Re: [PATCH v2 5/5] cpufreq: Catch double invocations of cpufreq_freq_transition_begin/end

["Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>]

On 04/29/2014 12:19 PM, Viresh Kumar wrote:
> On 29 April 2014 11:46, Srivatsa S. Bhat
> <srivatsa.bhat@linux.vnet.ibm.com> wrote:
>> Yes, I'm aware that this corner case doesn't work well with my debug
> 
> Don't know if its a corner case, it may be the most obvious case for
> some :)
>

Yeah, it could be.
 
>> patch. I tried to avoid this but couldn't think of any solution.
> 
> The problem is not that it wouldn't work for these systems, but we will
> get WARN_ON() when they shouldn't have come :)
> 

Yes, I thought about this, and I agree that this is not acceptable.

>> (One big-hammer way to avoid this is to exclude this infrastructure
>> for all ASYNC_NOTIFICATION drivers, but I didn't want to go with that
>> approach, since it makes it look ugly). Do you have any better ideas
>> to deal with this scenario?
> 
> Can't think of anything better than this:
> 
> +       WARN_ON(!(cpufreq_driver->flags & CPUFREQ_ASYNC_NOTIFICATION)
>                              && (current == policy->transition_task));
> 
> which you already mentioned.

Yeah, I think we should just go with this. I thought we needed lots of
if-conditions to do exclude these drivers (which would have made it ugly),
but as you pointed above, just this one would suffice.

Besides, the cpufreq core doesn't automatically invoke _begin() and 
_end() for ASYNC_NOTIFICATION drivers. So that means the probability
that such drivers will hit this problem is extremely low, since the
driver alone is responsible for invoking _begin/_end and hence there
shouldn't be much of a conflict. So I think we should really just
skip ASYNC_NOTIFICATION drivers in this debug infrastructure.

> 
>> Also, do we really have cases in mind where a single thread does
>> multiple frequency transitions in one go? That too in such quick
>> successions? Echo's to sysfs, changing of governors from userspace etc
>> all do one frequency transition at a time per-task...
> 
> Its not really about if we can think of a real use case or not. The point
> is, governor is free to call transition calls one after the other (will always
> happen from a single thread) and it isn't supposed to wait for drivers
> to finish earlier transitions as ->target() has already returned.
> 

Yes, I agree now. Making bold assumptions in the cpufreq core about
how many frequency transitions a single task will do etc is potentially
*very* dangerous. Let's not do it that way.

I'll send a v2 excluding the ASYNC_NOTIFICATION drivers.
Thanks a lot for your inputs!

Regards,
Srivatsa S. Bhat