Re: [PATCH] soc/tegra: pmc: Fix "scheduling while atomic"

[Jon Hunter <jonathanh@nvidia.com>]

On 26/05/16 15:57, Dmitry Osipenko wrote:
> On 26.05.2016 17:32, Jon Hunter wrote:
>>
>> On 26/05/16 12:42, Dmitry Osipenko wrote:
>>> On 26.05.2016 11:42, Jon Hunter wrote:
>>>>
>>>> On 25/05/16 19:51, Dmitry Osipenko wrote:
>>>>> On 25.05.2016 18:09, Jon Hunter wrote:
>>>>
>>>> ...
>>>>
>>>>>> If you are able to reproduce this on v3.18, then it would be good if
>>>>>> you
>>>>>> could trace the CCF calls around this WARNING to see what is causing
>>>>>> the
>>>>>> contention.
>>>>>
>>>>> I managed to reproduce it with some CCF "tracing".
>>>>> Full kmsg log is here: https://bpaste.net/show/d8ab7b7534b7
>>>>>
>>>>> Looks like CPU freq governor thread yields during clk_set_rate() and
>>>>> then CPU idle kicks in, taking the same mutex.
>>>>
>>>> On the surface that sounds odd to me, but without understanding the
>>>> details, I guess I don't know if this is a valid thing to be doing or
>>>> even how that actually works!
>>>>
>>>
>>> The reason of that happening should be that I'm using clk PRE/POST rate
>>> change notifiers in my DVFS driver that takes other mutexes and they
>>> could be locked, causing schedule. I haven't mentioned it before, sorry.
>>
>> OK, but I am not sure how these "other mutexes" would be relevant here
>> without any more details.
>>
>>> From drivers/clk/clk.c:
>>>
>>> static struct task_struct *prepare_owner;
>>>
>>> ...
>>>
>>> /***           locking             ***/
>>> static void clk_prepare_lock(void)
>>> {
>>>     if (!mutex_trylock(&prepare_lock)) {
>>>         if (prepare_owner == current) {
>>>             prepare_refcnt++;
>>>             return;
>>>         }
>>>         mutex_lock(&prepare_lock);
>>>     }
>>>
>>> You can see that it would lock the mutex if prepare_owner != current, in
>>> my case it's idle thread != interactive gov. thread.
>>
>> Right, but that would imply that someone else is actively doing
>> something with a clock. However, if we are entering LP2, then that
>> implies that all CPUs are idle and so I still don't understand the
>> scenario where this would be locked in that case. May be there is
>> something I am overlooking here?
>>
>>>>> However, cpufreq_interactive governor is android specific governor and
>>>>> isn't in upstream kernel yet. Quick googling shows that recent
>>>>> "upstreaming" patch uses same cpufreq_interactive_speedchange_task:
>>>>> https://lkml.org/lkml/2016/5/20/41
>>>>
>>>> Do you know if this version they are upstreaming could also yield
>>>> during
>>>> the clk_set_rate()?
>>>>
>>>
>>> I think it should be assumed that any clk_set_rate() potentially could.
>>> Please correct me if I'm wrong.
>>>
>>>>> I'm not aware of other possibility to reproduce this issue, it needs
>>>>> some CCF interaction from a separate task. So the current upstream
>>>>> kernel shouldn't be affected, I guess.
>>>>
>>>> What still does not make sense to me is why any frequency changes have
>>>> not completed before we attempt to enter the LP2 state?
>>>>
>>> Why not? I don't see any CPUIDLE <-> CPUFREQ interlocking. Do you think
>>> it could be harmful somehow?
>>
>> Like I said before, I still don't understand that scenario that is
>> causing this and without being able to fully understand it, I have no
>> idea what the exact problem we are trying to fix here is.
>>
> 
> That's how I see it:
> 
> +----------------------------------------------+
> |                    CPU 0                     |
> +-------------------+--------------------------+
> |    Idle thread    | Interactive gov. thread  |
> +----------------------------------------------+
> |     inactive      |                          |
> |                   |                          |
> |                   |   CPU freq. change       |
> |                   |                          |
> |                   |   clk_set_rate()         |
> |                   |                          |
> |       ...         |   clk_prepare_lock()     |
> |                   |                          |
> |                   |   PRE rate notifier call |
> |                   |                          |
> |                   |   schedule               |

What is this notifier doing? Is there some sort of hardware activity
that it is waiting for to complete?

Cheers
Jon

-- 
nvpublic