Re: [PATCH] cpufreq: Avoid attempts to create duplicate symbolic links

Viresh Kumar
2015-07-24T14:09:54Z

On 23-07-15, 23:14, Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
>
> After commit 87549141d516 (cpufreq: Stop migrating sysfs files on
> hotplug) there is a problem with CPUs that share cpufreq policy
> objects with other CPUs and are initially offline.
>
> Say CPU1 shares a policy with CPU0 which is online and is registered
> first. As part of the registration process, cpufreq_add_dev() is
> called for it. It creates the policy object and a symbolic link
> to it from the CPU1's sysfs directory. If CPU1 is registered
> subsequently and it is offline at that time, cpufreq_add_dev() will
> attempt to create a symbolic link to the policy object for it, but
> that link is present already, so a warning about that will be
> triggered.
>
> To avoid that warning, make cpufreq use an additional CPU mask
> containing related CPUs that are actually present for each policy
> object. That mask is initialized when the policy object is populated
> after its creation (for the first online CPU using it) and it includes
> CPUs from the "policy CPUs" mask returned by the cpufreq driver's
> ->init() callback that are physically present at that time. Symbolic
> links to the policy are created only for the CPUs in that mask.
>
> If cpufreq_add_dev() is invoked for an offline CPU, it checks the
> new mask and only creates the symlink if the CPU was not in it (the
> CPU is added to the mask at the same time).
>
> In turn, cpufreq_remove_dev() drops the given CPU from the new mask,
> removes its symlink to the policy object and returns, unless it is
> the CPU owning the policy object. In that case, the policy object
> is moved to a new CPU's sysfs directory or deleted if the CPU being
> removed was the last user of the policy.
>
> While at it, notice that cpufreq_remove_dev() can't fail, because
> its return value is ignored, so make it ignore return values from
> __cpufreq_remove_dev_prepare() and __cpufreq_remove_dev_finish()
> and prevent these functions from aborting on errors returned by
> __cpufreq_governor().
>
> Fixes: 87549141d516 (cpufreq: Stop migrating sysfs files on hotplug)
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> Reported-by: Russell King <linux@arm.linux.org.uk>
> ---
>
> This is supposed to replace the other patches sent so far to address the issue
> at hand.

Lets take this one and leave my patches. They are generating more
diff and actually doing part of the general improvements Russell
suggested.

> + if (!zalloc_cpumask_var(&policy->real_cpus, GFP_KERNEL))

I was wondering if we should use cpumask_t type variables, so that we
don't have to allocate these masks. They are always with policies.

> @@ -1307,6 +1316,9 @@ static int cpufreq_add_dev(struct device
> /* related cpus should atleast have policy->cpus */
> cpumask_or(policy->related_cpus, policy->related_cpus, policy->cpus);
>
> + cpumask_and(policy->cpus, policy->cpus, cpu_present_mask);
> + cpumask_or(policy->real_cpus, policy->real_cpus, policy->cpus);
> +

I will do this differently:
cpumask_and(policy->real_cpus, policy->cpus, cpu_present_mask);

policy->cpus is anyway going to be anded with online mask.

> /*
> * affected cpus must always be the one, which are online. We aren't
> * managing offline cpus here.

> static int cpufreq_remove_dev(struct device *dev, struct subsys_interface *sif)
> {

> - ret = __cpufreq_remove_dev_prepare(dev, sif);
> + if (cpu != policy->kobj_cpu) {
> + remove_cpu_dev_symlink(policy, cpu);
> + } else {
> + /*
> + * This is the CPU owning the policy object. Move it to another
> + * suitable CPU.
> + */
> + unsigned int new_cpu = cpumask_first(policy->real_cpus);
> + struct device *new_dev = get_cpu_device(new_cpu);
>
> - if (!ret)
> - ret = __cpufreq_remove_dev_finish(dev, sif);
> + dev_dbg(dev, "%s: Moving policy object to CPU%u\n", __func__, new_cpu);
>
> - return ret;
> + policy->kobj_cpu = new_cpu;

You need to remove the link for the target cpu, like what I did in my
patch:

sysfs_remove_link(&new_dev->kobj, "cpufreq");

> + WARN_ON(kobject_move(&policy->kobj, &new_dev->kobj));
> + }
> +
> + return 0;
> }
>
> static void handle_update(struct work_struct *work)

--
viresh