Messages in this thread | | | Date | Fri, 30 Jun 2023 18:37:57 -0700 | From | Eduardo Valentin <> | Subject | [PATCH 1/1] thermal: sysfs: avoid actual readings from sysfs |
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On Fri, Jun 30, 2023 at 10:16:38AM +0200, Rafael J. Wysocki wrote: > > > > On Wed, Jun 28, 2023 at 11:10 PM Eduardo Valentin <evalenti@kernel.org> wrote: > > > > On Fri, Jun 23, 2023 at 07:31:43PM +0200, Rafael J. Wysocki wrote: > > > > > [cut] > > > > > > > Regardless of where the problem is etc, if my understanding of the > > > patch is correct, it is proposing to change the behavior of a > > > well-known sysfs interface in a way that is likely to be incompatible > > > with at least some of its users. This is an obvious no-go in kernel > > > development and I would expect you to be well aware of it. > > > > yeah I get it. > > > > > > > > IOW, if you want the cached value to be returned, a new interface (eg. > > > a new sysfs attribute) is needed. > > > > Yeah, I am fine with either a new sysfs entry to return the cached value, > > or a new sysfs entry to change the behavior of the existing /temp, as I > > mentioned before, either way works for me, if changing the existing one > > is too intrusive. > > > > > > > > And I think that the use case is not really i2c sensors in general, > > > > I2C was just the factual example I had, but you are right, the use case > > is not isolated to I2C sensor. Rather, to be clear I am not blaming I2C, > > the actual issue just happen to be easier to see when I2C devices, slower > > than typical MMIO devices, are being used as input for the control. > > > > > because at least in some cases they work just fine AFAICS, but a > > > platform with a control loop running in the kernel that depends on > > > sensor reads carried out at a specific, approximately constant, rate > > > that can be disturbed by user space checking the sensor temperature > > > via sysfs at "wrong" times. If at the same time the user space > > > program in question doesn't care about the most recent values reported > > > by the sensor, it may very well use the values cached by the in-kernel > > > control loop. > > > > That is right, the balance between supporting user space reads and > > running the control timely is the actual original concern. The problem > > fades out a bit when you have device reads in the us / ns time scale > > and control update is in 100s of ms. But becomes more apparent on slower > > devices, when reads are in ms and policy update is in the 100s ms, that is > > why the I2C case was quoted. But nothing wrong with I2C, or supporting > > I2C on the thermal subsystem as we do today via the hwmon interface REGISTER_TZ, > > the problem is on having to support the control in kernel and a read in > > userspace that can create jitter to the control. > > > > And as you properly stated, for this use case, the userspace does not care > > about the most recent value of the device, so that is why the change > > proposes to give cached values. > > > > On the flip side though, there may be user space based policies that > > require the most recent device value. But in this case, the expectation > > is to disable the in kernel policy and switch the thermal zone to > > mode == disabled. And that is also why this patch will go the path > > to request the most recent device value when the /temp sysfs entry > > is read and the mode is disabled. > > > > I would suggest to have an addition sysfs entry that sets the > > thermal zone into cached mode or not, let's say for the sake of the > > discussion, we call it 'cached_values', with default to 'not cached'. > > This way, we could support: > > > > a) Default, current situation, where all reads in /temp are always backed up > > with an actual device .get_temp(). Nothing changes here, keeps reading > > under /temp, and so long system designer is satisfied with jittering, > > no need to change anything. > > > > b) When one has control in kernel, and frequent userspace reads on /temp > > but system designer wants to protect the control in kernel to avoid jittering. > > Just keep reading from /temp but set the new sysfs property 'cached_values' to 'cached'. > > Then userspace will get updated values as frequent as the kernel control has > > and the kernel control will not be disturbed by frequent device reads. > > > > c) When one has control in userspace, and wants to have the most frequent > > device read. Here, one can simply disable the in kernel control by > > setting the 'mode' sysfs entry to 'disabled', and making sure the new sysfs property is set > > to 'not cached'. Well in fact, the way I thought this originally in this patch > > was to simply always read the device when /temp is read is 'mode' is 'disabled'. > > > > I believe you proposed to have another sysfs entry sysfs entry for reading cached temperature. > > Something like 'temp_cached'. Thinking of it, as I mentioned before, it will work. > > The only possible downside is to have two entries to read temperature. > > > > Strong opinions on any of the above? > > So what about adding a new zone attribute that can be used to specify > the preferred caching time for the temperature? > > That is, if the time interval between two consecutive updates of the > cached temperature value is less than the value of the new attribute, > the cached temperature value will be returned by "temp". Otherwise, > it will cause the sensor to be read and the value obtained from it > will be returned to user space and cached. > > If the value of the new attribute is 0, everything will work as it > does now (which will also need to be the default behavior).
Yeah, that makes sense to me. I can cook something up in the next version.
-- All the best, Eduardo Valentin
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