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

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Date:	Thu, 26 May 2016 14:42:33 +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 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.

 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.

>> 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?

> OK, well may be we will hold off on this change for the moment.

-- 
Dmitry