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

[Jon Hunter <jonathanh@nvidia.com>]

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.

Cheers
Jon

-- 
nvpublic