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

[Brice Goglin <brice.goglin@gmail.com>]

Le 07/10/2020 à 07:15, Greg Kroah-Hartman a écrit :
> 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?  You would
> have to create it by traversing the directories you are creating anyway,
> so it's not much different, right?


Hello

Sorry for the late reply. As the first userspace consumer of this
interface [1], I can confirm that reading a single file to get the mask
would be better, at least for performance reason. On large platforms, we
already have to read thousands of sysfs files to get CPU topology and
cache information, I'd be happy not to read one more file per cpu.

Reading these sysfs files is slow, and it does not scale well when
multiple processes read them in parallel. There are ways to avoid this
multiple discoveries by sharing hwloc info through XML or shmem, but it
will take years before all developers of different runtimes all
implement this :)

Thanks

Brice

[1] hwloc and lstopo are going to expose this hybrid info to HPC
runtimes (https://www.open-mpi.org/projects/hwloc/)