Re: [PATCHv7 01/11] clockevents: Prefer CPU local devices over globaldevices

From: Daniel Lezcano
Date: Thu Jun 13 2013 - 05:33:43 EST

On 06/12/2013 11:44 PM, Stephen Boyd wrote:
> On 06/06, Stephen Boyd wrote:
>> On 06/07, Daniel Lezcano wrote:
>>> On 06/06/2013 08:04 PM, Stephen Boyd wrote:
>>>> On 06/06, Daniel Lezcano wrote:
>>>>> On 06/03/2013 10:33 PM, Stephen Boyd wrote:
>>>>>> On an SMP system with only one global clockevent and a dummy
>>>>>> clockevent per CPU we run into problems. We want the dummy
>>>>>> clockevents to be registered as the per CPU tick devices, but
>>>>>> we can only achieve that if we register the dummy clockevents
>>>>>> before the global clockevent or if we artificially inflate the
>>>>>> rating of the dummy clockevents to be higher than the rating
>>>>>> of the global clockevent. Failure to do so leads to boot
>>>>>> hangs when the dummy timers are registered on all other CPUs
>>>>>> besides the CPU that accepted the global clockevent as its tick
>>>>>> device and there is no broadcast timer to poke the dummy
>>>>>> devices.
>>>>>> If we're registering multiple clockevents and one clockevent is
>>>>>> global and the other is local to a particular CPU we should
>>>>>> choose to use the local clockevent regardless of the rating of
>>>>>> the device. This way, if the clockevent is a dummy it will take
>>>>>> the tick device duty as long as there isn't a higher rated tick
>>>>>> device and any global clockevent will be bumped out into
>>>>>> broadcast mode, fixing the problem described above.
>>>>> It is not clear the connection between the changelog, the patch and the
>>>>> comment. Could you clarify a bit ?
>>>> There is one tick device per-cpu and one broadcast device. The
>>>> broadcast device can only be a global clockevent, whereas the
>>>> per-cpu tick device can be a global clockevent or a per-cpu
>>>> clockevent. The code tries hard to keep per-cpu clockevents in
>>>> the tick device slots but it has an ordering/rating requirement
>>>> that doesn't work when there are only dummy per-cpu devices and
>>>> one global device.
>>>> Perhaps an example will help. Let's say you only have one global
>>>> clockevent such as the sp804, and you have SMP enabled. To
>>>> support SMP we have to register dummy clockevents on each CPU so
>>>> that the sp804 can go into broadcast mode. If we don't do this,
>>>> only the CPU that registered the sp804 will get interrupts while
>>>> the other CPUs will be left with no tick device and thus no
>>>> scheduling. To fix this we register dummy clockevents on all the
>>>> CPUs _before_ we register the sp804 to force the sp804 into the
>>>> broadcast slot. Or we give the dummy clockevents a higher rating
>>>> than the sp804 so that when we register them after the sp804 the
>>>> sp804 is bumped out to broadcast duty.
>>>> If the dummy devices are registered before the sp804 we can give
>>>> the dummies a low rating and the sp804 will still go into the
>>>> broadcast slot due to this code:
>>>> /*
>>>> * If we have a cpu local device already, do not replace it
>>>> * by a non cpu local device
>>>> */
>>>> if (curdev && cpumask_equal(curdev->cpumask, cpumask_of(cpu)))
>>>> goto out_bc;
>>>> If we register the sp804 before the dummies we're also fine as
>>>> long as the rating of the dummy is more than the sp804. Playing
>>>> games with the dummy rating is not very nice so this patch fixes
>>>> it by allowing the per-cpu device to replace the global device no
>>>> matter what the rating of the global device is.
>>>> This fixes the sp804 case when the dummy is rated lower than
>>>> sp804 and it removes any ordering requirement from the
>>>> registration of clockevents. It also completes the logic above
>>>> where we prefer cpu local devices over non cpu local devices.
>>> Thanks for the detailed explanation.
>>> Did Thomas reacted to this patch ?
>> So far there has been no response from Thomas.
> Will you ack this patch anyway? Or do we need Thomas to review
> this patch? It seems that this patch series has stalled again.

I prefer Thomas to have a look at it and ack it. I changed Cc to To for

-- Daniel

<> â Open source software for ARM SoCs

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