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

[Viresh Kumar]

On 27-11-19, 12:08, Sudeep Holla wrote:
> On Wed, Nov 27, 2019 at 12:48:01PM +0100, Dietmar Eggemann wrote:
> > Since commit ca74b316df96 ("arm: Use common cpu_topology structure and
> > functions.") the core cpumask has to be modified during cpu hotplug
> > operations.
> >
> > ("arm: Fix topology setup in case of CPU hotplug for CONFIG_SCHED_MC")
> > [1] fixed that but revealed another issue on TC2, i.e in its cpufreq
> > driver.
> >
> > During CPU hp stress operations on multiple CPUs, policy->related_cpus
> > can be altered. This is wrong since this cpumask should contain the
> > online and offline CPUs.
> >
> > The WARN_ON(!cpumask_test_cpu(cpu, policy->related_cpus)) in
> > cpufreq_online() triggers in this case.
> >
> > The core cpumask can't be used to set the policy->cpus in
> > ve_spc_cpufreq_init() anymore in case it is called via
> > cpuhp_cpufreq_online()->cpufreq_online()->cpufreq_driver->init().
> >
> > An empty online() callback can be used to avoid that the init()
> > driver function is called during CPU hotplug in so that
> > policy->related_cpus will not be changed.
> >
> 
> 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.

- Boot board with cpufreq driver as module.
- Offline all CPUs except CPU0.
- insert cpufreq driver.
- online all CPUs.

Now there is no guarantee that the last online will get the mask
properly, if I have understood the problem well :)

But yeah, who does this kind of messy work anyway :)

FWIW, we need a proper way (may be from architecture code) to find
list of all CPUs that share clock line.

-- 
viresh