Re: [PATCH] nohz1: Documentation

From: Steven Rostedt
Date: Wed Mar 20 2013 - 19:32:26 EST

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.
> 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 ?


> 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.
> 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.

> 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.

> 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.
> 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

I think this needs to be explained better.

> 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
> 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.
> 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,

> 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."

> 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

> 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!

-- Steve

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