doc: Add CPU Isolation documentation

(quoted diff) +76 -0

+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

Review

Quoted 3 lines · Frederic Weisbecker

+- :ref:`Documentation/admin-guide/cgroup-v2.rst <Cpuset v2 "isolated"

(quoted diff) +2 -0

+  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 ----

Documentation/admin-guide/cpu-isolation.rst +6 -7

index 250027acf7b26f..aef0b53b0ad5e6 100644
@@ -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

Quoted 77 lines

(quoted diff) +76 -0

+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

Review

Quoted 10 lines

(quoted diff) +10 -0

+  # 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.

nohz_full was introduced in v3.10 in 2013, which means this documentation is overdue for 12 years.

The shoemaker's children always go barefoot. And working on timers hasn't made me arriving on time either.

Fortunately Paul wrote a part of the needed documentation a while ago, especially concerning nohz_full in Documentation/timers/no_hz.rst and also about per-CPU kthreads in Documentation/admin-guide/kernel-per-CPU-kthreads.rst

Introduce a new page that gives an overview of CPU isolation in general.

Signed-off-by Frederic Weisbecker <[email protected]>

---
 Documentation/admin-guide/cpu-isolation.rst | 338 ++++++++++++++++++++
 Documentation/admin-guide/index.rst         |   1 +
 2 files changed, 339 insertions(+)
 create mode 100644 Documentation/admin-guide/cpu-isolation.rst

Documentation/admin-guide/cpu-isolation.rst new file +0 -0

new file mode 100644
index 000000000000..250027acf7b2

Documentation/admin-guide/cpu-isolation.rst new file +15 -0

+As a side effect the CPU is also isolated from unbound workqueues and
+unbound kthreads.
 
+Checklist
+=========
+
+You have set up each of the above isolation features but you still
+observe jitters that trash your workload? Make sure to check a few
+elements before proceeding.
+
+Some of these checklist items are similar to those of real time
+workloads:
+
+- Use mlock() to prevent your pages from being swapped away. Page
+  faults are usually not compatible with jitter sensitive workloads.
+

+- dynticks-testing does something similar but in userspace. It is available

Documentation/admin-guide/cpu-isolation.rst new file +1 -0

+  at git://git.kernel.org/pub/scm/linux/kernel/git/frederic/dynticks-testing.git

Documentation/admin-guide/index.rst +1 -1

index 259d79fbeb94..b5f1fc7d5290 100644
@@ -94,6 +94,7 @@ likely to be of interest on almost any system.
 
    cgroup-v2
    cgroup-v1/index
+   cpu-isolation
    cpu-load
    mm/index
    module-signing
--

2.50.1

Quoted 3 lines · Frederic Weisbecker

nohz_full was introduced in v3.10 in 2013, which means this documentation is overdue for 12 years.

12 years is not that bad, it's not old enough to drink (legally) yet!

Quoted 24 lines

The shoemaker's children always go barefoot. And working on timers hasn't made me arriving on time either.

Fortunately Paul wrote a part of the needed documentation a while ago, especially concerning nohz_full in Documentation/timers/no_hz.rst and also about per-CPU kthreads in Documentation/admin-guide/kernel-per-CPU-kthreads.rst

Introduce a new page that gives an overview of CPU isolation in general.

Signed-off-by Frederic Weisbecker <[email protected]>

---
 Documentation/admin-guide/cpu-isolation.rst | 338 ++++++++++++++++++++
 Documentation/admin-guide/index.rst         |   1 +
 2 files changed, 339 insertions(+)
 create mode 100644 Documentation/admin-guide/cpu-isolation.rst

Documentation/admin-guide/cpu-isolation.rst new file +0 -0

new file mode 100644
index 000000000000..250027acf7b2

Documentation/admin-guide/cpu-isolation.rst line 8

Quoted 8 lines

Documentation/admin-guide/cpu-isolation.rst new file +8 -0

@@ -0,0 +1,338 @@
+=============
+CPU Isolation
+=============
+
+Introduction
+============
+
+"CPU Isolation" means leaving a CPU exclusive to a given userspace

                                                            ^^^^^^^^^

Eh I'm being nitpicky, but this doesn't have to be userspace stuff right? "someone" could e.g. affine some IRQ to an isolated CPU to have the irqthread run undisturbed there, or somesuch.

Documentation/admin-guide/cpu-isolation.rst line 91

Quoted 7 lines

Documentation/admin-guide/cpu-isolation.rst new file +7 -0

+
+Scheduler domain isolation
+--------------------------
+
+This feature isolates a CPU from the scheduler topology. As a result,
+the target isn't part of the load balancing. Tasks won't migrate
+neither from nor to it unless affine explicitly.

