This is the 5th and final version of this series. We got some good improvements, like adding self-tests, using "N" as "just another number" that could be used anywhere, and making things not CPU specific.
But now it is time to close this review out since is down to just hand-wringing over hypothetical use cases, bikeshedding on upper/lower case, and a wild goose chase on trying to avoid adding a function arg. So, once again - thanks to all who provided input; it was all considered even if not all of it was used. And in that vein, just to be clear: 1) There will be no adaptive modifying or guessing what the user meant if a range turns out to be invalid. The caller will be responsible for handling the -EINVAL just as things are currently today. 2) There will be no use of "L" or lower case "n" because there is simply no need for it. Yes, it would be simple enough to add, but it complicates things and would also be impossible to remove later, once it went mainline. The original text from v4 follows: The basic objective here was to add support for "nohz_full=8-N" and/or "rcu_nocbs="4-N" -- essentially introduce "N" as a portable reference to the last core, evaluated at boot 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. Originally I did this at the CPU subsys level, but Yury suggested it be moved down further to bitmap level itself, which made the core implementation smaller and less complex, but the series longer. New self tests are added to better exercise what bitmap range/region currently supports, and new tests are added for the new "N" support. Also tested boot arg and the post-boot cgroup use case as per below: root@hackbox:~# cat /proc/cmdline BOOT_IMAGE=/boot/bzImage root=/dev/sda1 rcu_nocbs=2,3,8-N:1/2 root@hackbox:~# dmesg|grep Offl rcu: Offload RCU callbacks from CPUs: 2-3,8,10,12,14. root@hackbox:/sys/fs/cgroup/cpuset/foo# cat cpuset.cpus root@hackbox:/sys/fs/cgroup/cpuset/foo# /bin/echo 10-N > cpuset.cpus root@hackbox:/sys/fs/cgroup/cpuset/foo# cat cpuset.cpus 10-15 root@hackbox:/sys/fs/cgroup/cpuset/foo# /bin/echo N-N:N/N > cpuset.cpus root@hackbox:/sys/fs/cgroup/cpuset/foo# cat cpuset.cpus 15 This was on a 16 core machine with CONFIG_NR_CPUS=16 in .config file. Note that "N" is a dynamic quantity, and can change scope if the bitmap is changed in size. So at the risk of stating the obvious, don't use it for "burn_eFuse=128-N" or "secure_erase_firmware=32-N" type stuff. Paul. --- [v5: go back to v3 location of "nbits" in region. Add acks/reviewed.] [v4: pair nbits with region, instead of inside it. Split EINVAL and ERANGE tests. Don't handle start/end/offset within a macro to abstract away nbits usage. Added some Reviwed-by/Ack tags.] https://lore.kernel.org/lkml/20210209225907.78405-1-paul.gortma...@windriver.com/ [v3: Allow "N" to be used anywhere in the region spec, i.e. "N-N:N/N" vs. just being allowed at end of range like "0-N". Add new self-tests. Drop "all" and "none" aliases as redundant and not worth the extra complication. ] https://lore.kernel.org/lkml/20210126171141.122639-1-paul.gortma...@windriver.com/ [v2: push code down from cpu subsys to core bitmap code as per Yury's comments. Change "last" to simply be "N" as per PeterZ.] https://lore.kernel.org/lkml/20210121223355.59780-1-paul.gortma...@windriver.com/ [v1: https://lore.kernel.org/lkml/20210106004850.GA11682@paulmck-ThinkPad-P72/ Cc: Li Zefan <lize...@huawei.com> Cc: Ingo Molnar <mi...@kernel.org> Cc: Yury Norov <yury.no...@gmail.com> Cc: Thomas Gleixner <t...@linutronix.de> Cc: Josh Triplett <j...@joshtriplett.org> Cc: Peter Zijlstra <pet...@infradead.org> Cc: "Paul E. McKenney" <paul...@kernel.org> Cc: Frederic Weisbecker <fweis...@gmail.com> Cc: Rasmus Villemoes <li...@rasmusvillemoes.dk> Cc: Andy Shevchenko <andriy.shevche...@linux.intel.com> Paul Gortmaker (8): lib: test_bitmap: clearly separate ERANGE from EINVAL tests. lib: test_bitmap: add tests to trigger ERANGE case. lib: test_bitmap: add more start-end:offset/len tests lib: bitmap: fold nbits into region struct lib: bitmap: move ERANGE check from set_region to check_region lib: bitmap: support "N" as an alias for size of bitmap lib: test_bitmap: add tests for "N" alias rcu: deprecate "all" option to rcu_nocbs= .../admin-guide/kernel-parameters.rst | 7 +++ .../admin-guide/kernel-parameters.txt | 4 +- kernel/rcu/tree_plugin.h | 6 +-- lib/bitmap.c | 49 +++++++++++-------- lib/test_bitmap.c | 46 ++++++++++++++--- 5 files changed, 79 insertions(+), 33 deletions(-) -- 2.30.0