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

[Dmitry Osipenko]

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