Re: [PATCH 1/4] drivers core: Introduce CPU type sysfs interface

[Ricardo Neri]

On Wed, Oct 07, 2020 at 07:15:46AM +0200, Greg Kroah-Hartman wrote:
> On Tue, Oct 06, 2020 at 08:14:47PM -0700, Ricardo Neri wrote:
> > On Tue, Oct 06, 2020 at 09:37:44AM +0200, Greg Kroah-Hartman wrote:
> > > On Mon, Oct 05, 2020 at 05:57:36PM -0700, Ricardo Neri wrote:
> > > > On Sat, Oct 03, 2020 at 10:53:45AM +0200, Greg Kroah-Hartman wrote:
> > > > > On Fri, Oct 02, 2020 at 06:17:42PM -0700, Ricardo Neri wrote:
> > > > > > Hybrid CPU topologies combine CPUs of different microarchitectures in the
> > > > > > same die. Thus, even though the instruction set is compatible among all
> > > > > > CPUs, there may still be differences in features (e.g., some CPUs may
> > > > > > have counters that others CPU do not). There may be applications
> > > > > > interested in knowing the type of micro-architecture topology of the
> > > > > > system to make decisions about process affinity.
> > > > > > 
> > > > > > While the existing sysfs for capacity (/sys/devices/system/cpu/cpuX/
> > > > > > cpu_capacity) may be used to infer the types of micro-architecture of the
> > > > > > CPUs in the platform, it may not be entirely accurate. For instance, two
> > > > > > subsets of CPUs with different types of micro-architecture may have the
> > > > > > same capacity due to power or thermal constraints.
> > > > > > 
> > > > > > Create the new directory /sys/devices/system/cpu/types. Under such
> > > > > > directory, create individual subdirectories for each type of CPU micro-
> > > > > > architecture. Each subdirectory will have cpulist and cpumap files. This
> > > > > > makes it convenient for user space to read all the CPUs of the same type
> > > > > > at once without having to inspect each CPU individually.
> > > > > > 
> > > > > > Implement a generic interface using weak functions that architectures can
> > > > > > override to indicate a) support for CPU types, b) the CPU type number, and
> > > > > > c) a string to identify the CPU vendor and type.
> > > > > > 
> > > > > > For example, an x86 system with one Intel Core and four Intel Atom CPUs
> > > > > > would look like this (other architectures have the hooks to use whatever
> > > > > > directory naming convention below "types" that meets their needs):
> > > > > > 
> > > > > > user@host:~$: ls /sys/devices/system/cpu/types
> > > > > > intel_atom_0  intel_core_0
> > > > > > 
> > > > > > user@host:~$ ls /sys/devices/system/cpu/types/intel_atom_0
> > > > > > cpulist cpumap
> > > > > > 
> > > > > > user@host:~$ ls /sys/devices/system/cpu/types/intel_core_0
> > > > > > cpulist cpumap
> > > > > > 
> > > > > > user@host:~$ cat /sys/devices/system/cpu/types/intel_atom_0/cpumap
> > > > > > 0f
> > > > > > 
> > > > > > user@host:~$ cat /sys/devices/system/cpu/types/intel_atom_0/cpulist
> > > > > > 0-3
> > > > > > 
> > > > > > user@ihost:~$ cat /sys/devices/system/cpu/types/intel_core_0/cpumap
> > > > > > 10
> > > > > > 
> > > > > > user@host:~$ cat /sys/devices/system/cpu/types/intel_core_0/cpulist
> > > > > > 4
> > > > 
> > > > Thank you for the quick and detailed Greg!
> > > > 
> > > > > 
> > > > > The output of 'tree' sometimes makes it easier to see here, or:
> > > > > 	grep -R . *
> > > > > also works well.
> > > > 
> > > > Indeed, this would definitely make it more readable.
> > > > 
> > > > > 
> > > > > > On non-hybrid systems, the /sys/devices/system/cpu/types directory is not
> > > > > > created. Add a hook for this purpose.
> > > > > 
> > > > > Why should these not show up if the system is not "hybrid"?
> > > > 
> > > > My thinking was that on a non-hybrid system, it does not make sense to
> > > > create this interface, as all the CPUs will be of the same type.
> > > 
> > > Why not just have this an attribute type in the existing cpuX directory?
> > > Why do this have to be a totally separate directory and userspace has to
> > > figure out to look in two different spots for the same cpu to determine
> > > what it is?
> > 
> > But if the type is located under cpuX, usespace would need to traverse
> > all the CPUs and create its own cpu masks. Under the types directory it
> > would only need to look once for each type of CPU, IMHO.
> 
> What does a "mask" do?  What does userspace care about this? 

It would only need to consume the cpu masks that sysfs provides; but it
would have to implement the extra step of looking at another directory.

> You would have to create it by traversing the directories you are creating anyway,
> so it's not much different, right?

True, that is a good point, userspace would still end up traversing
diretories anyways.

> 
> > > That feels wasteful, it should be much simpler to use the existing
> > > object, right?
> > > 
> > > That way, you also show the "type" of all cpus, no matter if they are
> > > "hybrid" or not, again, making userspace deal with things in a much
> > > simpler manner.
> > 
> > Indeed, that would be simpler to implement, and perhaps a natural extension
> > of the existing interface. 
> > 
> > Lastly, legacy and non-hybrid parts will not have a type defined. Thus,
> > it may not make sense for them to expose a type in sysfs.
> 
> That's fine, not having a sysfs file if you don't know the type is fine.
> Or you can fix that up and show the type of those as well, why wouldn't
> you want to?

What information should be used for the fixup? On legacy x86 parts,
CPUID would not have the leaf that provides this information. Maybe an
"unknown" type?

> 
> > > > I thought of this as maybe intel_atcm vs a potential amd_minizen. I
> > > > guess arch is sufficient, as there will never be an amd_atom.
> > > 
> > > Even if there is, that can be part of the cpu "type" that you have for
> > > your enumerated type, right?
> > 
> > Indeed. The interface I propose lets archs define their own string
> > names.
> 
> Arches _should_ define their own names, otherwise who would?

Yes, ineed.

Thanks and BR,
Ricardo