nohz_full: Update based on Sedat Dilek review

Make it more clear that there are three options, and give hints as
to which of the three is most likely to be useful in different
situations.

Reported-by: Sedat Dilek <sedat.dilek@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

Documentation/timers/NO_HZ.txt

@@ -7,21 +7,59 @@ 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 main contexts in which the number of scheduling-clock
-interrupts can be reduced compared to the old-school approach of sending
-a scheduling-clock interrupt to all CPUs every jiffy whether they need
-it or not (CONFIG_HZ_PERIODIC=y or CONFIG_NO_HZ=n for older kernels):
+There are three main ways of managing scheduling-clock interrupts
+(also known as "scheduling-clock ticks" or simply "ticks"):
 
-1.	Idle CPUs (CONFIG_NO_HZ_IDLE=y or CONFIG_NO_HZ=y for older kernels).
+1.	Never omit scheduling-clock ticks (CONFIG_HZ_PERIODIC=y or
+	CONFIG_NO_HZ=n for older kernels).  You normally will -not-
+	want to choose this option.
 
-2.	CPUs having only one runnable task (CONFIG_NO_HZ_FULL=y).
+2.	Omit scheduling-clock ticks on idle CPUs (CONFIG_NO_HZ_IDLE=y or
+	CONFIG_NO_HZ=y for older kernels).  This is the most common
+	approach, and should be the default.
 
-These two cases are described in the following two sections, followed
+3.	Omit scheduling-clock ticks on CPUs that are either idle or that
+	have only one runnable task (CONFIG_NO_HZ_FULL=y).  Unless you
+	are running realtime applications or certain types of HPC
+	workloads, you will normally -not- want this option.
+
+These three cases are described in the following three sections, followed
 by a third section on RCU-specific considerations and a fourth and final
 section listing known issues.
 
 
-IDLE CPUs
+NEVER OMIT SCHEDULING-CLOCK TICKS
+
+Very old versions of Linux from the 1990s and the very early 2000s
+are incapable of omitting scheduling-clock ticks.  It turns out that
+there are some situations where this old-school approach is still the
+right approach, for example, in heavy workloads with lots of tasks
+that use short bursts of CPU, where there are very frequent idle
+periods, but where these idle periods are also quite short (tens or
+hundreds of microseconds).  For these types of workloads, scheduling
+clock interrupts will normally be delivered any way because there
+will frequently be multiple runnable tasks per CPU.  In these cases,
+attempting to turn off the scheduling clock interrupt will have no effect
+other than increasing the overhead of switching to and from idle and
+transitioning between user and kernel execution.
+
+This mode of operation can be selected using CONFIG_HZ_PERIODIC=y (or
+CONFIG_NO_HZ=n for older kernels).
+
+However, if you are instead running a light workload with long idle
+periods, failing to omit scheduling-clock interrupts will result in
+excessive power consumption.  This is especially bad on battery-powered
+devices, where it results in extremely short battery lifetimes.  If you
+are running light workloads, you should therefore read the following
+section.
+
+In addition, if you are running either a real-time workload or an HPC
+workload with short iterations, the scheduling-clock interrupts can
+degrade your applications performance.  If this describes your workload,
+you should read the following two sections.
+
+
+OMIT SCHEDULING-CLOCK TICKS FOR 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
@@ -59,10 +97,12 @@ By default, CONFIG_NO_HZ_IDLE=y kernels boot with "nohz=on", enabling
 dyntick-idle mode.
 
 
-CPUs WITH ONLY ONE RUNNABLE TASK
+OMIT SCHEDULING-CLOCK TICKS FOR CPUs WITH ONLY ONE RUNNABLE TASK
 
 If a CPU has only one runnable task, there is little point in sending it
 a scheduling-clock interrupt because there is no other task to switch to.
+Note that omitting scheduling-clock ticks for CPUs with only one runnable
+task implies also omitting them for idle CPUs.
 
 The CONFIG_NO_HZ_FULL=y Kconfig option causes the kernel to avoid
 sending scheduling-clock interrupts to CPUs with a single runnable task,