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SubjectRe: [PATCH V3 4/4] cpufreq: CPPC: Add support for frequency invariance
Hey,

On Tuesday 29 Jun 2021 at 10:02:44 (+0530), Viresh Kumar wrote:
> On 28-06-21, 11:49, Ionela Voinescu wrote:
> > To be honest I would like to have more time on this before you merge the
> > set, to better understand Qian's results and some observations I have
> > for Thunder X2 (I will share in a bit).
>
> Ideally, this code was already merged in 5.13 and would have required
> us to fix any problems as we encounter them. I did revert it because
> it caused a kernel crash and I wasn't sure if there was a sane/easy
> way of fixing that so late in the release cycle. That was the right
> thing to do then.
>
> All those issues are gone now, we may have an issue around rounding of
> counters or some hardware specific issues, it isn't clear yet.
>
> But the stuff works fine otherwise, doesn't make the kernel crash and
> it is controlled with a CONFIG_ option, so those who don't want to use
> it can still disable it.
>
> The merge window is here now, if we don't merge it now, it gets
> delayed by a full cycle (roughly two months) and if we merge it now
> and are able to narrow down the rounding issues, if there are any, we
> will have full two months to make a fix for that and still push it in
> 5.14 itself.
>
> And so I would like to get it merged in this merge window itself, it
> also makes sure more people would get to test it, like Qian was able
> to figure out a problem here for us.
>

Okay, makes sense. I have not seen this code actually do anything wrong
so far, and the issues I see on ThunderX2 point more to misbehaving
counters for this purpose. This being said, I would have probably
preferred for this feature to be disabled by default, until we've tested
more, but that won't give the chance to anyone else to test.

> > For the code, I think it's fine. I have a single observation regarding
> > the following code:
> >
> > > +static void cppc_cpufreq_cpu_fie_init(struct cpufreq_policy *policy)
> > > +{
> > > + struct cppc_freq_invariance *cppc_fi;
> > > + int cpu, ret;
> > > +
> > > + if (cppc_cpufreq_driver.get == hisi_cppc_cpufreq_get_rate)
> > > + return;
> > > +
> > > + for_each_cpu(cpu, policy->cpus) {
> > > + cppc_fi = &per_cpu(cppc_freq_inv, cpu);
> > > + cppc_fi->cpu = cpu;
> > > + cppc_fi->cpu_data = policy->driver_data;
> > > + kthread_init_work(&cppc_fi->work, cppc_scale_freq_workfn);
> > > + init_irq_work(&cppc_fi->irq_work, cppc_irq_work);
> > > +
> > > + ret = cppc_get_perf_ctrs(cpu, &cppc_fi->prev_perf_fb_ctrs);
> > > + if (ret) {
> > > + pr_warn("%s: failed to read perf counters for cpu:%d: %d\n",
> > > + __func__, cpu, ret);
> > > + return;
> > > + }
> >
> > For this condition above, think about a scenario where reading counters
> > for offline CPUs returns an error. I'm not sure if that can happen, to
> > be honest. That would mean here that you will never initialise the freq
> > source unless all CPUs in the policy are online at policy creation.
> >
> > My recommendation is to warn about the failed read of perf counters but
> > only return from this function if the target CPU is online as well when
> > reading counters fails.
> >
> > This is probably a nit, so I'll let you decide if you want to do something
> > about this.
>
> That is a very good observation actually. Thanks for that. This is how
> I fixed it.
>
> diff --git a/drivers/cpufreq/cppc_cpufreq.c b/drivers/cpufreq/cppc_cpufreq.c
> index d688877e8fbe..f6540068d0fe 100644
> --- a/drivers/cpufreq/cppc_cpufreq.c
> +++ b/drivers/cpufreq/cppc_cpufreq.c
> @@ -171,7 +171,13 @@ static void cppc_cpufreq_cpu_fie_init(struct cpufreq_policy *policy)
> if (ret) {
> pr_warn("%s: failed to read perf counters for cpu:%d: %d\n",
> __func__, cpu, ret);
> - return;
> +
> + /*
> + * Don't abort if the CPU was offline while the driver
> + * was getting registered.
> + */
> + if (cpu_online(cpu))
> + return;
> }
> }
>
> --

Thanks!

Reviewed-by: Ionela Voinescu <ionela.voinescu@arm.com>

Ionela.

> viresh

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