Re: CPUFREQ-CPUHOTPLUG: Possible circular locking dependency

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]


On Thu, Nov 30, 2006 at 12:03:15PM +0100, Ingo Molnar wrote:
> * Gautham R Shenoy <[email protected]> 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 <fn> once all CPUs have been
>    'gathered' - and release all CPUs once <fn> 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);

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

> 	Ingo

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!"
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to [email protected]
More majordomo info at
Please read the FAQ at

[Index of Archives]     [Kernel Newbies]     [Netfilter]     [Bugtraq]     [Photo]     [Stuff]     [Gimp]     [Yosemite News]     [MIPS Linux]     [ARM Linux]     [Linux Security]     [Linux RAID]     [Video 4 Linux]     [Linux for the blind]     [Linux Resources]
  Powered by Linux