                                 ^^^^^^

s/affine/affined/

Quoted 23 lines

Documentation/admin-guide/cpu-isolation.rst new file +21 -0

+As a side effect the CPU is also isolated from unbound workqueues and
+unbound kthreads.
 
+Checklist
+=========
+
+You have set up each of the above isolation features but you still
+observe jitters that trash your workload? Make sure to check a few
+elements before proceeding.
+
+Some of these checklist items are similar to those of real time
+workloads:
+
+- Use mlock() to prevent your pages from being swapped away. Page
+  faults are usually not compatible with jitter sensitive workloads.
+
+- Avoid SMT to prevent your hardware thread from being "preempted"
+  by another one.
+
+- CPU frequency changes may induce subtle sorts of jitter in a
+  workload. Cpufreq should be used and tuned with caution.
+

Documentation/admin-guide/cpu-isolation.rst line 206

Quoted 4 lines

+- Deep C-states may result in latency issues upon wake-up. If this

Documentation/admin-guide/cpu-isolation.rst new file +3 -0

+  happens to be a problem, C-states can be limited via kernel boot
+  parameters such as processor.max_cstate or intel_idle.max_cstate.
+

Nitpickery again, I know it's not an exhaustive listing, but I'd rather point to the sysfs cpuidle interface (or just mention it too), since that means deep C-states can be left enabled for HK CPUs.

Should we also mention BIOS/firmware fuckery like SMIs?

"""
- Your system may be subject to firmware-originating interrupts - x86 has System Management Interrupts (SMIs) for example. Check your system BIOS to disable such interference, and with some luck your vendor will have a BIOS tuning guidance for low-latency operations.
"""

Quoted 4 lines

Documentation/admin-guide/cpu-isolation.rst new file +3 -0

+Debugging
+=========
+

Documentation/admin-guide/cpu-isolation.rst line 330

Quoted 10 lines

Documentation/admin-guide/cpu-isolation.rst new file +10 -0

+Of course things are never so easy, especially on this matter.
+Chances are that actual noise will be observed in the aforementioned
+trace.7 file.
+
+The best way to investigate further is to enable finer grained
+tracepoints such as those of subsystems producing asynchronous
+events: workqueue, timer, irq_vector, etc... It also can be
+interesting to enable the tick_stop event to diagnose why the tick is
+retained when that happens.
+

I'd also list the 'ipi_send*' family, although that's emitted from the HK CPU, not the disturbed isolated CPU.

Documentation/admin-guide/cpu-isolation.rst line 336

Quoted 7 lines

Documentation/admin-guide/cpu-isolation.rst new file +6 -0

+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.
+

I'd want to point to hwnoise and timerlat as well, so maybe point to rtla.rst?

Quoted 17 lines

+- dynticks-testing does something similar but in userspace. It is available

Documentation/admin-guide/cpu-isolation.rst new file +1 -0

+  at git://git.kernel.org/pub/scm/linux/kernel/git/frederic/dynticks-testing.git

Documentation/admin-guide/index.rst +1 -1

index 259d79fbeb94..b5f1fc7d5290 100644
@@ -94,6 +94,7 @@ likely to be of interest on almost any system.
 
    cgroup-v2
    cgroup-v1/index
+   cpu-isolation
    cpu-load
    mm/index
    module-signing
--

2.50.1

nohz_full was introduced in v3.10 in 2013, which means this documentation is overdue for 12 years.

The shoemaker's children always go barefoot. And working on timers hasn't made me arriving on time either.

Fortunately Paul wrote a part of the needed documentation a while ago, especially concerning nohz_full in Documentation/timers/no_hz.rst and also about per-CPU kthreads in Documentation/admin-guide/kernel-per-CPU-kthreads.rst

Introduce a new page that gives an overview of CPU isolation in general.

Signed-off-by Frederic Weisbecker <[email protected]>

---
  Documentation/admin-guide/cpu-isolation.rst | 338 ++++++++++++++++++++
  Documentation/admin-guide/index.rst         |   1 +
  2 files changed, 339 insertions(+)

create mode 100644 Documentation/admin-guide/cpu-isolation.rst

Documentation/admin-guide/cpu-isolation.rst new file +11 -0

new file mode 100644
index 000000000000..250027acf7b2
@@ -0,0 +1,338 @@
+=============
+CPU Isolation
+=============
+
+Introduction
+============
+
+"CPU Isolation" means leaving a CPU exclusive to a given userspace
+workload without any undesired code interference from the kernel.
+
+Those interferences, commonly pointed out as "noise", can be triggered

Quoted 6 lines · Frederic Weisbecker

nohz_full was introduced in v3.10 in 2013, which means this documentation is overdue for 12 years.

