Re: [PATCH V2 1/5] sched: idle: cpuidle: Check the latency req before idle

From: Daniel Lezcano
Date: Thu Nov 06 2014 - 07:27:42 EST

On 11/06/2014 05:08 AM, Preeti U Murthy wrote:
On 11/05/2014 07:58 PM, Daniel Lezcano wrote:
On 10/29/2014 03:01 AM, Preeti U Murthy wrote:
On 10/29/2014 12:29 AM, Daniel Lezcano wrote:
On 10/28/2014 04:51 AM, Preeti Murthy wrote:
Hi Daniel,

On Thu, Oct 23, 2014 at 2:31 PM, Daniel Lezcano
<daniel.lezcano@xxxxxxxxxx> wrote:
When the pmqos latency requirement is set to zero that means "poll in
all the
cases".

That is correctly implemented on x86 but not on the other archs.

As how is written the code, if the latency request is zero, the
governor will
return zero, so corresponding, for x86, to the poll function, but for
the
others arch the default idle function. For example, on ARM this is
wait-for-
interrupt with a latency of '1', so violating the constraint.

This is not true actually. On PowerPC the idle state 0 has an
exit_latency of 0.


In order to fix that, do the latency requirement check *before*
calling the
cpuidle framework in order to jump to the poll function without
entering
cpuidle. That has several benefits:

Doing so actually hurts on PowerPC. Because the idle loop defined for
idle state 0 is different from what cpu_relax() does in
cpu_idle_loop().
The spinning is more power efficient in the former case. Moreover we
also set
certain register values which indicate an idle cpu. The ppc_runlatch
bits
do precisely this. These register values are being read by some user
space
tools. So we will end up breaking them with this patch

My suggestion is very well keep the latency requirement check in
kernel/sched/idle.c
like your doing in this patch. But before jumping to cpu_idle_loop
verify if the
idle state 0 has an exit_latency > 0 in addition to your check on the
latency_req == 0.
If not, you can fall through to the regular path of calling into the
cpuidle driver.
The scheduler can query the cpuidle_driver structure anyway.

What do you think?

Thanks for reviewing the patch and spotting this.

Wouldn't make sense to create:

void __weak_cpu_idle_poll(void) ?

and override it with your specific poll function ?


No this would become ugly as far as I can see. A weak function has to be
defined under arch/* code. We will either need to duplicate the idle
loop that we already have in the drivers or point the weak function to
the first idle state defined by our driver. Both of which is not
desirable (calling into the driver from arch code is ugly). Another
reason why I don't like the idea of a weak function is that if you have
missed looking at a specific driver and they have an idle loop with
features similar to on powerpc, you will have to spot it yourself and
include the arch specific cpu_idle_poll() for them.

Yes, I agree this is a fair point. But actually I don't see the interest
of having the poll loop in the cpuidle driver. These cleanups are

We can't do that simply because the idle poll loop has arch specific
bits on powerpc.

I am not sure.

Could you describe what is the difference between the arch_cpu_idle function in arch/arm/powerpc/kernel/idle.c and the 0th power PC idle state ?

Is it kind of duplicate ?

And for polling, do you really want to use while (...); cpu_relax(); as it is x86 specific ? instead of the powerpc's arch_idle ?

Today, if latency_req == 0, it returns the 0th idle state, so polling.

If we jump to the arch_cpu_idle_poll, the result will be the same for all architecture.

preparing the removal of the CPUIDLE_DRIVER_STATE_START macro which
leads to a lot of mess in the cpuidle code.

How is the suggestion to check the exit_latency of idle state 0 when
latency_req == 0 going to hinder this removal?

It sounds a bit hackish. I prefer to sort out the current situation.

And by the way, what is the reasoning behind having a target_residency / exit_latency equal to zero for an idle state ?

All this sounds really fuzzy for me.

With the removal of this macro, we should be able to move the select
loop from the menu governor and use it everywhere else. Furthermore,
this state which is flagged with TIME_VALID, isn't because the local
interrupt are enabled so we are measuring the interrupt time processing.
Beside that the idle loop for x86 is mostly not used.

So the idea would be to extract those idle loop from the drivers and use
them directly when:
1. the idle selection fails (use the poll loop under certain
circumstances we have to redefine)

This behavior will not change as per my suggestion.

2. when the latency req is zero

Its only here that I suggested you also verify state 0's exit_latency.
For the reason that the arch may have a more optimized idle poll loop,
which we cannot override with the generic cpuidle poll loop.

Regards
Preeti U Murthy

That will result in a cleaner code in cpuidle and in the governor.

Do you agree with that ?

But by having a check on the exit_latency, you are claiming that since
the driver's 0th idle state is no better than the generic idle loop in
cases of 0 latency req, we are better off calling the latter, which
looks reasonable. That way you don't have to bother about worsening the
idle loop behavior on any other driver.








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