| Message ID | cover.1628579170.git.viresh.kumar@linaro.org |
|---|---|
| Headers | show |
| Series | cpufreq: Auto-register with energy model | expand |
Hi Viresh, I like the idea, only small comments here in the cover letter. On 8/10/21 8:36 AM, Viresh Kumar wrote: > Provide a cpufreq driver flag so drivers can ask the cpufreq core to register > with the EM core on their behalf. This allows us to get rid of duplicated code > in the drivers and fix the unregistration part as well, which none of the > drivers have done until now. The EM is never freed for CPUs by design. The unregister function was introduced for devfreq devices. > > This would also make the registration with EM core to happen only after policy > is fully initialized, and the EM core can do other stuff from in there, like > marking frequencies as inefficient (WIP). Though this patchset is useful without > that work being done and should be merged nevertheless. > > This doesn't update scmi cpufreq driver for now as it is a special case and need > to be handled differently. Though we can make it work with this if required. The scmi cpufreq driver uses direct EM API, which provides flexibility and should stay as is. Let me review the patches. Regards, Lukasz
On 10-08-21, 10:17, Lukasz Luba wrote: > Hi Viresh, > > I like the idea, only small comments here in the cover letter. > > On 8/10/21 8:36 AM, Viresh Kumar wrote: > > Provide a cpufreq driver flag so drivers can ask the cpufreq core to register > > with the EM core on their behalf. This allows us to get rid of duplicated code > > in the drivers and fix the unregistration part as well, which none of the > > drivers have done until now. > > The EM is never freed for CPUs by design. The unregister function was > introduced for devfreq devices. I see. So if a cpufreq driver unregisters and registers again, it will be required to use the entries created by the registration itself, right ? Technically speaking, it is better to unregister and free any related resources and parse everything again. Lets say, just for fun, I want to test two copies of a cpufreq driver (providing different set of freq-tables). I build both of them as modules, insert the first version, remove it, insert the second one. Ideally, this should just work as expected. But I don't think it will in this case as you never parse the EM stuff again. Again, since the routine is there already, I think it is better/fine to just use it. > > This would also make the registration with EM core to happen only after policy > > is fully initialized, and the EM core can do other stuff from in there, like > > marking frequencies as inefficient (WIP). Though this patchset is useful without > > that work being done and should be merged nevertheless. > > > > This doesn't update scmi cpufreq driver for now as it is a special case and need > > to be handled differently. Though we can make it work with this if required. > > The scmi cpufreq driver uses direct EM API, which provides flexibility > and should stay as is. Right, so I left it as is for now.
On Tuesday 10 Aug 2021 at 14:25:15 (+0100), Lukasz Luba wrote: > The way I see this is that the flag in cpufreq avoids > mistakes potentially made by driver developer. It will automaticaly > register the *simple* EM model via dev_pm_opp_of_register_em() on behalf > of drivers (which is already done manually by drivers). The developer > would just set the flag similarly to CPUFREQ_IS_COOLING_DEV and be sure > it will register at the right time. Well tested flag approach should be > safer, easier to understand, maintain. I would agree with all that if calling dev_pm_opp_of_register_em() was complicated, but that is not really the case. I don't think we ever call PM_OPP directly from cpufreq core ATM, which makes a lot of sense if you consider PM_OPP arch-specific. I could understand that we might accept a little 'violation' of the abstraction with this series if there were real benefits, but I just don't see them.
