[PATCH v3 0/8] support for bitmap (and hence CPU) list "N" abbreviation

[Paul Gortmaker]

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.gortmaker@xxxxxxxxxxxxx/

[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 <lizefan@xxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Yury Norov <yury.norov@xxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
Cc: Frederic Weisbecker <fweisbec@xxxxxxxxx>
Cc: Rasmus Villemoes <linux@xxxxxxxxxxxxxxxxxx>
Cc: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>

---

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