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Message-ID: <aJcrzjAz6VafUsqz@archie.me>
Date: Sat, 9 Aug 2025 18:06:54 +0700
From: Bagas Sanjaya <bagasdotme@...il.com>
To: Frederic Weisbecker <frederic@...nel.org>,
LKML <linux-kernel@...r.kernel.org>
Cc: Anna-Maria Behnsen <anna-maria@...utronix.de>,
Gabriele Monaco <gmonaco@...hat.com>,
Ingo Molnar <mingo@...nel.org>, Jonathan Corbet <corbet@....net>,
Marcelo Tosatti <mtosatti@...hat.com>,
Marco Crivellari <marco.crivellari@...e.com>,
Michal Hocko <mhocko@...nel.org>,
"Paul E . McKenney" <paulmck@...nel.org>,
Peter Zijlstra <peterz@...radead.org>, Phil Auld <pauld@...hat.com>,
Steven Rostedt <rostedt@...dmis.org>,
Thomas Gleixner <tglx@...utronix.de>,
Valentin Schneider <vschneid@...hat.com>,
Vlastimil Babka <vbabka@...e.cz>, Waiman Long <longman@...hat.com>,
linux-doc@...r.kernel.org
Subject: Re: [PATCH] doc: Add CPU Isolation documentation
On Sat, Aug 09, 2025 at 11:42:47AM +0200, Frederic Weisbecker wrote:
> +- :ref:`Documentation/admin-guide/cgroup-v2.rst <Cpuset v2 "isolated"
> + partitions>`
> + are recommended because they are tunable at runtime.
Anchor link target and name are mistakenly swapped so I have to correct them
back to their appropriate places:
---- >8 ----
diff --git a/Documentation/admin-guide/cpu-isolation.rst b/Documentation/admin-guide/cpu-isolation.rst
index 250027acf7b26f..aef0b53b0ad5e6 100644
--- a/Documentation/admin-guide/cpu-isolation.rst
+++ b/Documentation/admin-guide/cpu-isolation.rst
@@ -107,9 +107,8 @@ are extracted from the global load balancing.
Interface
~~~~~~~~~
-- :ref:`Documentation/admin-guide/cgroup-v2.rst <Cpuset v2 "isolated"
- partitions>`
- are recommended because they are tunable at runtime.
+- :doc:`cgroup cpuset isolated partitions <cgroup-v2>` are recommended because
+ they are tunable at runtime.
- The 'isolcpus=' kernel boot parameter with the 'domain' flag is a
less flexible alternative that doesn't allow for runtime
@@ -124,7 +123,8 @@ target CPUs.
Interface
~~~~~~~~~
-- /proc/irq/*/smp_affinity as explained :ref:`Documentation/core-api/irq/irq-affinity.rst <here>` in detail.
+- /proc/irq/\*/smp_affinity as explained in
+ Documentation/core-api/irq/irq-affinity.rst.
- The "irqaffinity=" kernel boot parameter for a default setting.
@@ -330,9 +330,8 @@ retained when that happens.
Some tools may also be useful for higher level analysis:
-- :ref:`Documentation/tools/rtla/rtla-osnoise.rst <rtla-osnoise>` runs a kernel
- tracer that analyzes and output a
- summary of the noises.
+- :doc:`rtla-osnoise </tools/rtla/rtla-osnoise>` runs a kernel tracer that
+ analyzes and output a summary of the noises.
- dynticks-testing does something similar but in userspace. It is available
at git://git.kernel.org/pub/scm/linux/kernel/git/frederic/dynticks-testing.git
> +The full command line is then:
> +
> + nohz_full=7 irqaffinity=0-6 isolcpus=managed_irq,7 nosmt
> +
> +CPUSET configuration (cgroup v2)
> +--------------------------------
> +
> +Assuming cgroup v2 is mounted to /sys/fs/cgroup, the following script
> +isolates CPU 7 from scheduler domains.
> +
> + cd /sys/fs/cgroup
> + # Activate the cpuset subsystem
> + echo +cpuset > cgroup.subtree_control
> + # Create partition to be isolated
> + mkdir test
> + cd test
> + echo +cpuset > cgroup.subtree_control
> + # Isolate CPU 7
> + echo 7 > cpuset.cpus
> + echo "isolated" > cpuset.cpus.partition
> +
> +The userspace workload
> +----------------------
> +
> +Fake a pure userspace workload, the below program runs a dummy
> +userspace loop on the isolated CPU 7.
> +
> + #include <stdio.h>
> + #include <fcntl.h>
> + #include <unistd.h>
> + #include <errno.h>
> + int main(void)
> + {
> + // Move the current task to the isolated cpuset (bind to CPU 7)
> + int fd = open("/sys/fs/cgroup/test/cgroup.procs", O_WRONLY);
> + if (fd < 0) {
> + perror("Can't open cpuset file...\n");
> + return 0;
> + }
> +
> + write(fd, "0\n", 2);
> + close(fd);
> +
> + // Run an endless dummy loop until the launcher kills us
> + while (1)
> + ;
> +
> + return 0;
> + }
> +
> +Build it and save for later step:
> +
> + # gcc user_loop.c -o user_loop
> +
> +The launcher
> +------------
> +
> +The below launcher runs the above program for 10 seconds and traces
> +the noise resulting from preempting tasks and IRQs.
> +
> + TRACING=/sys/kernel/tracing/
> + # Make sure tracing is off for now
> + echo 0 > $TRACING/tracing_on
> + # Flush previous traces
> + echo > $TRACING/trace
> + # Record disturbance from other tasks
> + echo 1 > $TRACING/events/sched/sched_switch/enable
> + # Record disturbance from interrupts
> + echo 1 > $TRACING/events/irq_vectors/enable
> + # Now we can start tracing
> + echo 1 > $TRACING/tracing_on
> + # Run the dummy user_loop for 10 seconds on CPU 7
> + ./user_loop &
> + USER_LOOP_PID=$!
> + sleep 10
> + kill $USER_LOOP_PID
> + # Disable tracing and save traces from CPU 7 in a file
> + echo 0 > $TRACING/tracing_on
> + cat $TRACING/per_cpu/cpu7/trace > trace.7
> +
> +If no specific problem arose, the output of trace.7 should look like
> +the following:
> +
> + <idle>-0 [007] d..2. 1980.976624: sched_switch: prev_comm=swapper/7 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=user_loop next_pid=1553 next_prio=120
> + user_loop-1553 [007] d.h.. 1990.946593: reschedule_entry: vector=253
> + user_loop-1553 [007] d.h.. 1990.946593: reschedule_exit: vector=253
Wrap these snippets above in literal code blocks.
Thanks.
--
An old man doll... just what I always wanted! - Clara
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