lkml.org 
[lkml]   [2014]   [Jul]   [11]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
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


\
 
 \ /
  Last update: 2014-07-12 05:21    [W:0.487 / U:0.060 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site