Re: [PATCHv7 01/11] clockevents: Prefer CPU local devices overglobal devices
From: Stephen Boyd
Date: Thu Jun 06 2013 - 18:38:55 EST
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.
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