doc: Add CPU Isolation documentation

nohz_full was introduced in v3.10 in 2013, which means this documentation is overdue for 13 years.

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]>

---
v2:
- Fix links and code blocks (Bagas and Sebastian)
- Isolation is not only about userspace, rephrase accordingly (Valentin)
- Paste BIOS issues suggestion from Valentin
- Include the whole rtla suite (Valentin)
- Rephrase a few details (Waiman)
- Talk about RCU induced overhead rather than slower RCU (Sebastian)

  Documentation/admin-guide/cpu-isolation.rst | 357 ++++++++++++++++++++
  Documentation/admin-guide/index.rst         |   1 +
  2 files changed, 358 insertions(+)

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

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

new file mode 100644
index 000000000000..886dec79b056
@@ -0,0 +1,357 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+=============
+CPU Isolation
+=============
+
+Introduction
+============
+
+"CPU Isolation" means leaving a CPU exclusive to a given 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
+component of the requested kernel service. And asynchronous events are
+either sufficiently well distributed by the scheduler when executed

Documentation/admin-guide/cpu-isolation.rst new file +27 -0

+
+In order to run a CPU without or with limited kernel noise, the
+related housekeeping work needs to be either shutdown, migrated or
+offloaded.
+
+Housekeeping
+============
+
+In the CPU isolation terminology, housekeeping is the work, often
+asynchronous, that the kernel needs to process in order to maintain
+all its services. It matches the noises and disturbances enumerated
+above except when at least one CPU is isolated. Then housekeeping may
+make use of further coping mechanisms if CPU-tied work must be
+offloaded.
+
+Housekeeping CPUs are the non-isolated CPUs where the kernel noise
+is moved away from isolated CPUs.
+
+The isolation can be implemented in several ways depending on the
+nature of the noise:
+
+- Unbound work, where "unbound" means not tied to any CPU, can be
+  simply migrated away from isolated CPUs to housekeeping CPUs.
+  This is the case of unbound workqueues, kthreads and timers.
+
+- Bound work, where "bound" means tied to a specific CPU, usually
+  can't be moved away as-is by nature. Either:

Documentation/admin-guide/cpu-isolation.rst new file +99 -0

+	  with the constraint of running a single task on the CPU. A
+	  significant cost penalty is added on kernel entry/exit and
+	  a residual 1Hz scheduler tick is offloaded to housekeeping
+	  CPUs.
+
+In any case, housekeeping work has to be handled, which is why there
+must be at least one housekeeping CPU in the system, preferrably more
+if the machine runs a lot of CPUs. For example one per node on NUMA
+systems.
+
+Also CPU isolation often means a tradeoff between noise-free isolated
+CPUs and added overhead on housekeeping CPUs, sometimes even on
+isolated CPUs entering the kernel.
+
+Isolation features
+==================
+
+Different levels of isolation can be configured in the kernel, each of
+which having their own drawbacks and tradeoffs.
+
+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 affined explicitly.
+
+As a side effect the CPU is also isolated from unbound workqueues and
+unbound kthreads.
+
+Requirements
+~~~~~~~~~~~~
+
+- CONFIG_CPUSETS=y for the cpusets based interface
+
+Tradeoffs
+~~~~~~~~~
+
+By nature, the system load is overall less distributed since some CPUs
+are extracted from the global load balancing.
+
+Interface
+~~~~~~~~~
+
+- Documentation/admin-guide/cgroup-v2.rst cpuset isolated partitions 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
+  reconfiguration.
+
+IRQs isolation
+--------------
+
+Isolate the IRQs whenever possible, so that they don't fire on the
+target CPUs.
+
+Interface
+~~~~~~~~~
+
+- The file /proc/irq/\*/smp_affinity as explained in detail in
+  Documentation/core-api/irq/irq-affinity.rst page.
+
+- The "irqaffinity=" kernel boot parameter for a default setting.
+
+- The "managed_irq" flag in the "isolcpus=" kernel boot parameter
+  tries a best effort affinity override for managed IRQs.
+
+Full Dynticks (aka nohz_full)
+-----------------------------
+
+Full dynticks extends the dynticks idle mode, which stop the tick when
+the CPU is idle, to CPUs running a single task in userspace. That is,
+the timer tick is stopped if the environment allows it.
+
+Global timer callbacks are also isolated from the nohz_full CPUs.
+
+Requirements
+~~~~~~~~~~~~
+
+- CONFIG_NO_HZ_FULL=y
+
+Constraints
+~~~~~~~~~~~
+
+- The isolated CPUs must run a single task only. Multitask requires
+  the tick to maintain preemption. This is usually fine since the
+  workload usually can't stand the latency of random context switches.
+
+- No call to the kernel from isolated CPUs, at the risk of triggering
+  random noise.
+
+- No use of posix CPU timers on isolated CPUs.
+
+- Architecture must have a stable and reliable clocksource (no
+  unreliable TSC that requires the watchdog).
+
+
+Tradeoffs

