Re: [PATCH V3 2/6] sched: idle: cpuidle: Check the latency req before idle

Daniel Lezcano
2014-11-12T17:52:40Z

On 11/12/2014 04:02 PM, Peter Zijlstra wrote:
> On Wed, Nov 12, 2014 at 02:53:10PM +0100, Daniel Lezcano wrote:
>
>>>> The governors are just ignoring it, except for a small timer optimization in
>>>> menu.c (and I am still not convinced it is worth to have it). I don't see
>>>> the point to add a state we don't want to use.
>>>
>>> The ignoring it is _wrong_. Make that go away and you'll get rid of most
>>> of the STATE_START crap.
>>>
>>> The governors are the place where we combine the QoS constraints with
>>> idle predictors and pick an idle state, polling is a valid state to
>>> pick, and given QoS constraints it might be the only state to pick.
>>
>> Well, I understand your point of view but I still disagree. IMO, the poll
>> loop shouldn't be considered as a valid idle state for different reasons:
>>
>> 0. thermal issue if wrongly selected from any of the governor
>
> That seems like a QoS issue and should be fixed there, no?

It could be seen as a QoS issue if we stick the poll loop with the zero
latency req. But as soon as we introduce the poll loop in the cpuidle
framework, the issue could have multiple sources (see below at the end
of the message).

>> 1. a polling loop does not have a valid time measurements even if the
>> TIME_VALID flag has been added
>
> Ah, right you are. It does not. We _could_ fix that, not sure its worth
> the hassle but see below.
>
>> 2. entering the idle governors is not free, especially the menu governor,
>> which is contradictory with zero latency req
>
> Well, you could add this 'fast' path to the cpuidle code (before calling
> into the actual governors) too. Also, since the governors actually use
> this state it makes sense for it to be available.

Actually the governors do not use this state or do whatever they can to
prevent to use it. The ladder governor just ignore it, and the menu
governor will default to C1 (not poll) if no state is found. It will use
the poll loop only if a timer is about to expire within 5us and all this
dance is around this micro optimization.

You are suggesting to add the fast in the cpuidle framework. IIUC we
should have:

int cpuidle_select(drv, dev)
{
...
if (latency_req == 0)
return ????;
}

The '????' suggests we duplicate everywhere an 0th idle state (more than
17 drivers and that breaks the idle states DT binding.

>> 3. what is the meaning of having a zero latency (target + exit) idle state ?
>> We know it will always succeed if the other fails
>
> Not quite sure I follow; you seem to have answered your own question?

Yeah, right :)

>> 4. IIUC, you are suggesting to add the poll loop for all the cpuidle
>> drivers. This is a *lot* of changes, I am not afraid about the work to do
>> but there is a significant code impact and the entire behavior of the
>> cpuidle framework for all the arch will be changed.
>
> I'm not sure it would be a lot of work, add it in the common cpuidle
> code before calling the driver init?

>> So given the points above, why not have one poll function, generic, and if
>> we fail to find an idle state or if the req is zero, then fallback to the
>> poll loop ?
>
> I could agree but only if we keep the poll loop generic, we cannot go
> add yet more arch hooks there.

I understand. The arch hook just give an opportunity to specify an
arch's specific poll loop, that does not imply it has to.

>>> Because the latter is actually arch specific, whereas the idle
>>> polling thing is not. You can _always_ poll idle, its generic, its
>>> valid, and its guaranteed the most responsive method.
>>
>> I agree with this point but this kind of loop is hardware optimized for x86.
>
> well 'optimized' is a strong word there, the rep nop, or pause
> instruction isn't really much at all and is mostly a SMT hint afaik.
> ia64, sparc64 and ppc64 have similar magic and s390 and hexagon use a vm
> yield like construct.

>> On the other arch, where today this loop is never used, if we change the
>> behavior of the cpuidle framework and introduces this loop, it may be
>> selected and stay on this state for a long time (resulting from a bad
>> prediction), I am afraid that can lead to some thermal issues.
>
> Because of 1), right? Yes that's a problem.

Well, 1) won't help, but also for different reasons:

By removing the STATE_START macro and adding the poll as the 0th idle state:

ladder governor: it was never, ever, using the poll loop, just ignoring
the 0th idle state by using the STATE_START macro to stop demoting. That
won't be the case anymore, and we may fall in the poll loop

menu governor: it was almost never using the poll state. With the change
it will select the state. If the prediction is bad and the poll idle
loop is selected but the cpu stays idle longer (that could be several
seconds). That can happens with a bad predictions or task migration when
this one is doing a lot of IO.

On the ARM embedded systems where the poll loop is not used, except when
specified in the kernel command line. That may lead to some thermal
issues and big troubles.

I like your patch below that will help to re-evaluate the idle state but
I am not sure it will solve the issue introduced with the generic idle
loop as the 0th idle state.

Thanks Peter by taking the time to review this patch.

-- Daniel

> ---
> kernel/sched/idle.c | 6 +++++-
> kernel/softirq.c | 7 +++++++
> 2 files changed, 12 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
> index c47fce75e666..9c76ae92650f 100644
> --- a/kernel/sched/idle.c
> +++ b/kernel/sched/idle.c
> @@ -42,12 +42,16 @@ static int __init cpu_idle_nopoll_setup(char *__unused)
> __setup("hlt", cpu_idle_nopoll_setup);
> #endif
>
> +DEFINE_PER_CPU(unsigned int, int_seq);
> +
> static inline int cpu_idle_poll(void)
> {
> + unsigned int seq = this_cpu_read(int_seq);
> +
> rcu_idle_enter();
> trace_cpu_idle_rcuidle(0, smp_processor_id());
> local_irq_enable();
> - while (!tif_need_resched())
> + while (!tif_need_resched() && seq == this_cpu_read(int_seq))
> cpu_relax();
> trace_cpu_idle_rcuidle(PWR_EVENT_EXIT, smp_processor_id());
> rcu_idle_exit();
> diff --git a/kernel/softirq.c b/kernel/softirq.c
> index 0699add19164..bc8d43545964 100644
> --- a/kernel/softirq.c
> +++ b/kernel/softirq.c
> @@ -370,6 +370,8 @@ static inline void tick_irq_exit(void)
> #endif
> }
>
> +DECLARE_PER_CPU(unsigned int, int_seq);
> +
> /*
> * Exit an interrupt context. Process softirqs if needed and possible:
> */
> @@ -386,6 +388,11 @@ void irq_exit(void)
> if (!in_interrupt() && local_softirq_pending())
> invoke_softirq();
>
> +#ifdef TIG_POLLING_NRFLAG
> + if (test_thread_flag(TIF_POLLING_NRFLAG))
> +#endif
> + this_cpu_inc(int_seq);
> +
> tick_irq_exit();
> rcu_irq_exit();
> trace_hardirq_exit(); /* must be last! */
>

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