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    SubjectRe: [PATCH v2 0/4] support for global CPU list abbreviations
    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@gmail.com>
    > Cc: "Paul E. McKenney" <paulmck@kernel.org>
    > Cc: Josh Triplett <josh@joshtriplett.org>
    > Cc: Thomas Gleixner <tglx@linutronix.de>
    > Cc: Ingo Molnar <mingo@kernel.org>
    > Cc: Li Zefan <lizefan@huawei.com>
    >
    > 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
    >

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