On 11-08-21, 10:48, Viresh Kumar wrote: > On 10-08-21, 13:35, Quentin Perret wrote: > > This series adds more code than it removes, > > Sadly yes :( > > > and the unregistration is > > not a fix as we don't ever remove the EM tables by design, so not sure > > either of these points are valid arguments. > > I think that design needs to be looked over again, it looks broken to > me everytime I land onto this code. I wonder why we don't unregister > stuff. Coming back to this series. We have two options, based on what I proposed here: https://lore.kernel.org/linux-pm/20210811050327.3yxrk4kqxjjwaztx@vireshk-i7/ 1. Let cpufreq core register with EM on behalf of cpufreq drivers. 2. Update drivers to use ->ready() callback to do this stuff. I am fine with both :) -- viresh
On Wednesday 11 Aug 2021 at 10:48:59 (+0530), Viresh Kumar wrote: > On 10-08-21, 13:35, Quentin Perret wrote: > > On Tuesday 10 Aug 2021 at 13:06:47 (+0530), Viresh Kumar wrote: > > > Provide a cpufreq driver flag so drivers can ask the cpufreq core to register > > > with the EM core on their behalf. > > > > Hmm, that's not quite what this does. This asks the cpufreq core to > > use *PM_OPP* to register an EM, which I think is kinda wrong to do from > > there IMO. The decision to use PM_OPP or another mechanism to register > > an EM belongs to platform specific code (drivers), so it is odd for the > > PM_OPP registration to have its own cpufreq flag but not the other ways. > > > > As mentioned in another thread, the very reason to have PM_EM is to not > > depend on PM_OPP, so I'm worried about the direction of travel with this > > series TBH. > > I had to use the pm-opp version, since almost everyone was using that. > > On the other hand, there isn't a lot of OPP specific stuff in > dev_pm_opp_of_register_em(). It just uses dev_pm_opp_get_opp_count(), > that's all. This ended up in the OPP core, nothing else. Maybe we can > now move it back to the EM core and name it differently ? Well it also uses dev_pm_opp_find_freq_ceil() and dev_pm_opp_get_voltage(), so not sure how easy it will be to move, but if it is possible no objection from me. > > > This allows us to get rid of duplicated code > > > in the drivers and fix the unregistration part as well, which none of the > > > drivers have done until now. > > > > This series adds more code than it removes, > > Sadly yes :( > > > and the unregistration is > > not a fix as we don't ever remove the EM tables by design, so not sure > > either of these points are valid arguments. > > I think that design needs to be looked over again, it looks broken to > me everytime I land onto this code. I wonder why we don't unregister > stuff. > > Lets say, I am working on the cpufreq driver and I want to test that > on my ARM machine. Rebooting a simpler board to test stuff out is > easy, but if I am working on an ARM server which is running lots of > other userspace stuff as well, I won't want to reboot the machine just > to test a different versions of the driver. I will rather want to > build the driver as module and insert/remove it again and again. > > If the frequency table changes in between versions, this just breaks > as EM won't be updated again. > > This breaks one of the most basic rules of Linux Kernel. Inserting a > module should have exactly the same final behavior every single time. > This model doesn't guarantee it. It simply looks broken. Right but the EM is a description of the hardware, so it seemed fair to assume this wouldn't change across the lifetime of the OS, similar to the DT which we can't reload at run-time. Yes it can be a little odd if you load/unload your driver module, but note that you generally can't load two completely different drivers on a single system. You'll just load the same one again and the hardware hasn't changed in the meantime, so the previously loaded EM will still be correct. I hear your argument about cpufreq driver development, but the locking involved to allow 'just' that is pretty involved, and nobody has complained about this specific issue so far, so that didn't seem worth it. If we do have good reasons to change the EM at runtime, then yes I think we should do it, it just didn't seem like that was the case until now.
On Wednesday 11 Aug 2021 at 14:43:21 (+0530), Viresh Kumar wrote: > On 11-08-21, 09:37, Quentin Perret wrote: > > On Wednesday 11 Aug 2021 at 10:48:59 (+0530), Viresh Kumar wrote: > > > I had to use the pm-opp version, since almost everyone was using that. > > > > > > On the other hand, there isn't a lot of OPP specific stuff in > > > dev_pm_opp_of_register_em(). It just uses dev_pm_opp_get_opp_count(), > > > that's all. This ended up in the OPP core, nothing else. Maybe we can > > > now move it back to the EM core and name it differently ? > > > > Well it also uses dev_pm_opp_find_freq_ceil() and > > dev_pm_opp_get_voltage(), so not sure how easy it will be to move, but > > if it is possible no objection from me. > > What uses these routines ? dev_pm_opp_of_register_em() ? I am not able > to see that at least :( Yep, it's not immediately obvious, but see how it sets the struct em_data_callback to point at _get_power() where the actual energy calculation is done. So strictly speaking _get_power() is what uses these routines, but it goes in hand with dev_pm_opp_of_register_em() so I guess the same reasoning applies. > > Right but the EM is a description of the hardware, so it seemed fair > > to assume this wouldn't change across the lifetime of the OS, similar > > to the DT which we can't reload at run-time. Yes it can be a little odd > > if you load/unload your driver module, but note that you generally can't > > load two completely different drivers on a single system. You'll just > > load the same one again and the hardware hasn't changed in the meantime, > > so the previously loaded EM will still be correct. > > Yeah, it will be the same driver but a different version of it, which > may have updated the freq table. For me the EM is attached to the > freq-table, and the freq-table is not available anymore after the > driver is gone. > > Anyway, I will leave that for you guys to decide :) IIUC Lukasz is working on something that should allow changing the EM at run-time, so hopefully it'll enable this use-case as well, but we'll see :)