Documentation/admin-guide/cpu-isolation.rst new file +181 -0

+
+- Kernel entry/exit through syscalls, exceptions and IRQs are more
+  costly due to fully ordered RmW operations that maintain userspace
+  as RCU extended quiescent state. Also the CPU time is accounted on
+  kernel boundaries instead of periodically from the tick.
+
+- Housekeeping CPUs must run a 1Hz residual remote scheduler tick
+  on behalf of the isolated CPUs.
+
+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
+  happens to be a problem, C-states can be limited via kernel boot
+  parameters such as processor.max_cstate or intel_idle.max_cstate.
+  More finegrained tunings are described in
+  Documentation/admin-guide/pm/cpuidle.rst page
+
+- 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.
+
+
+Full isolation example
+======================
+
+In this example, the system has 8 CPUs and the 8th is to be fully
+isolated. Since CPUs start from 0, the 8th CPU is CPU 7.
+
+Kernel parameters
+-----------------
+
+Set the following kernel boot parameters to disable SMT and setup tick
+and IRQ isolation:
+
+- Full dynticks: nohz_full=7
+
+- IRQs isolation: irqaffinity=0-6
+
+- Managed IRQs isolation: isolcpus=managed_irq,7
+
+- Prevent from SMT: nosmt
+
+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
+
+That is, no specific noise triggered between the first trace and the
+second during 10 seconds when user_loop was running.
+
+Debugging
+=========
+
+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.
+
+Some tools may also be useful for higher level analysis:
+
+- Documentation/tools/rtla/rtla.rst provides a suite of tools to analyze
+  latency and noise in the system. For example Documentation/tools/rtla/rtla-osnoise.rst
+  runs a kernel tracer that analyzes and output a summary of the noises.
+
+- dynticks-testing does something similar to rtla-osnoise but in userspace. It is available
+  at git://git.kernel.org/pub/scm/linux/kernel/git/frederic/dynticks-testing.git

Documentation/admin-guide/index.rst +0 -0

index b734f8a2a2c4..cd28dfe91b06 100644

Quoted 51 lines · Frederic Weisbecker

nohz_full was introduced in v3.10 in 2013, which means this documentation is overdue for 13 years.

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]>

---
v2:
- Fix links and code blocks (Bagas and Sebastian)
- Isolation is not only about userspace, rephrase accordingly (Valentin)
- Paste BIOS issues suggestion from Valentin
- Include the whole rtla suite (Valentin)
- Rephrase a few details (Waiman)
- Talk about RCU induced overhead rather than slower RCU (Sebastian)

  Documentation/admin-guide/cpu-isolation.rst | 357 ++++++++++++++++++++
  Documentation/admin-guide/index.rst         |   1 +
  2 files changed, 358 insertions(+)

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

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

new file mode 100644
index 000000000000..886dec79b056
@@ -0,0 +1,357 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+=============
+CPU Isolation
+=============
+
+Introduction
+============
+
+"CPU Isolation" means leaving a CPU exclusive to a given 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
+component of the requested kernel service. And asynchronous events are
+either sufficiently well distributed by the scheduler when executed

Documentation/admin-guide/cpu-isolation.rst line 30

Quoted 10 lines

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

+as tasks or reasonably fast when executed as interrupt. The timer
+interrupt can even execute 1024 times per seconds without a significant
+and measurable impact most of the time.
+
+However some rare and extreme workloads can be quite sensitive to
+those kinds of noise. This is the case, for example, with high
+bandwidth network processing that can't afford losing a single packet
+or very low latency network processing. Typically those usecases
+involve DPDK, bypassing the kernel networking stack and performing
+direct access to the networking device from userscace.

