OpenCloudOS-Kernel/kernel/rcu/Kconfig

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# SPDX-License-Identifier: GPL-2.0-only
#
# RCU-related configuration options
#
menu "RCU Subsystem"
config TREE_RCU
bool
default y if SMP
# Dynticks-idle tracking
select CONTEXT_TRACKING_IDLE
help
This option selects the RCU implementation that is
designed for very large SMP system with hundreds or
thousands of CPUs. It also scales down nicely to
smaller systems.
config PREEMPT_RCU
bool
default y if PREEMPTION
select TREE_RCU
help
This option selects the RCU implementation that is
designed for very large SMP systems with hundreds or
thousands of CPUs, but for which real-time response
is also required. It also scales down nicely to
smaller systems.
Select this option if you are unsure.
config TINY_RCU
bool
default y if !PREEMPTION && !SMP
help
This option selects the RCU implementation that is
designed for UP systems from which real-time response
is not required. This option greatly reduces the
memory footprint of RCU.
config RCU_EXPERT
bool "Make expert-level adjustments to RCU configuration"
default n
help
This option needs to be enabled if you wish to make
expert-level adjustments to RCU configuration. By default,
no such adjustments can be made, which has the often-beneficial
side-effect of preventing "make oldconfig" from asking you all
sorts of detailed questions about how you would like numerous
obscure RCU options to be set up.
Say Y if you need to make expert-level adjustments to RCU.
Say N if you are unsure.
config TINY_SRCU
bool
default y if TINY_RCU
help
This option selects the single-CPU non-preemptible version of SRCU.
config TREE_SRCU
bool
default y if !TINY_RCU
help
This option selects the full-fledged version of SRCU.
srcu: Create an srcu_read_lock_nmisafe() and srcu_read_unlock_nmisafe() On strict load-store architectures, the use of this_cpu_inc() by srcu_read_lock() and srcu_read_unlock() is not NMI-safe in TREE SRCU. To see this suppose that an NMI arrives in the middle of srcu_read_lock(), just after it has read ->srcu_lock_count, but before it has written the incremented value back to memory. If that NMI handler also does srcu_read_lock() and srcu_read_lock() on that same srcu_struct structure, then upon return from that NMI handler, the interrupted srcu_read_lock() will overwrite the NMI handler's update to ->srcu_lock_count, but leave unchanged the NMI handler's update by srcu_read_unlock() to ->srcu_unlock_count. This can result in a too-short SRCU grace period, which can in turn result in arbitrary memory corruption. If the NMI handler instead interrupts the srcu_read_unlock(), this can result in eternal SRCU grace periods, which is not much better. This commit therefore creates a pair of new srcu_read_lock_nmisafe() and srcu_read_unlock_nmisafe() functions, which allow SRCU readers in both NMI handlers and in process and IRQ context. It is bad practice to mix the existing and the new _nmisafe() primitives on the same srcu_struct structure. Use one set or the other, not both. Just to underline that "bad practice" point, using srcu_read_lock() at process level and srcu_read_lock_nmisafe() in your NMI handler will not, repeat NOT, work. If you do not immediately understand why this is the case, please review the earlier paragraphs in this commit log. [ paulmck: Apply kernel test robot feedback. ] [ paulmck: Apply feedback from Randy Dunlap. ] [ paulmck: Apply feedback from John Ogness. ] [ paulmck: Apply feedback from Frederic Weisbecker. ] Link: https://lore.kernel.org/all/20220910221947.171557773@linutronix.de/ Signed-off-by: Paul E. McKenney <paulmck@kernel.org> Acked-by: Randy Dunlap <rdunlap@infradead.org> # build-tested Reviewed-by: Frederic Weisbecker <frederic@kernel.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: John Ogness <john.ogness@linutronix.de> Cc: Petr Mladek <pmladek@suse.com>
