[PATCH documentation 1/2] nohz1: Add documentation.

[Paul E. McKenney]

From: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>

Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Cc: Frederic Weisbecker <fweisbec@xxxxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Arjan van de Ven <arjan@xxxxxxxxxxxxxxx>
Cc: Kevin Hilman <khilman@xxxxxxxxxx>
Cc: Christoph Lameter <cl@xxxxxxxxx>
---
 Documentation/timers/NO_HZ.txt | 245 +++++++++++++++++++++++++++++++++++++++++
 1 file changed, 245 insertions(+)
 create mode 100644 Documentation/timers/NO_HZ.txt

diff --git a/Documentation/timers/NO_HZ.txt b/Documentation/timers/NO_HZ.txt
new file mode 100644
index 0000000..6b33f6b
--- /dev/null
+++ b/Documentation/timers/NO_HZ.txt
@@ -0,0 +1,245 @@
+		NO_HZ: Reducing Scheduling-Clock Ticks
+
+
+This document describes Kconfig options and boot parameters that can
+reduce the number of scheduling-clock interrupts, thereby improving energy
+efficiency and reducing OS jitter.  Reducing OS jitter is important for
+some types of computationally intensive high-performance computing (HPC)
+applications and for real-time applications.
+
+There are two major aspects of scheduling-clock interrupt reduction:
+
+1.	Idle CPUs.
+
+2.	CPUs having only one runnable task.
+
+These two cases are described in the following sections.
+
+
+IDLE CPUs
+
+If a CPU is idle, there is little point in sending it a scheduling-clock
+interrupt.  After all, the primary purpose of a scheduling-clock interrupt
+is to force a busy CPU to shift its attention among multiple duties,
+but an idle CPU by definition has no duties to shift its attention among.
+
+The CONFIG_NO_HZ=y Kconfig option causes the kernel to avoid sending
+scheduling-clock interrupts to idle CPUs, which is critically important
+both to battery-powered devices and to highly virtualized mainframes.
+A battery-powered device running a CONFIG_NO_HZ=n kernel would drain
+its battery very quickly, easily 2-3x as fast as would the same device
+running a CONFIG_NO_HZ=y kernel.  A mainframe running 1,500 OS instances
+might find that half of its CPU time was consumed by scheduling-clock
+interrupts.  In these situations, there is strong motivation to avoid
+sending scheduling-clock interrupts to idle CPUs.  That said, dyntick-idle
+mode is not free:
+
+1.	It increases the number of instructions executed on the path
+	to and from the idle loop.
+
+2.	Many architectures will place dyntick-idle CPUs into deep sleep
+	states, which further degrades from-idle transition latencies.
+
+Therefore, systems with aggressive real-time response constraints
+often run CONFIG_NO_HZ=n kernels in order to avoid degrading from-idle
+transition latencies.
+
+An idle CPU that is not receiving scheduling-clock interrupts is said to
+be "dyntick-idle", "in dyntick-idle mode", "in nohz mode", or "running
+tickless".  The remainder of this document will use "dyntick-idle mode".
+
+There is also a boot parameter "nohz=" that can be used to disable
+dyntick-idle mode in CONFIG_NO_HZ=y kernels by specifying "nohz=off".
+By default, CONFIG_NO_HZ=y kernels boot with "nohz=on", enabling
+dyntick-idle mode.
+
+
+CPUs WITH ONLY ONE RUNNABLE TASK
+
+If a CPU has only one runnable task, there is again little point in
+sending it a scheduling-clock interrupt because there is nowhere else
+for a CPU with but one runnable task to shift its attention to.
+
+The CONFIG_NO_HZ_EXTENDED=y Kconfig option causes the kernel to avoid
+sending scheduling-clock interrupts to CPUs with a single runnable task,
+and such CPUs are said to be "adaptive-ticks CPUs".  This is important
+for applications with aggressive real-time response constraints because
+it allows them to improve their worst-case response times by the maximum
+duration of a scheduling-clock interrupt.  It is also important for
+computationally intensive iterative workloads with short iterations:  If
+any CPU is delayed during a given iteration, all the other CPUs will be
+forced to wait idle while the delayed CPU finished.  Thus, the delay is
+multiplied by one less than the number of CPUs.  In these situations,
