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Date:   Tue, 23 Feb 2021 17:54:38 -0800
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Yury Norov <yury.norov@gmail.com>
Cc:     Paul Gortmaker <paul.gortmaker@windriver.com>,
        linux-kernel@vger.kernel.org, Li Zefan <lizefan@huawei.com>,
        Ingo Molnar <mingo@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Josh Triplett <josh@joshtriplett.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Rasmus Villemoes <linux@rasmusvillemoes.dk>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Subject: Re: [PATCH v5 0/8] support for bitmap (and hence CPU) list "N"
 abbreviation
Message-ID: <20210224015438.GG2743@paulmck-ThinkPad-P72>
Reply-To: paulmck@kernel.org
References: <20210221080827.84862-1-paul.gortmaker@windriver.com>
 <20210223010506.GA1041464@yury-ThinkPad>
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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.norov@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.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. ]
> >  https://lore.kernel.org/lkml/20210126171141.122639-1-paul.gortmaker@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.gortmaker@windriver.com/
> > 
> > [v1: https://lore.kernel.org/lkml/20210106004850.GA11682@paulmck-ThinkPad-P72/
> > 
> > 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 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