2022-09-16 05:29:07 +08:00
config NEED_SRCU_NMI_SAFE
def_bool HAVE_NMI && !ARCH_HAS_NMI_SAFE_THIS_CPU_OPS && !TINY_SRCU
config TASKS_RCU_GENERIC
rcu-tasks: Add an RCU Tasks Trace to simplify protection of tracing hooks Because RCU does not watch exception early-entry/late-exit, idle-loop, or CPU-hotplug execution, protection of tracing and BPF operations is needlessly complicated. This commit therefore adds a variant of Tasks RCU that: o Has explicit read-side markers to allow finite grace periods in the face of in-kernel loops for PREEMPT=n builds. These markers are rcu_read_lock_trace() and rcu_read_unlock_trace(). o Protects code in the idle loop, exception entry/exit, and CPU-hotplug code paths. In this respect, RCU-tasks trace is similar to SRCU, but with lighter-weight readers. o Avoids expensive read-side instruction, having overhead similar to that of Preemptible RCU. There are of course downsides: o The grace-period code can send IPIs to CPUs, even when those CPUs are in the idle loop or in nohz_full userspace. This is mitigated by later commits. o It is necessary to scan the full tasklist, much as for Tasks RCU. o There is a single callback queue guarded by a single lock, again, much as for Tasks RCU. However, those early use cases that request multiple grace periods in quick succession are expected to do so from a single task, which makes the single lock almost irrelevant. If needed, multiple callback queues can be provided using any number of schemes. Perhaps most important, this variant of RCU does not affect the vanilla flavors, rcu_preempt and rcu_sched. The fact that RCU Tasks Trace readers can operate from idle, offline, and exception entry/exit in no way enables rcu_preempt and rcu_sched readers to do so. The memory ordering was outlined here: https://lore.kernel.org/lkml/20200319034030.GX3199@paulmck-ThinkPad-P72/ This effort benefited greatly from off-list discussions of BPF requirements with Alexei Starovoitov and Andrii Nakryiko. At least some of the on-list discussions are captured in the Link: tags below. In addition, KCSAN was quite helpful in finding some early bugs. Link: https://lore.kernel.org/lkml/20200219150744.428764577@infradead.org/ Link: https://lore.kernel.org/lkml/87mu8p797b.fsf@nanos.tec.linutronix.de/ Link: https://lore.kernel.org/lkml/20200225221305.605144982@linutronix.de/ Cc: Alexei Starovoitov <alexei.starovoitov@gmail.com> Cc: Andrii Nakryiko <andriin@fb.com> [ paulmck: Apply feedback from Steve Rostedt and Joel Fernandes. ] [ paulmck: Decrement trc_n_readers_need_end upon IPI failure. ] [ paulmck: Fix locking issue reported by rcutorture. ] Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
2020-03-10 10:56:53 +08:00
def_bool TASKS_RCU || TASKS_RUDE_RCU || TASKS_TRACE_RCU
help
This option enables generic infrastructure code supporting
task-based RCU implementations. Not for manual selection.
config FORCE_TASKS_RCU
bool "Force selection of TASKS_RCU"
depends on RCU_EXPERT
select TASKS_RCU
default n
help
This option force-enables a task-based RCU implementation
that uses only voluntary context switch (not preemption!),
idle, and user-mode execution as quiescent states. Not for
manual selection in most cases.
config TASKS_RCU
bool
default n
select IRQ_WORK
config FORCE_TASKS_RUDE_RCU
bool "Force selection of Tasks Rude RCU"
depends on RCU_EXPERT
select TASKS_RUDE_RCU
default n
help
This option force-enables a task-based RCU implementation
that uses only context switch (including preemption) and
user-mode execution as quiescent states. It forces IPIs and
context switches on all online CPUs, including idle ones,
so use with caution. Not for manual selection in most cases.
