Messages in this thread | | | Date | Mon, 8 Jun 2009 09:57:30 +0800 | From | Feng Tang <> | Subject | Re: [PATCH] tick: add check for the existence of broadcast clock event device |
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On Sun, 7 Jun 2009 00:18:55 +0800 Thomas Gleixner <tglx@linutronix.de> wrote:
> On Sat, 6 Jun 2009, Thomas Gleixner wrote: > > Feng, > > > > On Sat, 6 Jun 2009, Feng Tang wrote: > > > > If your percpu devices are always on (not affected by C3 stop) > > > > then you never dereference bc. So why do we need an extra check > > > > for !bc ? > > > > > > Hi tglx, > > > > > Thanks for the explanation. But we really ran into the NULL > > > pointer case, in our platform, there are 2 X86 CPUs which have > > > lapic, also it has 2 external timers which are pretty similar > > > with HPET timers, those 2 external timers will be used as per-cpu > > > timers (higher rating than lapic timer). In system's power cycle > > > of suspend and resume, disable_nontboot_cpus will be called > > > before goto suspend state,and enable_nonboot_cpus will be called > > > for the resume process, so lapic timer of cpu1 will be first > > > registered as per-cpu timer, and our external timer will be > > > registered later after get a CPU_ONLINE notifier (similar with > > > HPET), right in this time slot that lapic is the per-cpu timer, > > > when system get the CLOCK_EVT_BROADCAST_ENTER/EXIT msg, > > > tick_do_broadcast_on_off() is called and hit the NULL pointer > > > case. > > > > Ok, I can understand now why we need it. I'll apply your patch and > > add some more info into the commit msg so we do not look at it in a > > year and scratch our heads. :) > > Hmm, thought more about it. > > 1) How do you calibrate the local APIC timer if you do not have some > initial timer device ? Yes, we have external timer device with the name "apbt" > > 2) If you have some initial timer device (PIT/HPET) why isn't it > registered as broadcast device. We only have 2 available apbt, apbt0 for cpu0 and apbt1 for cpu1. And based on my understanding, there is no explicit API to register a timer as bc, and bc is setup during clockevents_register_device() --> tick_check_new_device() -->tick_check_broadcast_device(), and HPET case can just meet the condition: lapic's rating is 110, timer "hpet"is 50 and set to be the broadcast device, and other hpet(2/3/4/5..)'s are 150 and set to be the per-cpu timer
> > 3) When the CPU uses the local APIC before the external timer is > initialized what happens if the system goes into a deeper c-state ? Good point, for cpu1, this is not an issue, as cpu0 will register apbt1 for cpu1. there is a window cpu1 is in deep c-idle state and no timer wake it up, but window is small as cpu0 will send IPI soon. > > Thanks, > > tglx
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