lkml.org 
[lkml]   [2021]   [Jan]   [26]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    From
    Subject[PATCH v3 0/8] support for bitmap (and hence CPU) list "N" abbreviation
    Date
    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@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 <lizefan@huawei.com>
    Cc: Ingo Molnar <mingo@kernel.org>
    Cc: Yury Norov <yury.norov@gmail.com>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Cc: Josh Triplett <josh@joshtriplett.org>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: "Paul E. McKenney" <paulmck@kernel.org>
    Cc: Frederic Weisbecker <fweisbec@gmail.com>
    Cc: Rasmus Villemoes <linux@rasmusvillemoes.dk>
    Cc: Andy Shevchenko <andriy.shevchenko@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

    \
     
     \ /
      Last update: 2021-01-27 02:46    [W:4.506 / U:0.900 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site