[PATCH v2] cpufreq: Don't destroy/realloc policy/sysfs on hotplug/suspend
skannan at codeaurora.org
Mon Jul 14 12:08:24 PDT 2014
On 07/13/2014 11:09 PM, Viresh Kumar wrote:
> On 12 July 2014 08:14, Saravana Kannan <skannan at codeaurora.org> wrote:
>>>> 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.
> Because we are getting rid of much of the complexity now, I do not want
> policy->cpu to keep changing. Just fix it up to the cpu for which the policy
> gets created first. That's it. No more changes required. It doesn't matter at
> userspace which cpu owns it as symlinks would anyway duplicate it under
> every cpu.
I think you missed one my of comments in the email. I agree with what
you are saying here. I'll just do it as a separate patch to keep this
one simpler. I don't want to touch all the governors and other potential
uses of policy->cpu in this patch.
>> 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,
>> 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,
>> ======= Picks the per CPU struct of an offline CPU
>> ======= Dies trying to lock a destroyed mutex
> I am still not getting it. Why would we get into this if policy->cpu is fixed
> once at boot ?
Yeah, it definitely crashes if policy->cpu if an offline cpu. Because
the mutex would be uninitialized if it's stopped after boot or it would
never have been initialized (depending on how you fix policy->cpu at boot).
Look at this snippet on the actual tree and it should be pretty evident.
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