Re: CPUFREQ-CPUHOTPLUG: Possible circular locking dependency

Gautham R Shenoy
2006-11-30T11:43:42Z

On Thu, Nov 30, 2006 at 12:03:15PM +0100, Ingo Molnar wrote:
>
> * Gautham R Shenoy <ego@in.ibm.com> wrote:
>
> > a) cpufreq maintain's it's own cpumask in the variable
> > policy->affected_cpus and says : If a frequency change is issued to
> > any one of the cpu's in the affected_cpus mask, you change frequency
> > on all cpus in the mask. So this needs to be consistent with
> > cpu_online map and hence cpu hotplug aware. Furthermore, we don't want
> > cpus in this mask to go down when we are trying to change frequencies
> > on them. The function which drives the frequency change in
> > cpufreq-core is cpufreq_driver_target and it needs cpu-hotplug
> > protection.
>
> couldnt this complexity be radically simplified by having new kernel
> infrastructure that does something like:
>
> " 'gather' all CPUs mentioned in <mask> via scheduling a separate
> helper-kthread on every CPU that <mask> specifies, disable all
> interrupts, and execute function once all CPUs have been
> 'gathered' - and release all CPUs once has executed on each of
> them."
>
> ?

This is what is currently being done by cpufreq:

a) get_some_cpu_hotplug_protection() [use either some global mechanism
or a persubsystem mutex]

b) actual_freq_change_driver_function(mask)
/* You can check out cpufreq_p4_target() in
* arch/i386/kernel/cpu/cpufreq/p4-clockmod.c
*/

{
for_each_cpu_mask(i, mask) {
cpumask_t this_cpu = cpumask_of_cpu(i);
set_cpus_allowed(current, this_cpu);
function_to_change_frequency();

}
}

c) release_whatever_cpu_hotplug_protection()

>
> This would be done totally serialized and while holding the hotplug
> lock, so no CPU could go away or arrive while this operation is going
> on.
>

Isn't the above same as what you are suggesting? Or have I missed out
anything?

> Ingo

regards
gautham.
--
Gautham R Shenoy
Linux Technology Center
IBM India.
"Freedom comes with a price tag of responsibility, which is still a bargain,
because Freedom is priceless!"