used by the governor's select function. The patch above just removed 
this field and the condition on power for 'select' assuming the idle 
state are power ordered in the array.

> However this is not true as far as I can see in the menu governor. It
> acknowledges the dynamic ordering of idle states as can be seen in the
> menu_select() function in the menu governor, where the idle state for the
> CPU gets chosen.  You will notice that, even if it is found that the predicted
> idle time of the CPU is smaller than the target residency of an idle state,
> the governor continues to search for suitable idle states in the higher indexed
> states although it should have halted if the idle states' were ordered according
> to their target residency.. The same holds for exit_latency.

I am not sure to get the point. Actually, this loop should be just 
optimized to backward search the idle state like cpuidle_play_dead does.

There is also a patch proposed by Alex Shi about this loop.

[RFC PATCH] cpuidle: reduce unnecessary loop in c-state selection

http://comments.gmane.org/gmane.linux.power-management.general/42124

> Hence I think this patch would make sense only with additional information
> like exit_latency or target_residency is present for the scheduler. The idle
> state index alone will not be sufficient.

May be I misunderstood, but if you have the index, you can get the idle 
state, hence the exit_latency and the target_residency, no ?

> > > Also, we should probably create a pretty function to get that state,
> > > just like you did in patch 1.
> >
> >
> > Yes, right.
> >
> >
> > > > 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.
> >
> >
> > > Not dereferencing the state array saves hitting cold cachelines.
> >
> >
> > Yeah, always good to remind that. Should keep in mind for later.
> >
> > Thanks for your comments.
> >
> >    -- Daniel
> >
> >
> >
> >
> > --
> >   <http://www.linaro.org/> Linaro.org â Open source software for ARM SoCs
> >
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