On Mon, Feb 22, 2021 at 05:05:06PM -0800, Yury Norov wrote:
> 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 <yury.no...@gmail.com>
Applied, thank you all!
Thanx, 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