As also pointed by by Sashiko, there is a typo "userscace" -> "userspace". There are also typos reported in

https://sashiko.dev/#/patchset/20260326140055.41555-1-frederic%40kernel.org

Quoted 33 lines

Documentation/admin-guide/cpu-isolation.rst new file +32 -0

+
+In order to run a CPU without or with limited kernel noise, the
+related housekeeping work needs to be either shutdown, migrated or
+offloaded.
+
+Housekeeping
+============
+
+In the CPU isolation terminology, housekeeping is the work, often
+asynchronous, that the kernel needs to process in order to maintain
+all its services. It matches the noises and disturbances enumerated
+above except when at least one CPU is isolated. Then housekeeping may
+make use of further coping mechanisms if CPU-tied work must be
+offloaded.
+
+Housekeeping CPUs are the non-isolated CPUs where the kernel noise
+is moved away from isolated CPUs.
+
+The isolation can be implemented in several ways depending on the
+nature of the noise:
+
+- Unbound work, where "unbound" means not tied to any CPU, can be
+  simply migrated away from isolated CPUs to housekeeping CPUs.
+  This is the case of unbound workqueues, kthreads and timers.
+
+- Bound work, where "bound" means tied to a specific CPU, usually
+  can't be moved away as-is by nature. Either:
+
+	- The work must switch to a locked implementation. Eg: This is
+	  the case of RCU with CONFIG_RCU_NOCB_CPU.
+
+	- The related feature must be shutdown and considered

Documentation/admin-guide/cpu-isolation.rst line 67

Quoted 5 lines

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

+	  incompatible with isolated CPUs. Eg: Lockup watchdog,
+	  unreliable clocksources, etc...
+
+	- An elaborated and heavyweight coping mechanism stands as a
+	  replacement. Eg: the timer tick is shutdown on nohz_full but

"shutdown" should be 2 words as "shutdown" isn't a verb. Should we add CPU after "nohz_full" to make it more clear?

Quoted 10 lines

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

+	  with the constraint of running a single task on the CPU. A
+	  significant cost penalty is added on kernel entry/exit and
+	  a residual 1Hz scheduler tick is offloaded to housekeeping
+	  CPUs.
+

Documentation/admin-guide/index.rst likely to be of interest on almost any system. · line 99

Quoted 6 lines

Documentation/admin-guide/index.rst +1 -0

@@ -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

Other than the minor nits mentioned above,

Acked-by Waiman Long <[email protected]>

Quoted 65 lines · Waiman Long

Quoted 51 lines · Frederic Weisbecker

nohz_full was introduced in v3.10 in 2013, which means this documentation is overdue for 13 years.

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]>

---
v2:
- Fix links and code blocks (Bagas and Sebastian)
- Isolation is not only about userspace, rephrase accordingly (Valentin)
- Paste BIOS issues suggestion from Valentin
- Include the whole rtla suite (Valentin)
- Rephrase a few details (Waiman)
- Talk about RCU induced overhead rather than slower RCU (Sebastian)

   Documentation/admin-guide/cpu-isolation.rst | 357 ++++++++++++++++++++
   Documentation/admin-guide/index.rst         |   1 +
   2 files changed, 358 insertions(+)

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

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

new file mode 100644
index 000000000000..886dec79b056
@@ -0,0 +1,357 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+=============
+CPU Isolation
+=============
+
+Introduction
+============
+
+"CPU Isolation" means leaving a CPU exclusive to a given 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
+component of the requested kernel service. And asynchronous events are
+either sufficiently well distributed by the scheduler when executed

Documentation/admin-guide/cpu-isolation.rst line 30

Quoted 10 lines

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

+as tasks or reasonably fast when executed as interrupt. The timer
+interrupt can even execute 1024 times per seconds without a significant
+and measurable impact most of the time.
+
+However some rare and extreme workloads can be quite sensitive to
+those kinds of noise. This is the case, for example, with high
+bandwidth network processing that can't afford losing a single packet
+or very low latency network processing. Typically those usecases
+involve DPDK, bypassing the kernel networking stack and performing
+direct access to the networking device from userscace.

As also pointed by by Sashiko, there is a typo "userscace" -> "userspace". There are also typos reported in

Quoted 1 line

https://sashiko.dev/#/patchset/20260326140055.41555-1-frederic%40kernel.org

Thanks!

