Re: [PATCH] nohz1: Documentation

[Paul E. McKenney]

On Wed, Mar 20, 2013 at 07:32:18PM -0400, Steven Rostedt wrote:
> On Mon, 2013-03-18 at 15:25 -0700, Paul E. McKenney wrote:
> 
> > ------------------------------------------------------------------------
> > 
> > 		NO_HZ: Reducing Scheduling-Clock Ticks
> > 
> > 
> > This document covers Kconfig options and boot parameters used to reduce
> > the number of scheduling-clock interrupts.  These reductions can be
> > helpful in improving energy efficiency and in reducing "OS jitter",
> > the latter being very important for some types of computationally
> > intensive high-performance computing (HPC) applications and for real-time
> > applications.
> > 
> > Within the Linux kernel, 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=n kernel.  A mainframe running 1,500 OS instances could
> 
> So a device running CONFIG_NO_HZ=n would drain its battery 2-3x faster
> than the
> same device running CONFIG_NO_HZ=n ?
> 
> :-)

Good catch, fixed!

That said, there are two solutions as stated -- either the battery drains
immediately, or it takes infinitely long to drain.  ;-)

> > easily find that half of its CPU time was consumed by scheduling-clock
> > interrupts.  In these situations, there is therefore 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.  Recall that the primary
> > purpose of a scheduling-clock interrupt is to force a busy CPU to
> > shift its attention among many things requiring its attention -- and
> > there is nowhere else for a CPU with but one runnable task to shift its
> > attention to.
> > 
> > The CONFIG_NO_HZ_FULL=y Kconfig option causes the kernel to avoid
> > sending scheduling-clock interrupts to CPUs with a single runnable task.
> > 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 to CPUs that have but one runnable task that
> > is executing in user mode.
> > 
> > The "full_nohz=" boot parameter specifies which CPUs are to be
> > adaptive-ticks CPUs.  For example, "full_nohz=1,6-8" says that CPUs 1,
> 
> This is the first time you mention "adaptive-ticks". Probably should
> define it before just using it, even though one should be able to figure
> out what adaptive-ticks are, it does throw in a wrench when reading this
> if you have no idea what an "adaptive-tick" is.

Good point, changed the first sentence of this paragraph to read:

	The CONFIG_NO_HZ_FULL=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".

> > 6, 7, and 8 are to be adaptive-ticks CPUs.  By default, no CPUs will
> > be adaptive-ticks CPUs.  Not 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.
> > 
> > Note that if a given CPU is in adaptive-ticks mode while executing in
> > user mode, 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_FULL depends on CONFIG_NO_HZ, so you cannot run
> > 	adaptive ticks without also running dyntick idle.  This dependency
> > 	of CONFIG_NO_HZ_FULL 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_FULL.
> 
> Not a comment on this document, but on the implementation. As idle NO_HZ
> can hurt RT, but RT would want to have full NO_HZ, it's a shame that you
> can't have both (no idle but full). As we only care about not letting
> the CPU go into deep sleep, I wonder if it wouldn't be too hard to add
> something that keeps idle from going into nohz mode. Hmm, I think there
> may be an option to keep the CPU from going too deep into sleep. That
> may be a better approach.

Would the combination of CONFIG_NO_HZ=y, CONFIG_NO_HZ_FULL=y, and
idle=poll do the trick in this case?

If so, I do need to document it.

> > 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
> > be enabled by default across the board at the current time.
> > 
> > 
> > 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.
> 
> All CPUs don't have their RCU call backs on them? I'm a bit confused by
> this. Or is it that the scheduler picks one CPU to do call backs? Does
> this mean that to use rcu_ncbs= to be the only deciding factor, you
> select RCU_NCB_CPU_NONE?
> 
> I think this needs to be explained better.

Does this help?

	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.
> > 
> > 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.
> 
> It should be mentioned that only CPUs that are in adaptive-tick mode
> have this issue. Other CPUs are still using the tick based accounting,
> right?
> 
> > 
> > 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.
> 
> You should point out that the example does not enable adaptive-ticks.
> That point is hinted at, but not really expressed. That is, perhaps end
> the paragraph with: 
> 
> "Even though the SCHED_FIFO task is the only task running, because the
> SCHED_OTHER tasks are queued on the CPU, it currently will not enter
> adaptive tick mode."

Again, good point!

How about adding the following sentence at the end of this paragraph.

	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.
> 
> When you enable for all CPUs, how do you tell what CPUs you don't want
> the scheduler to pick for off loading? I mean, if you pick all CPUs, can
> you at run time pick which ones should always off load and which ones
> shouldn't?

I must defer to Frederic on this one.

> > 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	At least one CPU must keep the scheduling-clock interrupt going
> > 	in order to support accurate timekeeping.
> 
> Thanks for writing this up Paul!

And to many other people, including yourself, for doing the actual work!

							Thanx, Paul

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