[PATCH v2 0/4] support for global CPU list abbreviations

Paul Gortmaker
2020-11-10T04:07:21Z

RFC/v1 ---> v2:

commit #1:
leave one line stub behind for !SMP solving build failures.
Reported by Randy Dunlap and various build bots.

commit #4
manage to remember '\0' char in strlen from one line to the next.
Reported by Colin King.

Original description from v1/RFC below remains unchanged...

---

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-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
Cc: "Paul E. McKenney" <paulmck-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
Cc: Josh Triplett <josh-iaAMLnmF4UmaiuxdJuQwMA@public.gmane.org>
Cc: Thomas Gleixner <tglx-hfZtesqFncYOwBW4kG4KsQ@public.gmane.org>
Cc: Ingo Molnar <mingo-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
Cc: Li Zefan <lizefan-hv44wF8Li93QT0dZR+AlfA@public.gmane.org>

Paul Gortmaker (4):
cpumask: un-inline cpulist_parse for SMP; 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 | 8 ++
kernel/rcu/tree_plugin.h | 13 +-
lib/cpumask.c | 132 ++++++++++++++++++
5 files changed, 165 insertions(+), 12 deletions(-)

--
2.25.1