Re: Enable arm_global_timer for Zynq brakes boot

From: Daniel Lezcano
Date: Fri Aug 09 2013 - 10:19:11 EST

On 08/09/2013 12:32 PM, Srinivas KANDAGATLA wrote:
> On 08/08/13 18:11, SÃren Brinkmann wrote:
>> Hi Daniel,
>> On Thu, Aug 01, 2013 at 07:48:04PM +0200, Daniel Lezcano wrote:
>>> On 08/01/2013 07:43 PM, SÃren Brinkmann wrote:
>>>> On Thu, Aug 01, 2013 at 07:29:12PM +0200, Daniel Lezcano wrote:
>>>>> On 08/01/2013 01:38 AM, SÃren Brinkmann wrote:
>>>>>> On Thu, Aug 01, 2013 at 01:01:27AM +0200, Daniel Lezcano wrote:
>>>>>>> On 08/01/2013 12:18 AM, SÃren Brinkmann wrote:
>>>>>>>> On Wed, Jul 31, 2013 at 11:08:51PM +0200, Daniel Lezcano wrote:
>>>>>>>>> On 07/31/2013 10:58 PM, SÃren Brinkmann wrote:
>>>>>>>>>> On Wed, Jul 31, 2013 at 10:49:06PM +0200, Daniel Lezcano wrote:
>>>>>>>>>>> On 07/31/2013 12:34 AM, SÃren Brinkmann wrote:
>>>>>>>>>>>> On Tue, Jul 30, 2013 at 10:47:15AM +0200, Daniel Lezcano wrote:
>>>>>>>>>>>>> On 07/30/2013 02:03 AM, SÃren Brinkmann wrote:
>>>>>>>>>>>>>> Hi Daniel,
>>>>>>>>>>>>>> On Mon, Jul 29, 2013 at 02:51:49PM +0200, Daniel Lezcano wrote:
>>>>>>>>>>>>>> (snip)
>>>>>>>>>>>>>>> the CPUIDLE_FLAG_TIMER_STOP flag tells the cpuidle framework the local
>>>>>>>>>>>>>>> timer will be stopped when entering to the idle state. In this case, the
>>>>>>>>>>>>>>> cpuidle framework will call clockevents_notify(ENTER) and switches to a
>>>>>>>>>>>>>>> broadcast timer and will call clockevents_notify(EXIT) when exiting the
>>>>>>>>>>>>>>> idle state, switching the local timer back in use.
>>>>>>>>>>>>>> I've been thinking about this, trying to understand how this makes my
>>>>>>>>>>>>>> boot attempts on Zynq hang. IIUC, the wrongly provided TIMER_STOP flag
>>>>>>>>>>>>>> would make the timer core switch to a broadcast device even though it
>>>>>>>>>>>>>> wouldn't be necessary. But shouldn't it still work? It sounds like we do
>>>>>>>>>>>>>> something useless, but nothing wrong in a sense that it should result in
>>>>>>>>>>>>>> breakage. I guess I'm missing something obvious. This timer system will
>>>>>>>>>>>>>> always remain a mystery to me.
>>>>>>>>>>>>>> Actually this more or less leads to the question: What is this
>>>>>>>>>>>>>> 'broadcast timer'. I guess that is some clockevent device which is
>>>>>>>>>>>>>> common to all cores? (that would be the cadence_ttc for Zynq). Is the
>>>>>>>>>>>>>> hang pointing to some issue with that driver?
>>>>>>>>>>>>> If you look at the /proc/timer_list, which timer is used for broadcasting ?
>>>>>>>>>>>> So, the correct run results (full output attached).
>>>>>>>>>>>> The vanilla kernel uses the twd timers as local timers and the TTC as
>>>>>>>>>>>> broadcast device:
>>>>>>>>>>>> Tick Device: mode: 1
>>>>>>>>>>>> Broadcast device
>>>>>>>>>>>> Clock Event Device: ttc_clockevent
>>>>>>>>>>>> When I remove the offending CPUIDLE flag and add the DT fragment to
>>>>>>>>>>>> enable the global timer, the twd timers are still used as local timers
>>>>>>>>>>>> and the broadcast device is the global timer:
>>>>>>>>>>>> Tick Device: mode: 1
>>>>>>>>>>>> Broadcast device
>>>>>>>>>>>> Clock Event Device: arm_global_timer
>>>>>>>>>>>> Again, since boot hangs in the actually broken case, I don't see way to
>>>>>>>>>>>> obtain this information for that case.
>>>>>>>>>>> Can't you use the maxcpus=1 option to ensure the system to boot up ?
>>>>>>>>>> Right, that works. I forgot about that option after you mentioned, that
>>>>>>>>>> it is most likely not that useful.
>>>>>>>>>> Anyway, this are those sysfs files with an unmodified cpuidle driver and
>>>>>>>>>> the gt enabled and having maxcpus=1 set.
>>>>>>>>>> /proc/timer_list:
>>>>>>>>>> Tick Device: mode: 1
>>>>>>>>>> Broadcast device
>>>>>>>>>> Clock Event Device: arm_global_timer
>>>>>>>>>> max_delta_ns: 12884902005
