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

From: Dmitry Osipenko
Date: Thu May 26 2016 - 13:01:16 EST

On 26.05.2016 18:27, Jon Hunter wrote:

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
could trace the CCF calls around this WARNING to see what is causing

I managed to reproduce it with some CCF "tracing".
Full kmsg log is here:

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) {

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:

Do you know if this version they are upstreaming could also yield
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?

It changes regulator voltage if required. So at least I2C would cause scheduling on wait_for_completion_timeout().