On 08/05/2014 03:46 AM, Viresh Kumar wrote:

On 2 August 2014 01:06, Stephen Boyd [off-list ref] wrote:

quoted

I have the same options. The difference is that my driver has a governor
per policy. That's set with the CPUFREQ_HAVE_GOVERNOR_PER_POLICY flag.

You may call me stupid but I got a bit confused after looking into the code
again. Why does the crash dump depends on this flag?

Nope, not a stupid question.  After reproducing (finally!) yesterday I've been
wondering the same thing.

We *always* remove the governor specific directory while switching governors
(Ofcourse only if its updated for All CPUs). And so on a dual core platform,
where both CPU 0 & 1 share a clock line, switching of governors should result
in this crash dump?

I've been looking into *exactly* this.  On any platform where
cpu_weight(affected_cpus) == 1 for a particular cpu this lockdep trace should
happen.

I may know the answer to the stupid question I had, but not sure why that is a
problem. The only (and quite significant) difference that this flag makes
is the location of governor-specific directory:
- w/o this flag: /sys/devices/system/cpu/cpufreq/<here>
- w/ this flag: /sys/devices/system/cpu/cpu*/cpufreq/<here>

cpufreq_global_kobject vs the policy's kobject.

So, is there some issue with the sysfs lock for <cpu*/cpufreq/> node as while
switching governor we change  <cpu*/cpufreq/scaling_governor> at the same
location?

That's what I'm wondering too.  I'm going to instrument the code to find out
this morning.  I'm wondering if this comes down to a lockdep class issue
(perhaps lockdep puts globally defined locks like cpufreq_global_kobject in a
different class?).

P.