>>>>>>>>>> min_delta_ns: 1000
>>>>>>>>>> mult: 715827876
>>>>>>>>>> shift: 31
>>>>>>>>>> mode: 3
>>>>>>>>> Here the mode is 3 (CLOCK_EVT_MODE_ONESHOT)
>>>>>>>>> The previous timer_list output you gave me when removing the offending
>>>>>>>>> cpuidle flag, it was 1 (CLOCK_EVT_MODE_SHUTDOWN).
>>>>>>>>> Is it possible you try to get this output again right after onlining the
>>>>>>>>> cpu1 in order to check if the broadcast device switches to SHUTDOWN ?
>>>>>>>> How do I do that? I tried to online CPU1 after booting with maxcpus=1
>>>>>>>> and that didn't end well:
>>>>>>>> # echo 1 > online && cat /proc/timer_list
>>>>>>> Hmm, I was hoping to have a small delay before the kernel hangs but
>>>>>>> apparently this is not the case... :(
>>>>>>> I suspect the global timer is shutdown at one moment but I don't
>>>>>>> understand why and when.
>>>>>>> Can you add a stack trace in the "clockevents_shutdown" function with
>>>>>>> the clockevent device name ? Perhaps, we may see at boot time an
>>>>>>> interesting trace when it hangs.
>>>>>> I did this change:
>>>>>> diff --git a/kernel/time/clockevents.c b/kernel/time/clockevents.c
>>>>>> index 38959c8..3ab11c1 100644
>>>>>> --- a/kernel/time/clockevents.c
>>>>>> +++ b/kernel/time/clockevents.c
>>>>>> @@ -92,6 +92,8 @@ void clockevents_set_mode(struct clock_event_device *dev,
>>>>>> */
>>>>>> void clockevents_shutdown(struct clock_event_device *dev)
>>>>>> {
>>>>>> + pr_info("ce->name:%s\n", dev->name);
>>>>>> + dump_stack();
>>>>>> clockevents_set_mode(dev, CLOCK_EVT_MODE_SHUTDOWN);
>>>>>> dev->next_event.tv64 = KTIME_MAX;
>>>>>> }
>>>>>> It is hit a few times during boot, so I attach a full boot log. I really
>>>>>> don't know what to look for, but I hope you can spot something in it. I
>>>>>> really appreciate you taking the time.
>>>>> Thanks for the traces.
>>>> Sure.
>>>>> If you try without the ttc_clockevent configured in the kernel (but with
>>>>> twd and gt), does it boot ?
>>>> Absence of the TTC doesn't seem to make any difference. It hangs at the
>>>> same location.
>>> Ok, IMO there is a problem with the broadcast device registration (may
>>> be vs twd).
>> I have an idea, but no real evidence to prove it:
>> Some of the registers in the arm_global_timer are banked per CPU. I.e.
>> some code must be executed on the CPU the timer is associated with
>> (struct clock_event_device.cpumask) to have the intended effect
>> As far as I can tell, there is no guarantee, that the set_mode()
>> and program_next_event() calls execute on the correct CPU.
>> If this was correct, shutting down the timer for the CPU entering
>> idle might actually shut down the timer for the running CPU, if
>> set_mode() executes on the CPU which is _not_ about to enter idle.
> Hi SÃren,
> Am able to reproduce similar issue on StiH415 SOC by enabling both
> global_timer and twd and using cpuidle driver like zynq.
> When CPU0 goes to idle, I noticed that the global timer used for
> boardcast is actually scheduled on wrong cpu.
> My traces for printk like this
> printk("DEBUG: %s on CPU:%d CPUMASK:%s\n", __FUNCTION__,
> smp_processor_id(), scpumask);
> shows:
> DEBUG: gt_clockevent_set_mode on CPU:1 CPUMASK: 0
> DEBUG: gt_clockevent_set_next_event on CPU:1 CPUMASK:0
> Which indicates that setting the mode and next_event for a clkevent with
> cpumask 0 is scheduled on cpu1, this will generate an global timer
> interrupt on cpu1 rather than cpu0.
> This might be the reason for cpu0 not coming out of the cpu_idle_loop.

yes, but at least the broadcast mechanism should send an IPI to cpu0 to
wake it up, no ? As Stephen stated this kind of configuration should has
never been tested before so the tick broadcast code is not handling this
case properly IMHO.

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