The shoemaker's children always go barefoot. And working on timers hasn't made me arriving on time either.

Quoted 8 lines

Fortunately Paul wrote a part of the needed documentation a while ago, especially concerning nohz_full in Documentation/timers/no_hz.rst and also about per-CPU kthreads in Documentation/admin-guide/kernel-per-CPU-kthreads.rst

Introduce a new page that gives an overview of CPU isolation in general.

Signed-off-by Frederic Weisbecker <[email protected]>

Thanks for working on this document. It will be really useful for those customers that need CPU isolation.

Quoted 28 lines

---
  Documentation/admin-guide/cpu-isolation.rst | 338 ++++++++++++++++++++
  Documentation/admin-guide/index.rst         |   1 +
  2 files changed, 339 insertions(+)

create mode 100644 Documentation/admin-guide/cpu-isolation.rst

Documentation/admin-guide/cpu-isolation.rst new file +16 -0

new file mode 100644
index 000000000000..250027acf7b2
@@ -0,0 +1,338 @@
+=============
+CPU Isolation
+=============
+
+Introduction
+============
+
+"CPU Isolation" means leaving a CPU exclusive to a given userspace
+workload without any undesired code interference from the kernel.
+
+Those interferences, commonly pointed out as "noise", can be triggered
+by asynchronous events (interrupts, timers, scheduler preemption by
+workqueues and kthreads, ...) or synchronous events (syscalls and page
+faults).
+
+Such noise usually goes unnoticed. After all synchronous events are a

Documentation/admin-guide/cpu-isolation.rst line 21

Quoted 5 lines

Documentation/admin-guide/cpu-isolation.rst new file +5 -0

+component of the requested kernel service. And asynchronous events are
+either sufficiently well distributed by the scheduler when executed
+as tasks or reasonably fast when executed as interrupt. The timer
+interrupt can even execute 1024 times per seconds without
+significantly measurable impact most of the time.

"significantly measurable" looks odd to me. Maybe "without a significant and measurable impact most of the time".

Documentation/admin-guide/cpu-isolation.rst line 174

Quoted 13 lines

Documentation/admin-guide/cpu-isolation.rst new file +12 -0

+
+Tradeoffs
+~~~~~~~~~
+
+In terms of cost, this is the most invasive isolation feature. It is
+assumed to be used when the workload spends most of its time in
+userspace and doesn't rely on the kernel except for preparatory
+work because:
+
+- RCU is slower due to the locked, offloaded and threaded callbacks
+  processing (the same that would be obtained with "rcu_nocb=" boot
+  parameter).

Should be "rcu_nocbs". The '=' is optional.

Cheers,
Longman

Documentation/admin-guide/cpu-isolation.rst new file +3 -0

+=============
+CPU Isolation
+=============

Documentation/admin-guide/cpu-isolation.rst new file +13 -0

+Checklist
+=========
+
+You have set up each of the above isolation features but you still
+observe jitters that trash your workload? Make sure to check a few
+elements before proceeding.
+
+Some of these checklist items are similar to those of real time
+workloads:
+
+- Use mlock() to prevent your pages from being swapped away. Page
+  faults are usually not compatible with jitter sensitive workloads.
+

Documentation/admin-guide/cpu-isolation.rst

Quoted 3 lines · Frederic Weisbecker

Documentation/admin-guide/cpu-isolation.rst new file +0 -0

@@ -0,0 +1,338 @@

You could start with

.. SPDX-License-Identifier: GPL-2.0

at the top.

Quoted 4 lines

Documentation/admin-guide/cpu-isolation.rst new file +3 -0

+=============
+CPU Isolation
+=============

…

Documentation/admin-guide/cpu-isolation.rst line 111

Quoted 5 lines

Documentation/admin-guide/cpu-isolation.rst new file +5 -0

+Interface
+~~~~~~~~~
+
+- :ref:`Documentation/admin-guide/cgroup-v2.rst <Cpuset v2 "isolated"
+  partitions>`

I've been told by Jonathan once to just use the .rst file without anything around it and the HTML render will make the link on its own.

