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

[Randy Dunlap <rdunlap@infradead.org>]

On 04/11/2013 09:05 AM, Paul E. McKenney wrote:
> From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
>
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> Cc: Frederic Weisbecker <fweisbec@gmail.com>
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: Borislav Petkov <bp@alien8.de>
> Cc: Arjan van de Ven <arjan@linux.intel.com>
> Cc: Kevin Hilman <khilman@linaro.org>
> Cc: Christoph Lameter <cl@linux.com>
> ---
>   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

I would say:                                 finishes.


> +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

	prohibits a CPU or prohibits CPUs

> +	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.
>

Nicely written.

Reviewed-by: Randy Dunlap <rdunlap@infradead.org>


-- 
~Randy