Re: [PATCH 0/2] Add exit_prepare callback to the cpufreq_driver interface.

[Rafael J. Wysocki]

On Friday, March 14, 2014 11:29:04 AM Dirk Brandewie wrote:
> On 03/14/2014 10:07 AM, Viresh Kumar wrote:
> > On 14 March 2014 20:40, Dirk Brandewie <dirk.brandewie@xxxxxxxxx> wrote:
> >> Are you proposing adding cpufreq_generic_suspend() to the core I can not
> >> find
> >> it in the mainline code.
> >
> > Its already there in linux-next. I am suggesting to reuse that
> > infrastructure with
> > some necessary modification to support both suspend and hotplug.
> 
> Suspend and hotplug are two very different things and if we start
> crossing those wires bad things are going to happen IMHO.
> 
> In "normal" operation using the suspend path to do this work could
> work in principal but doesn't handle the case where the user does
>     echo 0 | sudo tee /sys/devices/system/cpu/cpuX/online
> 
> Trying force hotplug and suspend into a common mechanism would
> lead to a bunch of special case code or a significant rework of the
> core code IMHO.
> 
> 
> >
> >>> Over that I don't think Dirk's solution is going to work if we twist
> >>> the systems a bit.
> >>
> >> Could you explain what "twist the systems a bit" means?
> >
> > The one I explained in the below paragraph.
> >
> >>> For example, Dirk probably wanted to set P-STATE of every core to MIN
> >>> when it goes down. But his solution probably doesn't do that right now.
> >>>
> >>
> >> No, I wanted to set the core that was being off-lined to min P state.
> >
> > Sorry, probably my words were a bit confusing. I meant exactly what
> > you just wrote. Core going down will set its freq to min.
> >
> >>> As exit() is called only when the policy expires or all the CPUs of that
> >>> policy
> >>> are down. Suppose only one CPU out of 4 goes down from a policy, then
> >>> pstate driver would never know that happened. And that core wouldn't go
> >>> to min state.
> >>
> >> My patch does not change the semantics of exit() or when it is called.  For
> >> intel_pstate their is one cpu per policy so I moved all the cleanup to
> >
> > I didn't knew that its guaranteed by pstate driver. I thought it would still be
> > hardware dependent as some cores might share clock line.
> 
> This is guaranteed by the hardware.  Each core has its own MSR for P state
> request.  Any coordination that is required between cores to select the
> package P state is handled by the hardware.
> 
> >
> >> exit_prepare() if there were work I needed to do at CPU_POST_DEAD I would
> >> have
> >> continued to export the *optional* exit callback.
> >>
> >> The callback name exit_prepare() was probably unfortunate and might be
> >> causing
> >> some confusion.  I will be changing the name per Rafael's feedback.
> >
> > Don't know.. There is another problem here that exit_prepare() would be called
> > for each CPU whereas exit() would be called for each policy.
> 
> Granted but I don't see this as a problem in this case there is a 1:1
> relationship.  If a driver chooses to use the *optional* exit_prepare() callback
> and knows that there is a many:1 relationship between the policy and CPUs
> then it would have to deal with it.

Actually, I think we should make it clear that the new callback is for
->setpolicy drivers only, which will make things a bit clearer.

We seem to get caught by the difference between ->setpolicy and ->target
drivers on a regular basis, so it might be a good idea to make the distinction
more clear in the code.  I have an idea how to do that, but need some time
to prototype it.

> > And I strongly feel that we shouldn't give another callback here but instead
> > just set core to a specific freq as mentioned by driver with some other field.
> >
> >>> I think we have two solutions here:
> >>> - If its just about setting core a particular freq when it goes down, I
> >>> think it
> >>> looks a generic enough problem and so better fix core for that. Probably
> >>> with
> >>> help of flags field/suspend-freq (maybe renamed) and without calling
> >>> drivers
> >>> exit() at all..
> >>
> >>
> >> ATM the only thing that needs to be done in this case is to allow
> >> intel_pstate
> >> to set the P state on the core when it is going done.  My solution from the
> >> cores point of view is more generic, it allows any driver that needs to do
> >> work
> >> during CPU_DOWN_PREPARE to do it without adding any new logic to the core.
> >
> > Yeah, do we really need to give that freedom right now? Would be better
> > to get this into core as that would be more generic and people looking to set
> > core to some freq at shutdown wouldn't be replicating that code.

Question is if it needs to be more generic.

I honestly don't think that ->target drivers will ever do anything like it,
because they need the governor to "exit" before.  So we are talking about the
only two ->setpolicy drivers in the tree here.

> IMHO yes and it would be hard to be more generic, if your platform needs to
> do architecture specific during the PREPARE phase of cpu hotplug use this
> callback or not.
> 
> BTW now that you have added a path where the cpufreq_suspend() could fail
> it return a value and be checked in dpm_suspend() instead of printing an
> error and just continuing.

I'm not sure what you mean?  Are you saying that it might be a good idea
to allow cpufreq_suspend() to return error codes on failure?

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