What do you think about these lines of Sashiko's review:

"""
Does this script violate the cgroup v2 "no internal process" constraint? By enabling the cpuset controller on the test directory's cgroup.subtree_control file, the cgroup cannot also contain processes. """

That is confusing me...

nohz_full was introduced in v3.10 in 2013, which means this documentation is overdue for 13 years.

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]>

---
v2:
- Fix links and code blocks (Bagas and Sebastian)
- Isolation is not only about userspace, rephrase accordingly (Valentin)
- Paste BIOS issues suggestion from Valentin
- Include the whole rtla suite (Valentin)
- Rephrase a few details (Waiman)
- Talk about RCU induced overhead rather than slower RCU (Sebastian)

 Documentation/admin-guide/cpu-isolation.rst | 357 ++++++++++++++++++++
 Documentation/admin-guide/index.rst         |   1 +
 2 files changed, 358 insertions(+)
 create mode 100644 Documentation/admin-guide/cpu-isolation.rst

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

new file mode 100644
index 000000000000..886dec79b056
@@ -0,0 +1,357 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+=============

Documentation/admin-guide/cpu-isolation.rst new file +19 -0

+
+In order to run a CPU without or with limited kernel noise, the
+related housekeeping work needs to be either shutdown, migrated or
+offloaded.
+
+Housekeeping
+============
+
+In the CPU isolation terminology, housekeeping is the work, often
+asynchronous, that the kernel needs to process in order to maintain
+all its services. It matches the noises and disturbances enumerated
+above except when at least one CPU is isolated. Then housekeeping may
+make use of further coping mechanisms if CPU-tied work must be
+offloaded.
+
+Housekeeping CPUs are the non-isolated CPUs where the kernel noise
+is moved away from isolated CPUs.
+
+The isolation can be implemented in several ways depending on the

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

+
+- Documentation/admin-guide/cgroup-v2.rst cpuset isolated partitions 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

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

+the CPU is idle, to CPUs running a single task in userspace. That is,
+the timer tick is stopped if the environment allows it.
+
+Global timer callbacks are also isolated from the nohz_full CPUs.
+
+Requirements
+~~~~~~~~~~~~
+
+- CONFIG_NO_HZ_FULL=y
+
+Constraints

Documentation/admin-guide/cpu-isolation.rst new file +23 -0

+
+- Architecture must have a stable and reliable clocksource (no
+  unreliable TSC that requires the watchdog).
+
+
+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 adds more overhead due to the locked, offloaded and threaded
+  callbacks processing (the same that would be obtained with "rcu_nocb"
+  boot parameter).
+
+- Kernel entry/exit through syscalls, exceptions and IRQs are more
+  costly due to fully ordered RmW operations that maintain userspace
+  as RCU extended quiescent state. Also the CPU time is accounted on
+  kernel boundaries instead of periodically from the tick.
+
+- Housekeeping CPUs must run a 1Hz residual remote scheduler tick

Documentation/admin-guide/cpu-isolation.rst new file +32 -0

+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
+  happens to be a problem, C-states can be limited via kernel boot
+  parameters such as processor.max_cstate or intel_idle.max_cstate.
+  More finegrained tunings are described in
+  Documentation/admin-guide/pm/cpuidle.rst page
+
+- 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.
+
+
+Full isolation example
+======================
+
+In this example, the system has 8 CPUs and the 8th is to be fully
+isolated. Since CPUs start from 0, the 8th CPU is CPU 7.
+
+Kernel parameters
+-----------------
+

Documentation/admin-guide/cpu-isolation.rst new file +18 -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

Documentation/admin-guide/cpu-isolation.rst new file +84 -0

+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
+
+That is, no specific noise triggered between the first trace and the
+second during 10 seconds when user_loop was running.
+
+Debugging
+=========
+
+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

Quoted 1 line

+Those interferences, commonly pointed out as "noise", can be triggered

nit: "noise,"

Being read by C programmers, I would keep he ',' out of the quotes as it looks like a string literal "noun,".

Quoted 3 lines · On Thu, 26 Mar 2026 14:42:32 -0700 Randy Dunlap <[email protected]> wrote:

Quoted 1 line

+Those interferences, commonly pointed out as "noise", can be triggered

nit: "noise,"

Really?

Even if it's grammatically correct, it still looks horrible.

Those interferences, commonly pointed out as "noise", can be triggered

vs

Those interferences, commonly pointed out as "noise," can be triggered

As the quotes are going around the noun, and placing the comma into the quotes makes it look like the ',' is part of that noun and not a punctuation of the sentence.

