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
