Re: [PATCH] cpufreq: vexpress-spc: Fix wrong alternation of policy->related_cpus during CPU hp

[Dietmar Eggemann]

On 27/11/2019 14:32, Sudeep Holla wrote:
> On Wed, Nov 27, 2019 at 05:44:02PM +0530, Viresh Kumar wrote:
>> On 27-11-19, 12:08, Sudeep Holla wrote:
>>> On Wed, Nov 27, 2019 at 12:48:01PM +0100, Dietmar Eggemann wrote:

[...]

>>> Unlike DT based drivers, it not easy to get the fixed cpumask unless we
>>> add some mechanism to extract it based on clks/OPP added. I prefer
>>> this simple solution instead.
>>
>> I will call this a work-around for the problem and not really the
>> solution, though I won't necessarily oppose it. There are cases which
>> will break even with this solution.
>>
> 
> I agree and that's the reason I spoke out my thought aloud here :)
> 
>> - Boot board with cpufreq driver as module.
>> - Offline all CPUs except CPU0.
>> - insert cpufreq driver.
>> - online all CPUs.
>>
> 
> Indeed, not just boot anytime since it's a module :)
> 
>> Now there is no guarantee that the last online will get the mask
>> properly, if I have understood the problem well :)
>>
> 
> Yes
> 
>> But yeah, who does this kind of messy work anyway :)
>>
> 
> I won't bet on that ;)
> 
>> FWIW, we need a proper way (may be from architecture code) to find
>> list of all CPUs that share clock line.
>>
> 
> Yes but there's no architectural way. I need to revise and see tc2_pm.c
> to check if we can do any magic there.

I'm fine with finding a better solution to return a fixed topology core
cpumask or calling this patch a workaround. AFAICS, only TC2 is affected.

("arm: Fix topology setup in case of CPU hotplug for CONFIG_SCHED_MC")
is needed for other systems as well in case we have commit ca74b316df96
("arm: Use common cpu_topology structure and functions."). We probably
don't want to revert commit ca74b316df96?

We do CPU hp stress tests in our EAS mainline integration test suite
https://developer.arm.com/tools-and-software/open-source-software/linux-kernel/energy-aware-scheduling/eas-mainline-development
and there is where we initially encountered this issue on TC2.