Being read by C programmers, I would keep he ',' out of the quotes as it

looks like a string literal "noun,".

Stephen, all I can do is make recommendations. If it hurts your eyes, tough. ;)

Quoted 1 line · On Thu, 26 Mar 2026 16:03:18 -0700 Randy Dunlap <[email protected]> wrote:

Stephen, all I can do is make recommendations. If it hurts your eyes, tough. ;)

Who's this "Stephen" you are talking to?

Quoted 3 lines · Steven Rostedt

On Thu, 26 Mar 2026 16:03:18 -0700
Randy Dunlap <[email protected]> wrote:

Quoted 3 lines

Quoted 1 line

Stephen, all I can do is make recommendations. If it hurts your eyes, tough. ;)

Who's this "Stephen" you are talking to?

Oops. I guess it's the one who accidentally wrote "he" instead of "the".
(Do you like where that period ^ is,
even though it's incorrect? :)

(Just some small comments -- take them or not.)

Quoted 38 lines · Frederic Weisbecker

nohz_full was introduced in v3.10 in 2013, which means this documentation is overdue for 13 years.

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]>

---
v2:
- Fix links and code blocks (Bagas and Sebastian)
- Isolation is not only about userspace, rephrase accordingly (Valentin)
- Paste BIOS issues suggestion from Valentin
- Include the whole rtla suite (Valentin)
- Rephrase a few details (Waiman)
- Talk about RCU induced overhead rather than slower RCU (Sebastian)

 Documentation/admin-guide/cpu-isolation.rst | 357 ++++++++++++++++++++
 Documentation/admin-guide/index.rst         |   1 +
 2 files changed, 358 insertions(+)
 create mode 100644 Documentation/admin-guide/cpu-isolation.rst

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

new file mode 100644
index 000000000000..886dec79b056
@@ -0,0 +1,357 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+=============
+CPU Isolation
+=============
+
+Introduction

Documentation/admin-guide/cpu-isolation.rst line 13

Quoted 6 lines

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

+============
+
+"CPU Isolation" means leaving a CPU exclusive to a given workload
+without any undesired code interference from the kernel.
+
+Those interferences, commonly pointed out as "noise", can be triggered

nit: "noise,"

Quoted 6 lines

Documentation/admin-guide/cpu-isolation.rst line 13

Quoted 4 lines

+"CPU Isolation" means leaving a CPU exclusive to a given workload

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

+without any undesired code interference from the kernel.
+
+Those interferences, commonly pointed out as "noise", can be triggered  

nit: "noise,"

Thanks! I have applied all your suggestions, except this one for now because I don't really understand the typo rule behind. Any hint?

Quoted 9 lines · On Wed, 1 Apr 2026 18:27:03 +0200 Frederic Weisbecker <[email protected]>…

Quoted 6 lines

Documentation/admin-guide/cpu-isolation.rst line 13

Quoted 4 lines

+"CPU Isolation" means leaving a CPU exclusive to a given workload

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

+without any undesired code interference from the kernel.
+
+Those interferences, commonly pointed out as "noise", can be triggered  

nit: "noise,"

Thanks! I have applied all your suggestions, except this one for now because I don't really understand the typo rule behind. Any hint?

So this looks to be an American English thing (placing commas within the quote), but from what I read, British English places the comma outside the quote.

Here's one case I much rather go the British English way. This also means it's only incorrect to Americans ;-)

Quoted 7 lines · On Wed, 1 Apr 2026 18:27:03 +0200 Frederic Weisbecker <[email protected]>…

Quoted 6 lines

Documentation/admin-guide/cpu-isolation.rst line 13

Quoted 4 lines

+"CPU Isolation" means leaving a CPU exclusive to a given workload

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

+without any undesired code interference from the kernel.
+
+Those interferences, commonly pointed out as "noise", can be triggered  

nit: "noise,"

Quoted 2 lines

Thanks! I have applied all your suggestions, except this one for now because I don't really understand the typo rule behind. Any hint?

So this looks to be an American English thing (placing commas within the quote), but from what I read, British English places the comma outside the quote.

Here's one case I much rather go the British English way. This also means it's only incorrect to Americans ;-)

nohz_full was introduced in v3.10 in 2013, which means this documentation is overdue for 13 years.

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]>

nohz_full was introduced in v3.10 in 2013, which means this documentation is overdue for 13 years.

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]>