Messages in this thread | | | From | Yury Norov <> | Date | Tue, 26 Jan 2021 14:27:38 -0800 | Subject | Re: [PATCH v3 0/8] support for bitmap (and hence CPU) list "N" abbreviation |
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On Tue, Jan 26, 2021 at 9:12 AM Paul Gortmaker <paul.gortmaker@windriver.com> 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.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 >
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