Message-ID: <1864010.r8gSosnm8c@vostro.rjw.lan>
Date:	Thu, 01 Aug 2013 20:04:36 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
Cc:	Viresh Kumar <viresh.kumar@...aro.org>,
	Linux PM list <linux-pm@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>, cpufreq@...r.kernel.org,
	Lists linaro-kernel <linaro-kernel@...ts.linaro.org>
Subject: Re: [RFC][PATCH] cpufreq: Do not hold driver module references for additional policy CPUs

On Thursday, August 01, 2013 08:54:59 PM Srivatsa S. Bhat wrote:
> On 08/01/2013 08:14 PM, Viresh Kumar wrote:
> > On 1 August 2013 13:41, Srivatsa S. Bhat
> > <srivatsa.bhat@...ux.vnet.ibm.com> wrote:
> >> On 08/01/2013 05:38 AM, Rafael J. Wysocki wrote:
> >>> From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> >>>
> >>> The cpufreq core is a little inconsistent in the way it uses the
> >>> driver module refcount.
> >>>
> >>> Namely, if __cpufreq_add_dev() is called for a CPU without siblings
> >>> or generally a CPU for which a new policy object has to be created,
> >>> it grabs a reference to the driver module to start with, but drops
> >>> that reference before returning.  As a result, the driver module
> >>> refcount is then equal to 0 after __cpufreq_add_dev() has returned.
> >>>
> >>> On the other hand, if the given CPU is a sibling of some other
> >>> CPU already having a policy, cpufreq_add_policy_cpu() is called
> >>> to link the new CPU to the existing policy.  In that case,
> >>> cpufreq_cpu_get() is called to obtain that policy and grabs a
> >>> reference to the driver module, but that reference is not
> >>> released and the module refcount will be different from 0 after
> >>> __cpufreq_add_dev() returns (unless there is an error).  That
> >>> prevents the driver module from being unloaded until
> >>> __cpufreq_remove_dev() is called for all the CPUs that
> >>> cpufreq_add_policy_cpu() was called for previously.
> >>>
> >>> To remove that inconsistency make cpufreq_add_policy_cpu() execute
> >>> cpufreq_cpu_put() for the given policy before returning, which
> >>> decrements the driver module refcount so that it will be 0 after
> >>> __cpufreq_add_dev() returns,
> >>
> >> Removing the inconsistency is a good thing, but I think we should
> >> make it consistent the other way around - make a CPU-online increment
> >> the driver module refcount and decrement it only on CPU-offline.
> > 
> > I took time to review to this mail as I was looking at the problem
> > yesterday. I am sorry to say, but I have completely different views as
> > compared to You and Rafael both :)
> > 
> > First of all, Rafael's patch is incomplete as it hasn't fixed the issue
> > completely. When we have multiple CPUs per policy and
> > cpufreq_add_dev() is called for the first one, it call cpufreq_get_cpu()
> > for all cpus of this policy(), so count is == x (no. of CPUs in this policy)
> > + 1 (This is the initial value of .owner).
> > 
> > And so we still have module count getting incremented for other cpus :)
> >
> 
> Good catch!

Sorry, I don't see how this happens.

__cpufreq_add_dev() only directly calls cpufreq_cpu_get() at the beginning to
check if the policy is there and then immediately calls cpufreq_cpu_put() in
that case (for that policy).

Next, cpufreq_add_policy_cpu() calls cpufreq_cpu_get(), but that's what my
patch changes.

I don't see where else cpufreq_cpu_get() is called by __cpufreq_add_dev()
whether directly or indirectly.

Moreover, if I'm completely wrong and it is called there in an invisible
hush-hush way, it has to be explained why the module usage count as printed by
lsmod for acpi-cpufreq is 0 (in current linux-next).

> > Now few lines about My point of view to this whole thing. I believe we
> > should get rid of .owner field from struct cpufreq_driver completely. It
> > doesn't make sense to me in doing all this management at all. Surprised?
> > Shocked? Laughing at me? :)
> > 
> > Well I may be wrong but this is what I think:
> > - It looks stupid to me that I can't do this from userspace in one go:
> >   $ insmod cpufreq_driver.ko
> >   $ rmmod cpufreq_driver.ko
> > 
> > What the hell changed in between that isn't visible to user? It looked
> > completely stupid in that way..
> > 
> > Something like this sure makes sense:
> > $ insmod ondemand-governor.ko
> > $ change governor to ondemand for few CPUs
> > $ rmmod ondemand-governor.ko
> > 
> > as we have deliberately add few users of governor. And so without second
> > step, rmmod should really work smoothly. And it does.
> > 
> > Now, why shouldn't there be a problem with this approach? I will write
> > that inline to the problems you just described.
> > 
> >> The reason is, one should not be able to unload the back-end cpufreq
> >> driver module when some CPUs are still being managed. Nasty things
> >> will result if we allow that. For example, if we unload the module,
> >> and then try to do a CPU offline, then the cpufreq hotplug notifier
> >> won't even be called (because cpufreq_unregister_driver also
> >> unregisters the hotplug notifier). And that might be troublesome.
> > 
> > So what? Its simply equivalent to we have booted our system, haven't
> > inserted cpufreq module and taken out a cpu.

I'd put that differently.

With the current code as is it may cause problems to happen, but there are
two ways to change that in general:

(1) Disallow the removal of the cpufreq driver while there are any users, but
    for that we only need the driver to be refcounted *once* when a new policy
    is created (and not as many times as there are CPUs using that policy).
    Then, the reference can be dropped while removing the policy object.

(2) Allow the removal of the cpufreq driver, but harden the code against
    that.  [Maybe it doesn't have to be hardened any more as is, I haven't
    checked that.]

I agree with Viresh that (1) is kind of weird from the usability perspective,
because if we did that, it wouldn't be practically possible to remove cpufreq
driver modules after loading them (cpuidle currently has that problem).

> >> Even worse, if we unload a cpufreq driver module and load a new
> >> one and *then* try to offline the CPU, then the cpufreq_driver->exit()
> >> function that we call during CPU offline will end up calling the
> >> corresponding function of an entirely different driver! So the
> >> ->init() and ->exit() calls won't match.
> > 
> > That's not true. When we unload the module, it must call
> > cpufreq_unregister_driver() which should call cpufreq_remove_cpu()
> > for all cpus and so exit() is already called for last module.
> > 
> 
> Sorry, I missed this one.
> 
> > If we get something new now, it should simply work.
> > 
> 
> Yeah, I now see your point. It won't create any problems by
> unloading the module and loading a new one.
> 
> > What do you think gentlemen?
> > 
> 
> Well, I now agree that we don't have to keep the module refcount
> non-zero as long as CPUs are being managed (that was just my
> misunderstanding, sorry for the noise). However, I think the _get()
> and _put() used in the existing code is for synchronization: that
> is, to avoid races between trying to unload the cpufreq driver
> module and running a hotplug notifier (and calling the driver module's
> ->init() or ->exit() function).
> 
> With that being the case, I think we can retain the module refcounts
> and use them only for synchronization. And naturally the refcount
> should drop to zero after the critical section; no point keeping
> it incremented until the CPU is taken offline.
> 
> Or, am I confused again?

No, I don't think so.

In fact, the point of my patch was to make the module refcount stay 0
beyond critical sections, but it looks like I overlooked something.  What is
that?

Rafael


-- 
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.
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