i.MX6, PREEMPT-RT and interactive cpufreq governor

[Andrey Smirnov]

Hello everyone,

I am working on integrating PREEMPT-RT patches into 3.10.17 kernel BSP
release from Freescale which can be found at:

http://git.freescale.com/git/cgit.cgi/imx/linux-2.6-imx.git (tag
imx_3.10.17_1.0.0_ga)

and what I am finding is that if I select "interactive" cpufreq
governor I get a kernel the kernel that occasionally spouts this

BUG: scheduling while atomic: swapper/3/0/0x00000002
Modules linked in:
Preemption disabled at:[<  (null)>]   (null)

CPU: 3 PID: 0 Comm: swapper/3 Not tainted 3.10.17-rt12-80705-g232293e-dirty #3
[<800139c4>] (unwind_backtrace+0x0/0xf8) from [<80011420>]
(show_stack+0x10/0x14)
[<80011420>] (show_stack+0x10/0x14) from [<805bfbbc>] (__schedule_bug+0x78/0x9c)
[<805bfbbc>] (__schedule_bug+0x78/0x9c) from [<805c431c>]
(__schedule+0x398/0x49c)
[<805c431c>] (__schedule+0x398/0x49c) from [<805c44d0>] (schedule+0x34/0xa0)
[<805c44d0>] (schedule+0x34/0xa0) from [<805c5250>]
(rt_spin_lock_slowlock+0xc0/0x258)
[<805c5250>] (rt_spin_lock_slowlock+0xc0/0x258) from [<80031d44>]
(lock_timer_base+0x2c/0x4c)
[<80031d44>] (lock_timer_base+0x2c/0x4c) from [<80032024>]
(mod_timer+0x60/0x1c0)
[<80032024>] (mod_timer+0x60/0x1c0) from [<803fe860>]
(cpufreq_interactive_idle_notifier+0xa4/0x13c)
[<803fe860>] (cpufreq_interactive_idle_notifier+0xa4/0x13c) from
[<80048444>] (notifier_call_chain+0)
[<80048444>] (notifier_call_chain+0x44/0x84) from [<80048754>]
(__atomic_notifier_call_chain+0x38/0x)
[<80048754>] (__atomic_notifier_call_chain+0x38/0x4c) from
[<80048780>] (atomic_notifier_call_chain+)
[<80048780>] (atomic_notifier_call_chain+0x18/0x20) from [<80058df8>]
(cpu_startup_entry+0x68/0x1a4)
[....] [<80058df8>] (cpu_startup_entry+0x68/0x1a4) from [<105bc204>]
(0x105bc204)


and eventually crashes. After doing some digging I believe the
sequence of events leading to this is following:

secondary_start_kernel() calls preempt_disable(), then
cpu_startup_entry(CPUHP_ONLINE), which results in a call to
cpufreq_interactive_idle_notifier() which in turn tries to use
mod_timer(). Mod_timer() internally tries to acquire a spinlock, but
with RT
patches applied it unfolds into a rt_mutex, the attempt of acquisition
of which results in the call to schedule() and that's when we
see that backtrace.

Eventually I am hoping to disable any sorts of frequency scaling or
power management on our system, but I am still curious to know if that
is a known issue patches for which exist. Does anyone has any leads/suggestions?

Thank you,
Andrey Smirnov
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