On Tue, Jan 26, 2021 at 9:12 AM Paul Gortmaker
wrote:
>
> 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.
I think it's worth moving this sentence to the Documentation. Another
caveat with
N is that users' config may surprisingly become invalid, like if user
says 32-N, and
on some machine with a smaller bitmap this config fails to boot.
It doesn't mean of course that I'm against 'N'. I think it's very
useful especially in
such common cases like "N", "0-N", "1-N".
Would it make sense to treat the mask "32-N" when N < 32 as N-N,
and bark something in dmesg?
> 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
> 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 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
>
