Subject: Enable arm_global_timer for Zynq brakes boot

From: [email-masked] (Daniel Lezcano)
Date: Thu, 01 Aug 2013 01:01:27 +0200

On 08/01/2013 12:18 AM, S?ren Brinkmann wrote:
> On Wed, Jul 31, 2013 at 11:08:51PM +0200, Daniel Lezcano wrote:
>> On 07/31/2013 10:58 PM, S?ren Brinkmann wrote:
>>> On Wed, Jul 31, 2013 at 10:49:06PM +0200, Daniel Lezcano wrote:
>>>> On 07/31/2013 12:34 AM, S?ren Brinkmann wrote:
>>>>> On Tue, Jul 30, 2013 at 10:47:15AM +0200, Daniel Lezcano wrote:
>>>>>> On 07/30/2013 02:03 AM, S?ren 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.

> 	[ 4689.992658] CPU1: Booted secondary processor
> 	[ 4690.986295] CPU1: failed to come online
> 	sh: write error: Input/output error
> 	# [ 4691.045945] CPU1: thread -1, cpu 1, socket 0, mpidr 80000001
> 	[ 4691.045986] 
> 	[ 4691.052972] ===============================
> 	[ 4691.057349] [ INFO: suspicious RCU usage. ]
> 	[ 4691.061413] 3.11.0-rc3-00001-gc14f576-dirty #139 Not tainted
> 	[ 4691.067026] -------------------------------
> 	[ 4691.071129] kernel/sched/fair.c:5477 suspicious rcu_dereference_check() usage!
> 	[ 4691.078292] 
> 	[ 4691.078292] other info that might help us debug this:
> 	[ 4691.078292] 
> 	[ 4691.086209] 
> 	[ 4691.086209] RCU used illegally from offline CPU!
> 	[ 4691.086209] rcu_scheduler_active = 1, debug_locks = 0
> 	[ 4691.097216] 1 lock held by swapper/1/0:
> 	[ 4691.100968]  #0:  (rcu_read_lock){.+.+..}, at: [<c00679b4>] set_cpu_sd_state_idle+0x0/0x1e4
> 	[ 4691.109250] 
> 	[ 4691.109250] stack backtrace:
> 	[ 4691.113531] CPU: 1 PID: 0 Comm: swapper/1 Not tainted 3.11.0-rc3-00001-gc14f576-dirty #139
> 	[ 4691.121755] [<c0016a88>] (unwind_backtrace+0x0/0x128) from [<c0012d58>] (show_stack+0x20/0x24)
> 	[ 4691.130263] [<c0012d58>] (show_stack+0x20/0x24) from [<c045bd50>] (dump_stack+0x80/0xc4)
> 	[ 4691.138264] [<c045bd50>] (dump_stack+0x80/0xc4) from [<c007ad78>] (lockdep_rcu_suspicious+0xdc/0x118)
> 	[ 4691.147371] [<c007ad78>] (lockdep_rcu_suspicious+0xdc/0x118) from [<c0067ac0>] (set_cpu_sd_state_idle+0x10c/0x1e4)
> 	[ 4691.157605] [<c0067ac0>] (set_cpu_sd_state_idle+0x10c/0x1e4) from [<c0078238>] (tick_nohz_idle_enter+0x48/0x80)
> 	[ 4691.167583] [<c0078238>] (tick_nohz_idle_enter+0x48/0x80) from [<c006dc5c>] (cpu_startup_entry+0x28/0x388)
> 	[ 4691.177127] [<c006dc5c>] (cpu_startup_entry+0x28/0x388) from [<c0014acc>] (secondary_start_kernel+0x12c/0x144)
> 	[ 4691.187013] [<c0014acc>] (secondary_start_kernel+0x12c/0x144) from [<000081ec>] (0x81ec)
> 
> 
> 	S?ren
> 
> 

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