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

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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
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.


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()    |                          |
|                   |                          |
+-------------------+--------------------------+

--
Dmitry
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