…

Documentation/admin-guide/cpu-isolation.rst line 174

Quoted 12 lines

Documentation/admin-guide/cpu-isolation.rst new file +11 -0

+Tradeoffs
+~~~~~~~~~
+
+In terms of cost, this is the most invasive isolation feature. It is
+assumed to be used when the workload spends most of its time in
+userspace and doesn't rely on the kernel except for preparatory
+work because:
+
+- RCU is slower due to the locked, offloaded and threaded callbacks
+  processing (the same that would be obtained with "rcu_nocb=" boot
+  parameter).

You mean the callback invocation is delayed? It shouldn't affect grace period handling and so on.

…

Quoted 21 lines

Documentation/admin-guide/cpu-isolation.rst new file +20 -0

+Checklist
+=========
+
+You have set up each of the above isolation features but you still
+observe jitters that trash your workload? Make sure to check a few
+elements before proceeding.
+
+Some of these checklist items are similar to those of real time
+workloads:
+
+- Use mlock() to prevent your pages from being swapped away. Page
+  faults are usually not compatible with jitter sensitive workloads.
+
+- Avoid SMT to prevent your hardware thread from being "preempted"
+  by another one.
+
+- CPU frequency changes may induce subtle sorts of jitter in a
+  workload. Cpufreq should be used and tuned with caution.
+
+- Deep C-states may result in latency issues upon wake-up. If this

Documentation/admin-guide/cpu-isolation.rst line 206

Quoted 3 lines

Documentation/admin-guide/cpu-isolation.rst new file +3 -0

+  happens to be a problem, C-states can be limited via kernel boot
+  parameters such as processor.max_cstate or intel_idle.max_cstate.
+

I intended to make a similar list similar to this for the real time part but it somehow faded away. There is now the idea to identify kernel options which are not so optimal which include CPU frequency for instance. The requirements here are should be the same.

Quoted 9 lines

Documentation/admin-guide/cpu-isolation.rst line 111

Quoted 5 lines

Documentation/admin-guide/cpu-isolation.rst new file +5 -0

+Interface
+~~~~~~~~~
+
+- :ref:`Documentation/admin-guide/cgroup-v2.rst <Cpuset v2 "isolated"
+  partitions>`

I've been told by Jonathan once to just use the .rst file without anything around it and the HTML render will make the link on its own.

Quoted 1 line

…

Hmm, how would that look like? Just this?

+- Documentation/admin-guide/cgroup-v2.rst

Quoted 2 lines

Quoted 2 lines

+Checklist
+=========

We should probably have this to a seperate file that both isolation and rt could link to?

Thanks.

Documentation/admin-guide/cpu-isolation.rst new file +0 -0

@@ -0,0 +1,338 @@

Documentation/admin-guide/cpu-isolation.rst new file +6 -0

+CPUSET configuration (cgroup v2)
+--------------------------------
+
+Assuming cgroup v2 is mounted to /sys/fs/cgroup, the following script
+isolates CPU 7 from scheduler domains.
+

On 2025-08-09 11:42:47 [+0200], Frederic Weisbecker wrote:
…

Quoted 3 lines

Documentation/admin-guide/cpu-isolation.rst new file +0 -0

@@ -0,0 +1,338 @@

…

Quoted 6 lines

Documentation/admin-guide/cpu-isolation.rst new file +6 -0

+CPUSET configuration (cgroup v2)
+--------------------------------
+
+Assuming cgroup v2 is mounted to /sys/fs/cgroup, the following script
+isolates CPU 7 from scheduler domains.
+

Documentation/admin-guide/cpu-isolation.rst line 247

Quoted 10 lines

Documentation/admin-guide/cpu-isolation.rst new file +10 -0

+  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

while looking at this again, shouldn't you also do
echo 7 > cpuset.cpus.exclusive

to ensure the CPU isn't used somewhere else?

Quoted 4 lines

while looking at this again, shouldn't you also do
echo 7 > cpuset.cpus.exclusive

to ensure the CPU isn't used somewhere else?

For backport compatibility, the use of cpuset.cpus.exclusive is optional for creating a local partition underneath the cgroup root. The example should still work without setting cpuset.cpus.exclusive.

Cheers,
Longman

Quoted 5 lines · Waiman Long

Quoted 4 lines

while looking at this again, shouldn't you also do
echo 7 > cpuset.cpus.exclusive

to ensure the CPU isn't used somewhere else?

Quoted 3 lines

For backport compatibility, the use of cpuset.cpus.exclusive is optional for creating a local partition underneath the cgroup root. The example should still work without setting cpuset.cpus.exclusive.

I would have to double check but I think only after the cpuset.cpus.exclusive the CPU vanished from the cpumask of my current task.

Quoted 2 lines

Cheers,
Longman