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    SubjectRe: [PATCH v2] cpufreq: Don't destroy/realloc policy/sysfs on hotplug/suspend
    On 07/11/2014 03:52 AM, Viresh Kumar wrote:

    Just responding to one comment. The one about policy->cpu.

    >
    >>>> diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
    >
    >>>> static int cpufreq_add_dev_symlink(struct cpufreq_policy *policy)
    >>>> {
    >>>> - unsigned int j;
    >>>> + unsigned int j, first_cpu = cpumask_first(policy->related_cpus);
    >>>> int ret = 0;
    >>>>
    >>>> - for_each_cpu(j, policy->cpus) {
    >>>> + for_each_cpu(j, policy->related_cpus) {
    >>>> struct device *cpu_dev;
    >>>>
    >>>> - if (j == policy->cpu)
    >>>> + if (j == first_cpu)
    >>>
    >>> why?
    >>
    >> The first CPU is a cluster always own the real nodes.
    >
    > What I meant was, why not use policy->cpu?
    >
    >>>> +static int cpufreq_add_dev_interface(struct cpufreq_policy *policy)
    >>>> {
    >>>> struct freq_attr **drv_attr;
    >>>> + struct device *dev;
    >>>> int ret = 0;
    >>>>
    >>>> + dev = get_cpu_device(cpumask_first(policy->related_cpus));
    >>>> + if (!dev)
    >>>> + return -EINVAL;
    >>>> +
    >>>
    >>> Why?
    >>
    >> I'm just always adding the real nodes to the first CPU in a cluster
    >> independent of which CPU gets added first. Makes it easier to know which
    >> ones to symlink. See comment next to policy->cpu for full context.
    >
    > Yeah, and that is the order in which CPUs will boot and cpufreq_add_dev()
    > will be called. So, isn't policy->cpu the right CPU always?

    No, the "first" cpu in a cluster doesn't need to be the first one to be
    added. An example is 2x2 cluster system where the system is booted with
    max cpus = 2 and then cpu3 could be onlined first by userspace.

    >
    >>>> - if (has_target()) {
    >>>> + cpus = cpumask_weight(policy->cpus);
    >>>> + policy->cpu = cpumask_first(policy->cpus);
    >>>
    >>> why update it at all? Also, as per your logic what if cpus == 0?
    >>
    >> Yeah, I didn't write it this way at first. But the governors are making
    >> the assumption that policy->cpu is always an online CPU. So, they try to
    >
    > Are you sure? I had a quick look and failed to see that..
    >
    >> queue work there and use data structs of that CPU (even if they free it in
    >> the STOP event since it went offline).
    >
    > So, it queues work on all policy->cpus, not policy->cpu.
    > And the data structures
    > are just allocated with a CPU number, its fine if its offline.
    >
    > And where are we freeing that stuff in STOP ?
    >
    > Sorry if I am really really tired and couldn't read it correctly.

    Yeah, it is pretty convolution. But pretty much anywhere in the gov code
    where policy->cpu is used could cause this. The specific crash I hit was
    in this code:

    static void od_dbs_timer(struct work_struct *work)
    {
    struct od_cpu_dbs_info_s *dbs_info =
    container_of(work, struct od_cpu_dbs_info_s, cdbs.work.work);
    unsigned int cpu = dbs_info->cdbs.cur_policy->cpu;

    ======= CPU is policy->cpu here.

    struct od_cpu_dbs_info_s *core_dbs_info = &per_cpu(od_cpu_dbs_info,
    cpu);

    ======= Picks the per CPU struct of an offline CPU

    <snip>

    mutex_lock(&core_dbs_info->cdbs.timer_mutex);

    ======= Dies trying to lock a destroyed mutex

    >
    >> Another option is to leave policy->cpu unchanged and then fix all the
    >> governors. But this patch would get even more complicated. So, we can
    >> leave this as is, or fix that up in a separate patch.
    >
    > Since we are simplifying it here, I think we should NOT change policy->cpu
    > at all. It will make life simple (probably).

    I agree, but then I would have to fix up the governors. In the interest
    of keeping this patch small. I'll continue with what I'm doing and fix
    it up in another patch.

    -Saravana

    --
    The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum,
    hosted by The Linux Foundation


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