Re: [PATCH v4 39/63] Documentation: x86: convert topology.txt to reST

Changbin Du
2019-04-26T14:23:54Z

On Wed, Apr 24, 2019 at 02:44:07PM -0300, Mauro Carvalho Chehab wrote:
> Em Wed, 24 Apr 2019 00:29:08 +0800
> Changbin Du <changbin.du@gmail.com> escreveu:
>
> > This converts the plain text documentation to reStructuredText format and
> > add it to Sphinx TOC tree. No essential content change.
> >
> > Signed-off-by: Changbin Du <changbin.du@gmail.com>
> > ---
> > Documentation/x86/index.rst | 1 +
> > Documentation/x86/topology.rst | 228 +++++++++++++++++++++++++++++++++
> > Documentation/x86/topology.txt | 217 -------------------------------
> > 3 files changed, 229 insertions(+), 217 deletions(-)
> > create mode 100644 Documentation/x86/topology.rst
> > delete mode 100644 Documentation/x86/topology.txt
>
> Why? Please preserve as much as possible from the original file...
> it is really hard to see what you're doing. Most of those x86
> files are already almost at ReST format (like this one). There's
> absolutely **no reason** why you would do so much radical changes
> that would below the 50% similarity threshold that would make git
> to recognize as a change on the same file!
>
My editor changed the indent. I need to redo this conversion. Thanks.

