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@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 >
