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 [6/8] 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. --- [v1: https://lore.kernel.org/lkml/20210106004850.GA11682@paulmck-ThinkPad-P72/ [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/ [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. ] 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 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_getnum: separate arg into region and field 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 | 2 + .../admin-guide/kernel-parameters.txt | 4 +- kernel/rcu/tree_plugin.h | 6 +-- lib/bitmap.c | 46 ++++++++++-------- lib/test_bitmap.c | 48 ++++++++++++++++--- 5 files changed, 72 insertions(+), 34 deletions(-) -- 2.17.1
