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

Paul E. McKenney
2020-11-10T15:32:11Z

On Mon, Nov 09, 2020 at 11:07:21PM -0500, Paul Gortmaker wrote:
> 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...

Queued and this time kicking off testing that actually includes your
patches! ;-)

Thanx, Paul

> ---
>
> 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
>