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

From: Dmitry Osipenko
Date: Thu May 26 2016 - 10:58:00 EST

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 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 |
| | |
| irqs_disable() | |
| | |
| enter CPU idle | |
| | |
| clk_get_rate(pclk)| |
| | |
| clk_prepare_lock()| |
| | |
| schedule bug() | |
| | |