Re: [PATCH V4 2/3] tick/cpuidle: Initialize hrtimer mode of broadcast

[Daniel Lezcano <daniel.lezcano@linaro.org>]

On 02/11/2014 05:09 PM, Preeti U Murthy wrote:
> Hi Daniel,
>
> Thank you very much for the review.
>
> On 02/11/2014 03:46 PM, Daniel Lezcano wrote:
>> On 02/07/2014 09:06 AM, Preeti U Murthy wrote:
>>> From: Thomas Gleixner <tglx@linutronix.de>
>>>
>>> On some architectures, in certain CPU deep idle states the local
>>> timers stop.
>>> An external clock device is used to wakeup these CPUs. The kernel
>>> support for the
>>> wakeup of these CPUs is provided by the tick broadcast framework by
>>> using the
>>> external clock device as the wakeup source.
>>>
>>> However not all implementations of architectures provide such an external
>>> clock device. This patch includes support in the broadcast framework
>>> to handle
>>> the wakeup of the CPUs in deep idle states on such systems by queuing
>>> a hrtimer
>>> on one of the CPUs, which is meant to handle the wakeup of CPUs in
>>> deep idle states.
>>>
>>> This patchset introduces a pseudo clock device which can be registered
>>> by the
>>> archs as tick_broadcast_device in the absence of a real external clock
>>> device. Once registered, the broadcast framework will work as is for
>>> these
>>> architectures as long as the archs take care of the BROADCAST_ENTER
>>> notification failing for one of the CPUs. This CPU is made the stand
>>> by CPU to
>>> handle wakeup of the CPUs in deep idle and it *must not enter deep
>>> idle states*.
>>>
>>> The CPU with the earliest wakeup is chosen to be this CPU. Hence this
>>> way the
>>> stand by CPU dynamically moves around and so does the hrtimer which is
>>> queued
>>> to trigger at the next earliest wakeup time. This is consistent with
>>> the case where
>>> an external clock device is present. The smp affinity of this clock
>>> device is
>>> set to the CPU with the earliest wakeup.
>>
>> Hi Preeti,
>>
>> jumping a bit late in the thread...
>>
>> Setting the smp affinity on the earliest timer should be handled
>> automatically with the CLOCK_EVT_FEAT_DYNIRQ flag. Did you look at using
>> this flag ?
>
> This patch is not setting the smp affinity of the pseudo clock device at
> all. Its not required to for the reason that it does not exist.
>
> I mentioned this point because we assign a CPU with the earliest wakeup
> as standby. I compared this logic to the one used by the tick broadcast
> framework for archs which have an external clock device to set the smp
> affinity of the device.
>
> If these archs do not have the flag CLOCK_EVT_FEAT_DYNIRQ set for the
> external clock device, the tick broadcast framework sets the smp
> affinity of this device to the CPU with the earliest wakeup. We are
> using the same logic in this patchset as well to assign the stand by CPU.
>
>>
>> Another comment is the overall approach. We enter the cpuidle idle
>> framework with a specific state to go to and it is the tick framework
>> telling us we mustn't go to this state. IMO the logic is wrong, the
>> decision to not enter this state should be moved somewhere else.
>
> Its not the tick framework which tells us that we cannot enter deep idle
> state, its the *tick broadcast* framework specifically. The tick
> broadcast framework was introduced with the primary intention of
> handling wakeup of CPUs in deep idle states when the local timers become
> non-functional. Therefore there is a co-operation between this tick
> broadcast framework and cpuidle. This has always been the case.
>
> That is why just before cpus go into deep idle, they call into the
> broadcast framework. Till now it was assumed that the tick broadcast
> framework would find no problems with the cpus entering deep idle.
> Therefore cpuidle would simply assume that all is well and go ahead and
> enter deep idle state.
>    But today there is a scenario when there could be problems if all cpus
> enter deep idle states and the tick broadcast framework now notifies the
> cpuidle framework to hold back one cpu. This is just a simple extension
> of the current interaction between cpuidle and tick broadcast framework.
>
>>
>> Why don't you create a cpuidle driver with the shallow idle states
>> assigned to a cpu (let's say cpu0) and another one with all the deeper
>> idle states for the rest of the cpus ? Using the multiple cpuidle driver
>> support makes it possible. The timer won't be moving around and a cpu
>> will be dedicated to act as the broadcast timer.
>>
>
> Having a dedicated stand by cpu for broadcasting has some issues which
> were pointed to when I posted the initial versions of this patchset.
> https://lkml.org/lkml/2013/7/27/14
>
> 1. This could create power/thermal imbalance on the chip since only the
> standby cpu cannot enter deep idle state at all times.
>
> 2. If it is cpu0 it is fine, else with the logic that you suggest,
> hot-plugging out the dedicated stand by cpu would mean moving the work
> of broadcasting to another cpu and modifying the cpuidle state table for
> it. Even with cpu0, if support to hotplug it out is enabled (maybe it is
> already), we will face the same issue and this gets very messy.
>
>> Wouldn't make sense and be less intrusive than the patchset you proposed ?
>
> Actually this patchset brings in a solution that is as less intrusive as
> possible. It makes the problem nearly invisible except for a failed
> return from a call into the broadcast framework. It simply asks the
> archs which do not have an external clock device to register a pseudo
> device with the broadcast framework and from then on everything just
> falls in place to enable deep idle states for such archs.

Hi Preeti,

thanks for the detailed explanation. I understand better now why you did 
it this way. Furthermore, as Thomas pointed me, what I missed is one cpu 
can't setup a local timer for another cpu, so what I was talking about 
does not make sense.

   -- Daniel


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