On 10/11/2020 15:32, Paul E. McKenney wrote: > 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. shouldn't that be "Reported and fixed 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@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 >>
