From: Paul Gortmaker <paul.gortmaker@windriver.com>
To: linux-kernel@vger.kernel.org
Cc: cgroups@vger.kernel.org,
Paul Gortmaker <paul.gortmaker@windriver.com>,
Frederic Weisbecker <fweisbec@gmail.com>,
"Paul E. McKenney" <paulmck@kernel.org>,
Josh Triplett <josh@joshtriplett.org>,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@kernel.org>, Li Zefan <lizefan@huawei.com>
Subject: [PATCH 0/4] RFC: support for global CPU list abbreviations
Date: Sun, 8 Nov 2020 11:08:12 -0500 [thread overview]
Message-ID: <20201108160816.896881-1-paul.gortmaker@windriver.com> (raw)
The basic objective here was to add support for "nohz_full=8-last" and/or
"rcu_nocbs="4-last" -- essentially introduce "last" as a portable
reference evaluated at boot/runtime 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.
Looking around before starting, I noticed RCU already had a short-form
abbreviation "all" -- but if we want to treat CPU lists in a uniform
matter, then tokens shouldn't be implemented at a subsystem level and
hence be subsystem specific; each with their own variations.
So I moved "all" to global use - for boot args, and for cgroups. Then
I added the inverse "none" and finally, the one I wanted -- "last".
The use of "last" isn't a standalone word like "all" or "none". It will
be a part of a complete range specification, possibly with CSV separate
ranges, and possibly specified multiple times. So I had to be a bit
more careful with string matching - and hence un-inlined the parse
function as commit #1 in this series.
But it really is a generic support for "replace token ABC with known at
boot value XYZ" - for example, it would be trivial to extend support to
add "half" as a dynamic token to be replaced with 1/2 the core count,
even though I wouldn't suggest that has a use case like "last" does.
I tested the string matching with a bunch of intentionally badly crafted
strings in a user-space harness, and tested bootarg use with nohz_full
and rcu_nocbs, and also the post-boot cgroup use case as per below:
root@hackbox:/sys/fs/cgroup/cpuset# mkdir foo
root@hackbox:/sys/fs/cgroup/cpuset# cd foo
root@hackbox:/sys/fs/cgroup/cpuset/foo# cat cpuset.cpus
root@hackbox:/sys/fs/cgroup/cpuset/foo# /bin/echo 10-last > cpuset.cpus
root@hackbox:/sys/fs/cgroup/cpuset/foo# cat cpuset.cpus
10-15
root@hackbox:/sys/fs/cgroup/cpuset/foo# /bin/echo all > cpuset.cpus
root@hackbox:/sys/fs/cgroup/cpuset/foo# cat cpuset.cpus
0-15
root@hackbox:/sys/fs/cgroup/cpuset/foo# /bin/echo none > cpuset.cpus
root@hackbox:/sys/fs/cgroup/cpuset/foo# cat cpuset.cpus
root@hackbox:/sys/fs/cgroup/cpuset/foo#
This was on a 16 core machine with CONFIG_NR_CPUS=16 in .config file.
Note that the two use cases (boot and runtime) are why you see "early"
parameter in the code - I entertained just sticking the string copy on
the stack vs. the early alloc dance, but this felt more correct/robust.
The cgroup and modular code using cpulist_parse() are runtime cases.
---
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Li Zefan <lizefan@huawei.com>
Paul Gortmaker (4):
cpumask: un-inline cpulist_parse; prepare for ascii helpers
cpumask: make "all" alias global and not just RCU
cpumask: add a "none" alias to complement "all"
cpumask: add "last" alias for cpu list specifications
.../admin-guide/kernel-parameters.rst | 20 +++
.../admin-guide/kernel-parameters.txt | 4 +-
include/linux/cpumask.h | 12 +-
kernel/rcu/tree_plugin.h | 13 +-
lib/cpumask.c | 132 ++++++++++++++++++
5 files changed, 158 insertions(+), 23 deletions(-)
--
2.25.1
next reply other threads:[~2020-11-08 16:08 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-11-08 16:08 Paul Gortmaker [this message]
2020-11-08 16:08 ` [PATCH 1/4] cpumask: un-inline cpulist_parse; prepare for ascii helpers Paul Gortmaker
2020-11-08 16:08 ` [PATCH 2/4] cpumask: make "all" alias global and not just RCU Paul Gortmaker
2020-11-08 16:08 ` [PATCH 3/4] cpumask: add a "none" alias to complement "all" Paul Gortmaker
2020-11-08 16:08 ` [PATCH 4/4] cpumask: add "last" alias for cpu list specifications Paul Gortmaker
2020-11-08 18:02 ` [PATCH 0/4] RFC: support for global CPU list abbreviations Paul E. McKenney
2020-11-08 20:21 ` Paul Gortmaker
2020-11-08 21:24 ` Paul E. McKenney
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