+there is again strong motivation to avoid sending scheduling-clock
+interrupts.
+
+The "nohz_extended=" boot parameter specifies which CPUs are to be
+adaptive-ticks CPUs.  For example, "nohz_extended=1,6-8" says that CPUs
+1, 6, 7, and 8 are to be adaptive-ticks CPUs.  By default, no CPUs will
+be adaptive-ticks CPUs.  Note that you are prohibited from marking all
+of the CPUs as adaptive-tick CPUs:  At least one non-adaptive-tick CPU
+must remain online to handle timekeeping tasks in order to ensure that
+gettimeofday() returns sane values on adaptive-tick CPUs.
+
+Transitioning to kernel mode does not automatically force that CPU out
+of adaptive-ticks mode.  The CPU will exit adaptive-ticks mode only if
+needed, for example, if that CPU enqueues an RCU callback.
+
+Just as with dyntick-idle mode, the benefits of adaptive-tick mode do
+not come for free:
+
+1.	CONFIG_NO_HZ_EXTENDED depends on CONFIG_NO_HZ, so you cannot run
+	adaptive ticks without also running dyntick idle.  This dependency
+	of CONFIG_NO_HZ_EXTENDED on CONFIG_NO_HZ extends down into the
+	implementation.  Therefore, all of the costs of CONFIG_NO_HZ
+	are also incurred by CONFIG_NO_HZ_EXTENDED.
+
+2.	The user/kernel transitions are slightly more expensive due
+	to the need to inform kernel subsystems (such as RCU) about
+	the change in mode.
+
+3.	POSIX CPU timers on adaptive-tick CPUs may fire late (or even
+	not at all) because they currently rely on scheduling-tick
+	interrupts.  This will likely be fixed in one of two ways: (1)
+	Prevent CPUs with POSIX CPU timers from entering adaptive-tick
+	mode, or (2) Use hrtimers or other adaptive-ticks-immune mechanism
+	to cause the POSIX CPU timer to fire properly.
+
+4.	If there are more perf events pending than the hardware can
+	accommodate, they are normally round-robined so as to collect
+	all of them over time.  Adaptive-tick mode may prevent this
+	round-robining from happening.  This will likely be fixed by
+	preventing CPUs with large numbers of perf events pending from
+	entering adaptive-tick mode.
+
+5.	Scheduler statistics for adaptive-idle CPUs may be computed
+	slightly differently than those for non-adaptive-idle CPUs.
+	This may in turn perturb load-balancing of real-time tasks.
+
+6.	The LB_BIAS scheduler feature is disabled by adaptive ticks.
+
+Although improvements are expected over time, adaptive ticks is quite
+useful for many types of real-time and compute-intensive applications.
+However, the drawbacks listed above mean that adaptive ticks should not
+(yet) be enabled by default.
+
+
+RCU IMPLICATIONS
+
+There are situations in which idle CPUs cannot be permitted to
+enter either dyntick-idle mode or adaptive-tick mode, the most
+familiar being the case where that CPU has RCU callbacks pending.
+
+The CONFIG_RCU_FAST_NO_HZ=y Kconfig option may be used to cause such
+CPUs to enter dyntick-idle mode or adaptive-tick mode anyway, though a
+timer will awaken these CPUs every four jiffies in order to ensure that
+the RCU callbacks are processed in a timely fashion.
+
+Another approach is to offload RCU callback processing to "rcuo" kthreads
+using the CONFIG_RCU_NOCB_CPU=y.  The specific CPUs to offload may be
+selected via several methods:
+
+1.	One of three mutually exclusive Kconfig options specify a
+	build-time default for the CPUs to offload:
+
+	a.	The RCU_NOCB_CPU_NONE=y Kconfig option results in
+		no CPUs being offloaded.
+
+	b.	The RCU_NOCB_CPU_ZERO=y Kconfig option causes CPU 0 to
+		be offloaded.
+
+	c.	The RCU_NOCB_CPU_ALL=y Kconfig option causes all CPUs
+		to be offloaded.  Note that the callbacks will be
+		offloaded to "rcuo" kthreads, and that those kthreads
+		will in fact run on some CPU.  However, this approach
+		gives fine-grained control on exactly which CPUs the
+		callbacks run on, the priority that they run at (including
+		the default of SCHED_OTHER), and it further allows
+		this control to be varied dynamically at runtime.
+
+2.	The "rcu_nocbs=" kernel boot parameter, which takes a comma-separated
+	list of CPUs and CPU ranges, for example, "1,3-5" selects CPUs 1,
