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Message-ID: <51B991EF.70800@linaro.org> Date: Thu, 13 Jun 2013 11:33:35 +0200 From: Daniel Lezcano <daniel.lezcano@...aro.org> To: Stephen Boyd <sboyd@...eaurora.org>, Thomas Gleixner <tglx@...utronix.de> CC: linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org, linux-arm-msm@...r.kernel.org, John Stultz <john.stultz@...aro.org> Subject: Re: [PATCHv7 01/11] clockevents: Prefer CPU local devices over global devices 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 Thomas. Thanks -- Daniel -- <http://www.linaro.org/> Linaro.org │ Open source software for ARM SoCs Follow Linaro: <http://www.facebook.com/pages/Linaro> Facebook | <http://twitter.com/#!/linaroorg> Twitter | <http://www.linaro.org/linaro-blog/> Blog -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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