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

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Date:	Fri, 27 May 2016 17:43:45 +0300
From:	Dmitry Osipenko <digetx@...il.com>
To:	Jon Hunter <jonathanh@...dia.com>
Cc:	Stephen Warren <swarren@...dotorg.org>,
	Thierry Reding <thierry.reding@...il.com>,
	Alexandre Courbot <gnurou@...il.com>,
	Peter De Schrijver <pdeschrijver@...dia.com>,
	Prashant Gaikwad <pgaikwad@...dia.com>,
	linux-clk@...r.kernel.org, linux-tegra@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] soc/tegra: pmc: Fix "scheduling while atomic"

On 27.05.2016 15:46, Jon Hunter wrote:
>
> On 26/05/16 18:01, Dmitry Osipenko wrote:
>> On 26.05.2016 18:27, Jon Hunter wrote:
>>> On 26/05/16 15:57, Dmitry Osipenko wrote:
>
> ...
>
>>>> 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?
>>>
>>
>> It changes regulator voltage if required. So at least I2C would cause
>> scheduling on wait_for_completion_timeout().
>
> Yes, of course that would make sense. What is interesting/odd in this
> case is that the frequency is increasing (voltage scaled pre frequency
> change) but yet you are entering LP2. May be that is possible? I guess
> this problem may also occur on reducing frequency as well?
>

Sorry, possible what? Surely it might happen on the POST notify, I just used PRE 
for example.

There are no active tasks while CPU changing the frequency, so kernel enters 
idle mode while waiting for the HW completion - the interrupt. I don't see 
anything wrong here.

> What are you using in the v3.18 kernel for exit_latency and
> target_residency? The current mainline has 5000us and 10000us,
> respectively.
>

Default stock latencies, not sure why you are asking. It's essentially a 
mainline kernel with some added device drivers and android patches.

> It does seem that this could be triggered in the right circumstances and
> I have to say I don't like the fact that this could be fragile as it is
> today. Have you thought about adding a post clock notifier for pclk in
> the PMC driver as an alternative to the change you are suggesting?
>

No, I haven't. Sounds like a good idea, thanks for the suggestion! I'll try it 
and send V2 if it will be okay, otherwise will report the problem.

-- 
Dmitry