Re: [PATCH v8 4/8] PM / EM: add support for other devices than CPUs in Energy Model

["Rafael J. Wysocki" <rafael@kernel.org>]

On Wed, Jun 3, 2020 at 6:12 PM Lukasz Luba <lukasz.luba@arm.com> wrote:
>
>
>
> On 6/3/20 4:40 PM, Rafael J. Wysocki wrote:
> > On Wed, Jun 3, 2020 at 5:26 PM Lukasz Luba <lukasz.luba@arm.com> wrote:
> >>
> >>
> >>
> >> On 6/3/20 4:13 PM, Rafael J. Wysocki wrote:
> >>> On Tue, Jun 2, 2020 at 1:31 PM Lukasz Luba <lukasz.luba@arm.com> wrote:
> >>>>
> >>>> Hi Daniel,
> >>>>
> >>>> On 6/1/20 10:44 PM, Daniel Lezcano wrote:
> >>>>> On 27/05/2020 11:58, Lukasz Luba wrote:
> >>>>>> Add support for other devices than CPUs. The registration function
> >>>>>> does not require a valid cpumask pointer and is ready to handle new
> >>>>>> devices. Some of the internal structures has been reorganized in order to
> >>>>>> keep consistent view (like removing per_cpu pd pointers).
> >>>>>>
> >>>>>> Signed-off-by: Lukasz Luba <lukasz.luba@arm.com>
> >>>>>> ---
> >>>>>
> >>>>> [ ... ]
> >>>>>
> >>>>>>     }
> >>>>>>     EXPORT_SYMBOL_GPL(em_register_perf_domain);
> >>>>>> +
> >>>>>> +/**
> >>>>>> + * em_dev_unregister_perf_domain() - Unregister Energy Model (EM) for a device
> >>>>>> + * @dev             : Device for which the EM is registered
> >>>>>> + *
> >>>>>> + * Try to unregister the EM for the specified device (but not a CPU).
> >>>>>> + */
> >>>>>> +void em_dev_unregister_perf_domain(struct device *dev)
> >>>>>> +{
> >>>>>> +    if (IS_ERR_OR_NULL(dev) || !dev->em_pd)
> >>>>>> +            return;
> >>>>>> +
> >>>>>> +    if (_is_cpu_device(dev))
> >>>>>> +            return;
> >>>>>> +
> >>>>>> +    mutex_lock(&em_pd_mutex);
> >>>>>
> >>>>> Is the mutex really needed?
> >>>>
> >>>> I just wanted to align this unregister code with register. Since there
> >>>> is debugfs dir lookup and the device's EM existence checks I thought it
> >>>> wouldn't harm just to lock for a while and make sure the registration
> >>>> path is not used. These two paths shouldn't affect each other, but with
> >>>> modules loading/unloading I wanted to play safe.
> >>>>
> >>>> I can change it maybe to just dmb() and the end of the function if it's
> >>>> a big performance problem in this unloading path. What do you think?
> >>>
> >>> I would rather leave the mutex locking as is.
> >>>
> >>> However, the question to ask is what exactly may go wrong without that
> >>> locking in place?  Is there any specific race condition that you are
> >>> concerned about?
> >>>
> >>
> >> I tried to test this with module loading & unloading with panfrost
> >> driver and CPU hotplug (which should bail out quickly) and was OK.
> >> I don't see any particular race. I don't too much about the
> >> debugfs code, though. That's why I tried to protect from some
> >> scripts/services which try to re-load the driver.
> >>
> >> Apart from that, maybe just this dev->em = NULL to be populated to all
> >> CPUs, which mutex_unlock synchronizes for free here.
> >
> > If it may run concurrently with the registration for the same device,
> > the locking is necessary, but in that case the !dev->em_pd check needs
> > to go under the mutex too IMO, or you may end up leaking the pd if the
> > registration can run between that check and the point at which the
> > mutex is taken.
>
> They don't run concurrently for the same device and users of that EM are
> already gone.
> I just wanted to be sure that everything is cleaned and synced properly.
> Here is some example of the directories under
> /sys/kernel/debug/energy_model
> cpu0, cpu4, gpu, dsp, etc
>
> The only worry that I had was the debugfs dir name, which is a
> string from dev_name() and will be the same for the next registration
> if module is re-loaded.

OK, so that needs to be explained in a comment.

> So the 'name' is reused and debugfs_create_dir()
> and debugfs_remove_recursive() uses this fsnotify, but they are
> operating under inode_lock/unlock() on the parent dir 'energy_model'.
> Then there is also this debugfs_lookup() which is slightly different.
>
> That's why I put a mutex to separate all registration and unregistration
> for all devices.
> It should work without the mutex in unregister path, but I think it does
> not harm to take

Well, fair enough, but I still think that the !dev->em_pd check should
be done under the mutex or it will be confusing.

> it just in case and also have the CPU variable sync for free.

I'm not sure what you mean by the last part here?

> >
> > Apart from this your kerneldoc comments might me improved IMO.
> >
> > First of all, you can use @dev inside of a kerneldoc (if @dev
> > represents an argument of the documented function) and that will
> > produce the right output automatically.
>
> OK
>
> >
> > Second, it is better to avoid saying things like "Try to unregister
> > ..." in kerneldoc comments (the "Try to" part is redundant).  Simply
> > say "Unregister ..." instead.
>
> Good point, thanks, I will use "Unregister ..." then.
>
> >
> > Thanks!
> >
>
> Shell I send a 'resend patch' which changes these @dev and
> 'unregister' comments?

Yes, please, but see the comments above too.

> Or wait for you to finish reviewing the other patches and send v9?

That is not necessary, unless you want to make kerneldoc improvements
in the other patches.

Thanks!
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