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

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Thu, Nov 12, 2020 at 07:19:48AM +0100, Brice Goglin wrote:
> 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.

Really?  Where is the slowdown?  Would something like readfile() work
better for you for that?
	https://lore.kernel.org/linux-api/20200704140250.423345-1-gregkh@linuxfoundation.org/

How does multiple processes slow anything down, there shouldn't be any
shared locks here.

> 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 :)

I don't understand, what exactly are you suggesting we do here instead?

thanks,

greg k-h