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@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 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
