Re: [PATCH v2] cpufreq: Don't destroy/realloc policy/sysfs on hotplug/suspend

[Saravana Kannan]

On 07/13/2014 11:09 PM, Viresh Kumar wrote:
> On 12 July 2014 08:14, Saravana Kannan <skannan@xxxxxxxxxxxxxx> 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,
> > 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
>
> 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.

-Saravana

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