On 11-08-21, 10:34, Quentin Perret wrote: > Yep, it's not immediately obvious, but see how it sets the struct > em_data_callback to point at _get_power() where the actual energy > calculation is done. So strictly speaking _get_power() is what uses > these routines, but it goes in hand with dev_pm_opp_of_register_em() so > I guess the same reasoning applies. My bad. -- viresh
On Wednesday 11 Aug 2021 at 11:04:06 (+0530), Viresh Kumar wrote: > On 11-08-21, 10:48, Viresh Kumar wrote: > > On 10-08-21, 13:35, Quentin Perret wrote: > > > This series adds more code than it removes, > > > > Sadly yes :( > > > > > and the unregistration is > > > not a fix as we don't ever remove the EM tables by design, so not sure > > > either of these points are valid arguments. > > > > I think that design needs to be looked over again, it looks broken to > > me everytime I land onto this code. I wonder why we don't unregister > > stuff. > > Coming back to this series. We have two options, based on what I > proposed here: > > https://lore.kernel.org/linux-pm/20210811050327.3yxrk4kqxjjwaztx@vireshk-i7/ > > 1. Let cpufreq core register with EM on behalf of cpufreq drivers. If we're going that route, I think we should allow _all_ possible EM registration methods (via PM_OPP or else) to be done that way. Otherwise we're creating an inconsitency in how the EM is registered (e.g. from the ->init() cpufreq callback for some, or from cpufreq core for others) which is problematic as we risk building features that assume loading is done at a certain time, which won't work for some platforms. > 2. Update drivers to use ->ready() callback to do this stuff. I think this should work, but perhaps will be a bit tricky for cpufreq driver developers as they need to have a pretty good understanding of the stack to know that they should do the registration from here and not ->init() for instance. Suggested alternative: we introduce a ->register_em() callback to cpufreq_driver, and turn dev_pm_opp_of_register_em() into a valid handler for this callback. This should 'document' things a bit better, avoid some of the problems your other series tried to achieve, and allow us to call the EM registration in exactly the right place from cpufreq core. On the plus side, we could easily make this work for e.g. the SCMI driver which would only need to provide its own version of ->register_em(). Thoughts?
On 11-08-21, 10:48, Quentin Perret wrote: > I think this should work, but perhaps will be a bit tricky for cpufreq > driver developers as they need to have a pretty good understanding of > the stack to know that they should do the registration from here and not > ->init() for instance. Suggested alternative: we introduce a ->register_em() > callback to cpufreq_driver, and turn dev_pm_opp_of_register_em() into a > valid handler for this callback. This should 'document' things a bit > better, avoid some of the problems your other series tried to achieve, and > allow us to call the EM registration in exactly the right place from > cpufreq core. On the plus side, we could easily make this work for e.g. > the SCMI driver which would only need to provide its own version of > ->register_em(). > > Thoughts? I had exactly the same thing in mind, but was thinking of two callbacks, to register and unregister. But yeah, we aren't going to register for now at least :) I wasn't sure if that should be done or not, since we also have ready() callback. So was reluctant to suggest it earlier. But that can work well as well.
On Wednesday 11 Aug 2021 at 15:23:11 (+0530), Viresh Kumar wrote: > On 11-08-21, 10:48, Quentin Perret wrote: > > I think this should work, but perhaps will be a bit tricky for cpufreq > > driver developers as they need to have a pretty good understanding of > > the stack to know that they should do the registration from here and not > > ->init() for instance. Suggested alternative: we introduce a ->register_em() > > callback to cpufreq_driver, and turn dev_pm_opp_of_register_em() into a > > valid handler for this callback. This should 'document' things a bit > > better, avoid some of the problems your other series tried to achieve, and > > allow us to call the EM registration in exactly the right place from > > cpufreq core. On the plus side, we could easily make this work for e.g. > > the SCMI driver which would only need to provide its own version of > > ->register_em(). > > > > Thoughts? > > I had exactly the same thing in mind, but was thinking of two > callbacks, to register and unregister. But yeah, we aren't going to > register for now at least :) Ack, we probably want both once we unregister things. > I wasn't sure if that should be done or not, since we also have > ready() callback. So was reluctant to suggest it earlier. But that can > work well as well. I think using the ready() callback can work just fine as long as we document clearly it is important to register the EM from there and not anywhere else. The dedicated em_register() callback makes that a bit clearer and should avoid a bit of boilerplate in the driver, but it's not a big deal really, so I'm happy either way ;)