linux-kernel - Re: [PATCH v6 2/4] scmi-cpufreq: Move CPU initialisation to probe

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Date:   Thu, 14 Jan 2021 10:37:53 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     Nicola Mazzucato <nicola.mazzucato@....com>
Cc:     linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-pm@...r.kernel.org, sudeep.holla@....com, rjw@...ysocki.net,
        vireshk@...nel.org, cristian.marussi@....com,
        morten.rasmussen@....com, chris.redpath@....com
Subject: Re: [PATCH v6 2/4] scmi-cpufreq: Move CPU initialisation to probe

On 13-01-21, 11:55, Nicola Mazzucato wrote:
> On 1/12/21 11:17 AM, Viresh Kumar wrote:
> > This could have been done with a per-cpu variable instead.
> 
> sure, I can do a DEFINE_PER_CPU() for it if it makes it better.

If we don't go with the linked list approach, then yes.
 
> >> +	for_each_possible_cpu(cpu) {
> >> +		if (!zalloc_cpumask_var(&cpudata_table[cpu].scmi_shared_cpus,
> >> +					GFP_KERNEL))
> >> +			goto out;
> >> +	}
> > 
> > You are making a copy of the struct for each CPU and so for a 16 CPUs
> > sharing their clock lines, you will have 16 copies of the exact same
> > stuff.
> > 
> > An optimal approach would be to have a linked-list of this structure
> > and that will only have 1 node per cpufreq policy.
> 
> It is allocating space for the cpumask for each of the cpu. No data is copied yet.

Yes, I was talking about the whole design here.

> I understand the optimisation, but I don't see a linkage to cpufreq policy to be
> a good idea. This cpudata is for internal storage of scmi and opp-shared info
> and it is not tied to cpufreq policy.

Well, it is going to be the same information for all CPUs of a policy, isn't it
?

> We have moved all the cpu bits to probe
> and at this stage we have no knowledge of cpufreq polices.

Yes, but you are reading that information from scmi or DT (empty opp tables) and
so you know what the cpumasks are going to be set to. The linked list is the
right solution in my opinion, it is much more optimal.

> >> +static int scmi_init_device(const struct scmi_handle *handle, int cpu)
> >> +{
> >> +	struct device *cpu_dev;
> >> +	int ret, nr_opp;
> >> +	struct em_data_callback em_cb = EM_DATA_CB(scmi_get_cpu_power);
> >> +	bool power_scale_mw;
> >> +	cpumask_var_t scmi_cpus;
> >> +
> >> +	if (!zalloc_cpumask_var(&scmi_cpus, GFP_KERNEL))
> >> +		return -ENOMEM;
> >> +
> >> +	cpumask_set_cpu(cpu, scmi_cpus);
> >> +
> >> +	cpu_dev = get_cpu_device(cpu);
> >> +
> >> +	ret = scmi_get_sharing_cpus(cpu_dev, scmi_cpus);
> > 
> > Where do you expect the sharing information to come from in this case
> > ? DT ?
> 
> Coming from SCMI perf. The source of info has not changed.
> 
> > 
> >> +	if (ret) {
> >> +		dev_warn(cpu_dev, "failed to get sharing cpumask\n");
> >> +		goto free_cpumask;
> >> +	}
> >> +
> >> +	/*
> >> +	 * We get here for each CPU. Add OPPs only on those CPUs for which we
> >> +	 * haven't already done so, or set their OPPs as shared.
> >> +	 */
> >> +	nr_opp = dev_pm_opp_get_opp_count(cpu_dev);
> >> +	if (nr_opp <= 0) {
> >> +		ret = handle->perf_ops->device_opps_add(handle, cpu_dev);
> >> +		if (ret) {
> >> +			dev_warn(cpu_dev, "failed to add opps to the device\n");
> >> +			goto free_cpumask;
> >> +		}
> >> +
> >> +		ret = dev_pm_opp_set_sharing_cpus(cpu_dev, scmi_cpus);
> >> +		if (ret) {
> >> +			dev_err(cpu_dev, "%s: failed to mark OPPs as shared: %d\n",
> >> +				__func__, ret);
> >> +			goto free_cpumask;
> >> +		}
> >> +
> >> +		nr_opp = dev_pm_opp_get_opp_count(cpu_dev);
> > 
> > Shouldn't you do this just after adding the OPPs ?
> 
> This was suggested earlier. It was moved closer to em_registration to where the
> nr_opp is used. One way or the other as I don't have a strong preference for its
> place.

It is better to move it above as this will shorten the error path.

-- 
viresh