+	3, 4, and 5.  The specified CPUs will be offloaded in addition
+	to any CPUs specified as offloaded by RCU_NOCB_CPU_ZERO or
+	RCU_NOCB_CPU_ALL.
+
+The offloaded CPUs never have RCU callbacks queued, and therefore RCU
+never prevents offloaded CPUs from entering either dyntick-idle mode or
+adaptive-tick mode.  That said, note that it is up to userspace to
+pin the "rcuo" kthreads to specific CPUs if desired.  Otherwise, the
+scheduler will decide where to run them, which might or might not be
+where you want them to run.
+
+
+KNOWN ISSUES
+
+o	Dyntick-idle slows transitions to and from idle slightly.
+	In practice, this has not been a problem except for the most
+	aggressive real-time workloads, which have the option of disabling
+	dyntick-idle mode, an option that most of them take.  However,
+	some workloads will no doubt want to use adaptive ticks to
+	eliminate scheduling-clock-tick latencies.  Here are some
+	options for these workloads:
+
+	a.	Use PMQOS from userspace to inform the kernel of your
+		latency requirements (preferred).
+
+	b.	On x86 systems, use the "idle=mwait" boot parameter.
+
+	c.	On x86 systems, use the "intel_idle.max_cstate=" to limit
+	`	the maximum depth C-state depth.
+
+	d.	On x86 systems, use the "idle=poll" boot parameter.
+		However, please note that use of this parameter can cause
+		your CPU to overheat, which may cause thermal throttling
+		to degrade your latencies -- and that this degradation can
+		be even worse than that of dyntick-idle.  Furthermore,
+		this parameter effectively disables Turbo Mode on Intel
+		CPUs, which can significantly reduce maximum performance.
+
+o	Adaptive-ticks slows user/kernel transitions slightly.
+	This is not expected to be a problem for computational-intensive
+	workloads, which have few such transitions.  Careful benchmarking
+	will be required to determine whether or not other workloads
+	are significantly affected by this effect.
+
+o	Adaptive-ticks does not do anything unless there is only one
+	runnable task for a given CPU, even though there are a number
+	of other situations where the scheduling-clock tick is not
+	needed.  To give but one example, consider a CPU that has one
+	runnable high-priority SCHED_FIFO task and an arbitrary number
+	of low-priority SCHED_OTHER tasks.  In this case, the CPU is
+	required to run the SCHED_FIFO task until either it blocks or
+	some other higher-priority task awakens on (or is assigned to)
+	this CPU, so there is no point in sending a scheduling-clock
+	interrupt to this CPU.	However, the current implementation
+	prohibits CPU with a single runnable SCHED_FIFO task and multiple
+	runnable SCHED_OTHER tasks from entering adaptive-ticks mode,
+	even though it would be correct to allow it to do so.
+
+	Better handling of these sorts of situations is future work.
+
+o	A reboot is required to reconfigure both adaptive idle and RCU
+	callback offloading.  Runtime reconfiguration could be provided
+	if needed, however, due to the complexity of reconfiguring RCU
+	at runtime, there would need to be an earthshakingly good reason.
+	Especially given the option of simply offloading RCU callbacks
+	from all CPUs.
+
+o	Additional configuration is required to deal with other sources
+	of OS jitter, including interrupts and system-utility tasks
+	and processes.  This configuration normally involves binding
+	interrupts and tasks to particular CPUs.
+
+o	Some sources of OS jitter can currently be eliminated only by
+	constraining the workload.  For example, the only way to eliminate
+	OS jitter due to global TLB shootdowns is to avoid the unmapping
+	operations (such as kernel module unload operations) that result
+	in these shootdowns.  For another example, page faults and TLB
+	misses can be reduced (and in some cases eliminated) by using
+	huge pages and by constraining the amount of memory used by the
+	application.
+
+o	Unless all CPUs are idle, at least one CPU must keep the
+	scheduling-clock interrupt going in order to support accurate
+	timekeeping.
-- 
1.8.1.5

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