[PATCH 0/4] RFC: support for global CPU list abbreviations

[Paul Gortmaker]

The basic objective here was to add support for "nohz_full=8-last" and/or
"rcu_nocbs="4-last" -- essentially introduce "last" as a portable
reference evaluated at boot/runtime for anything using a CPU list.

The thinking behind this, is that people carve off a few early CPUs to
support housekeeping tasks, and perhaps dedicate one to a busy I/O
peripheral, and then the remaining pool of CPUs out to the end are a
part of a commonly configured pool used for the real work the user
cares about.

Extend that logic out to a fleet of machines - some new, and some
nearing EOL, and you've probably got a wide range of core counts to
contend with - even though the early number of cores dedicated to the
system overhead probably doesn't vary.

This change would enable sysadmins to have a common bootarg across all
such systems, and would also avoid any off-by-one fencepost errors that
happen for users who might briefly forget that core counts start at
zero.

Looking around before starting, I noticed RCU already had a short-form
abbreviation "all" -- but if we want to treat CPU lists in a uniform
matter, then tokens shouldn't be implemented at a subsystem level and
hence be subsystem specific; each with their own variations.

So I moved "all" to global use - for boot args, and for cgroups.  Then
I added the inverse "none" and finally, the one I wanted -- "last".

The use of "last" isn't a standalone word like "all" or "none".  It will
be a part of a complete range specification, possibly with CSV separate
ranges, and possibly specified multiple times.  So I had to be a bit
more careful with string matching - and hence un-inlined the parse
function as commit #1 in this series.

But it really is a generic support for "replace token ABC with known at
boot value XYZ" - for example, it would be trivial to extend support to
add "half" as a dynamic token to be replaced with 1/2 the core count,
even though I wouldn't suggest that has a use case like "last" does.

I tested the string matching with a bunch of intentionally badly crafted
strings in a user-space harness, and tested bootarg use with nohz_full
and rcu_nocbs, and also the post-boot cgroup use case as per below:

   root@hackbox:/sys/fs/cgroup/cpuset# mkdir foo
   root@hackbox:/sys/fs/cgroup/cpuset# cd foo
   root@hackbox:/sys/fs/cgroup/cpuset/foo# cat cpuset.cpus
   
   root@hackbox:/sys/fs/cgroup/cpuset/foo# /bin/echo 10-last > cpuset.cpus
   root@hackbox:/sys/fs/cgroup/cpuset/foo# cat cpuset.cpus
   10-15
   root@hackbox:/sys/fs/cgroup/cpuset/foo# /bin/echo all > cpuset.cpus
   root@hackbox:/sys/fs/cgroup/cpuset/foo# cat cpuset.cpus
   0-15
   root@hackbox:/sys/fs/cgroup/cpuset/foo# /bin/echo none > cpuset.cpus
   root@hackbox:/sys/fs/cgroup/cpuset/foo# cat cpuset.cpus
   
   root@hackbox:/sys/fs/cgroup/cpuset/foo#

This was on a 16 core machine with CONFIG_NR_CPUS=16 in .config file.

Note that the two use cases (boot and runtime) are why you see "early"
parameter in the code - I entertained just sticking the string copy on
the stack vs. the early alloc dance, but this felt more correct/robust.
The cgroup and modular code using cpulist_parse() are runtime cases.

---

Cc: Frederic Weisbecker <fweisbec@xxxxxxxxx>
Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
Cc: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Li Zefan <lizefan@xxxxxxxxxx>

Paul Gortmaker (4):
  cpumask: un-inline cpulist_parse; prepare for ascii helpers
  cpumask: make "all" alias global and not just RCU
  cpumask: add a "none" alias to complement "all"
  cpumask: add "last" alias for cpu list specifications

 .../admin-guide/kernel-parameters.rst         |  20 +++
 .../admin-guide/kernel-parameters.txt         |   4 +-
 include/linux/cpumask.h                       |  12 +-
 kernel/rcu/tree_plugin.h                      |  13 +-
 lib/cpumask.c                                 | 132 ++++++++++++++++++
 5 files changed, 158 insertions(+), 23 deletions(-)

-- 
2.25.1