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SubjectRe: [PATCH v5 0/8] support for bitmap (and hence CPU) list "N" abbreviation
On Sun, Feb 21, 2021 at 03:08:19AM -0500, Paul Gortmaker wrote:
> 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.
> ---

Acked-by: Yury Norov <>

> [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.]
> [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. ]
> [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.]
> [v1:
> Cc: Li Zefan <>
> Cc: Ingo Molnar <>
> Cc: Yury Norov <>
> Cc: Thomas Gleixner <>
> Cc: Josh Triplett <>
> Cc: Peter Zijlstra <>
> Cc: "Paul E. McKenney" <>
> Cc: Frederic Weisbecker <>
> Cc: Rasmus Villemoes <>
> Cc: Andy Shevchenko <>
> 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

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