> I'll give a quick review on this one, but it is really hard to be
> sure that something is missing, when the similarity is too low.
>
> >
> > diff --git a/Documentation/x86/index.rst b/Documentation/x86/index.rst
> > index 8f08caf4fbbb..2033791e53bc 100644
> > --- a/Documentation/x86/index.rst
> > +++ b/Documentation/x86/index.rst
> > @@ -9,3 +9,4 @@ Linux x86 Support
> > :numbered:
> >
> > boot
> > + topology
> > diff --git a/Documentation/x86/topology.rst b/Documentation/x86/topology.rst
> > new file mode 100644
> > index 000000000000..1df5f56f4882
> > --- /dev/null
> > +++ b/Documentation/x86/topology.rst
> > @@ -0,0 +1,228 @@
> > +.. SPDX-License-Identifier: GPL-2.0
> > +
> > +============
> > +x86 Topology
> > +============
> > +
> > +This documents and clarifies the main aspects of x86 topology modelling and
> > +representation in the kernel. Update/change when doing changes to the
> > +respective code.
> > +
> > +The architecture-agnostic topology definitions are in
> > +Documentation/cputopology.txt. This file holds x86-specific
> > +differences/specialities which must not necessarily apply to the generic
> > +definitions. Thus, the way to read up on Linux topology on x86 is to start
> > +with the generic one and look at this one in parallel for the x86 specifics.
> > +
> > +Needless to say, code should use the generic functions - this file is *only*
> > +here to *document* the inner workings of x86 topology.
> > +
> > +Started by Thomas Gleixner <tglx@linutronix.de> and Borislav Petkov <bp@alien8.de>.
> > +
> > +The main aim of the topology facilities is to present adequate interfaces to
> > +code which needs to know/query/use the structure of the running system wrt
> > +threads, cores, packages, etc.
> > +
> > +The kernel does not care about the concept of physical sockets because a
> > +socket has no relevance to software. It's an electromechanical component. In
> > +the past a socket always contained a single package (see below), but with the
> > +advent of Multi Chip Modules (MCM) a socket can hold more than one package. So
> > +there might be still references to sockets in the code, but they are of
> > +historical nature and should be cleaned up.
> > +
> > +The topology of a system is described in the units of:
> > +
> > + - packages
> > + - cores
> > + - threads
> > +
> > +Package
> > +=======
> > +
> > +Packages contain a number of cores plus shared resources, e.g. DRAM
> > +controller, shared caches etc.
> > +
> > +AMD nomenclature for package is 'Node'.
> > +
> > +Package-related topology information in the kernel:
> > +
> > + - cpuinfo_x86.x86_max_cores:
> > +
> > + The number of cores in a package. This information is retrieved via CPUID.
> > +
> > + - cpuinfo_x86.phys_proc_id:
> > +
> > + The physical ID of the package. This information is retrieved via CPUID
> > + and deduced from the APIC IDs of the cores in the package.
> > +
> > + - cpuinfo_x86.logical_id:
> > +
> > + The logical ID of the package. As we do not trust BIOSes to enumerate the
> > + packages in a consistent way, we introduced the concept of logical package
> > + ID so we can sanely calculate the number of maximum possible packages in
> > + the system and have the packages enumerated linearly.
> > +
> > + - topology_max_packages():
> > +
> > + The maximum possible number of packages in the system. Helpful for per
> > + package facilities to preallocate per package information.
> > +
> > + - cpu_llc_id:
> > +
> > + A per-CPU variable containing:
> > +
> > + - On Intel, the first APIC ID of the list of CPUs sharing the Last Level
> > + Cache.
> > +
> > + - On AMD, the Node ID or Core Complex ID containing the Last Level
> > + Cache. In general, it is a number identifying an LLC uniquely on the
> > + system.
> > +
> > +Cores
> > +=====
> > +
> > +A core consists of 1 or more threads. It does not matter whether the threads
> > +are SMT- or CMT-type threads.
> > +
> > +AMDs nomenclature for a CMT core is "Compute Unit". The kernel always uses
> > +"core".
> > +
> > +Core-related topology information in the kernel:
> > +
> > + - smp_num_siblings:
> > +
> > + The number of threads in a core. The number of threads in a package can be
> > + calculated by::
> > +
> > + threads_per_package = cpuinfo_x86.x86_max_cores * smp_num_siblings
> > +
> > +
> > +Threads
> > +=======
> > +
> > +A thread is a single scheduling unit. It's the equivalent to a logical Linux
> > +CPU.
> > +
> > +AMDs nomenclature for CMT threads is "Compute Unit Core". The kernel always
> > +uses "thread".
> > +
> > +Thread-related topology information in the kernel:
> > +
> > + - topology_core_cpumask():
> > +
> > + The cpumask contains all online threads in the package to which a thread
> > + belongs.
> > +
> > + The number of online threads is also printed in /proc/cpuinfo "siblings."
> > +
> > + - topology_sibling_cpumask():
> > +
> > + The cpumask contains all online threads in the core to which a thread
> > + belongs.
> > +
> > + - topology_logical_package_id():
> > +
> > + The logical package ID to which a thread belongs.
> > +
> > + - topology_physical_package_id():
> > +
> > + The physical package ID to which a thread belongs.
> > +
> > + - topology_core_id();
> > +
> > + The ID of the core to which a thread belongs. It is also printed in /proc/cpuinfo
> > + "core_id."
> > +
> > +
> > +
> > +System topology examples
> > +========================
> > +
> > +.. note:: The alternative Linux CPU enumeration depends on how the BIOS
> > + enumerates the threads. Many BIOSes enumerate all threads 0 first and
> > + then all threads 1. That has the "advantage" that the logical Linux CPU
> > + numbers of threads 0 stay the same whether threads are enabled or not.
> > + That's merely an implementation detail and has no practical impact.
> > +
> > +1) Single Package, Single Core
> > +::
>
> I would just place the :: on the above line. Same applies to similar
> cases on this file.
>
Sure.

> > +
> > + [package 0] -> [core 0] -> [thread 0] -> Linux CPU 0
> > +
> > +2) Single Package, Dual Core
> > +
> > + a) One thread per core
> > + ::
> > +
> > + [package 0] -> [core 0] -> [thread 0] -> Linux CPU 0
> > + -> [core 1] -> [thread 0] -> Linux CPU 1
>
> Something got broken here.
>
> > +
> > + b) Two threads per core
> > + ::
> > +
> > + [package 0] -> [core 0] -> [thread 0] -> Linux CPU 0
> > + -> [thread 1] -> Linux CPU 1
> > + -> [core 1] -> [thread 0] -> Linux CPU 2
> > + -> [thread 1] -> Linux CPU 3
>
> And here... This one, for example, should be, instead:
>
> [package 0] -> [core 0] -> [thread 0] -> Linux CPU 0
> -> [thread 1] -> Linux CPU 1
> -> [core 1] -> [thread 0] -> Linux CPU 2
> -> [thread 1] -> Linux CPU 3
>
> Clearly there's something that it is messing with tabs on your
> x86 conversion.
>
Sorry for such mistake. I will check them one by one.

> I'll stop my review here, as it sounds pointless to review it,
> as there are too many broken whitespace stuff on your
> conversion.
>
> Thanks,
> Mauro

--
Cheers,
Changbin Du