config TASKS_RUDE_RCU
bool
default n
select IRQ_WORK
config FORCE_TASKS_TRACE_RCU
bool "Force selection of Tasks Trace RCU"
depends on RCU_EXPERT
select TASKS_TRACE_RCU
default n
rcu-tasks: Add an RCU Tasks Trace to simplify protection of tracing hooks Because RCU does not watch exception early-entry/late-exit, idle-loop, or CPU-hotplug execution, protection of tracing and BPF operations is needlessly complicated. This commit therefore adds a variant of Tasks RCU that: o Has explicit read-side markers to allow finite grace periods in the face of in-kernel loops for PREEMPT=n builds. These markers are rcu_read_lock_trace() and rcu_read_unlock_trace(). o Protects code in the idle loop, exception entry/exit, and CPU-hotplug code paths. In this respect, RCU-tasks trace is similar to SRCU, but with lighter-weight readers. o Avoids expensive read-side instruction, having overhead similar to that of Preemptible RCU. There are of course downsides: o The grace-period code can send IPIs to CPUs, even when those CPUs are in the idle loop or in nohz_full userspace. This is mitigated by later commits. o It is necessary to scan the full tasklist, much as for Tasks RCU. o There is a single callback queue guarded by a single lock, again, much as for Tasks RCU. However, those early use cases that request multiple grace periods in quick succession are expected to do so from a single task, which makes the single lock almost irrelevant. If needed, multiple callback queues can be provided using any number of schemes. Perhaps most important, this variant of RCU does not affect the vanilla flavors, rcu_preempt and rcu_sched. The fact that RCU Tasks Trace readers can operate from idle, offline, and exception entry/exit in no way enables rcu_preempt and rcu_sched readers to do so. The memory ordering was outlined here: https://lore.kernel.org/lkml/20200319034030.GX3199@paulmck-ThinkPad-P72/ This effort benefited greatly from off-list discussions of BPF requirements with Alexei Starovoitov and Andrii Nakryiko. At least some of the on-list discussions are captured in the Link: tags below. In addition, KCSAN was quite helpful in finding some early bugs. Link: https://lore.kernel.org/lkml/20200219150744.428764577@infradead.org/ Link: https://lore.kernel.org/lkml/87mu8p797b.fsf@nanos.tec.linutronix.de/ Link: https://lore.kernel.org/lkml/20200225221305.605144982@linutronix.de/ Cc: Alexei Starovoitov <alexei.starovoitov@gmail.com> Cc: Andrii Nakryiko <andriin@fb.com> [ paulmck: Apply feedback from Steve Rostedt and Joel Fernandes. ] [ paulmck: Decrement trc_n_readers_need_end upon IPI failure. ] [ paulmck: Fix locking issue reported by rcutorture. ] Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
2020-03-10 10:56:53 +08:00
help
This option enables a task-based RCU implementation that uses
explicit rcu_read_lock_trace() read-side markers, and allows
these readers to appear in the idle loop as well as on the
CPU hotplug code paths. It can force IPIs on online CPUs,
including idle ones, so use with caution. Not for manual
selection in most cases.
config TASKS_TRACE_RCU
bool
default n
select IRQ_WORK
rcu-tasks: Add an RCU Tasks Trace to simplify protection of tracing hooks Because RCU does not watch exception early-entry/late-exit, idle-loop, or CPU-hotplug execution, protection of tracing and BPF operations is needlessly complicated. This commit therefore adds a variant of Tasks RCU that: o Has explicit read-side markers to allow finite grace periods in the face of in-kernel loops for PREEMPT=n builds. These markers are rcu_read_lock_trace() and rcu_read_unlock_trace(). o Protects code in the idle loop, exception entry/exit, and CPU-hotplug code paths. In this respect, RCU-tasks trace is similar to SRCU, but with lighter-weight readers. o Avoids expensive read-side instruction, having overhead similar to that of Preemptible RCU. There are of course downsides: o The grace-period code can send IPIs to CPUs, even when those CPUs are in the idle loop or in nohz_full userspace. This is mitigated by later commits. o It is necessary to scan the full tasklist, much as for Tasks RCU. o There is a single callback queue guarded by a single lock, again, much as for Tasks RCU. However, those early use cases that request multiple grace periods in quick succession are expected to do so from a single task, which makes the single lock almost irrelevant. If needed, multiple callback queues can be provided using any number of schemes. Perhaps most important, this variant of RCU does not affect the vanilla flavors, rcu_preempt and rcu_sched. The fact that RCU Tasks Trace readers can operate from idle, offline, and exception entry/exit in no way enables rcu_preempt and rcu_sched readers to do so. The memory ordering was outlined here: https://lore.kernel.org/lkml/20200319034030.GX3199@paulmck-ThinkPad-P72/ This effort benefited greatly from off-list discussions of BPF requirements with Alexei Starovoitov and Andrii Nakryiko. At least some of the on-list discussions are captured in the Link: tags below. In addition, KCSAN was quite helpful in finding some early bugs. Link: https://lore.kernel.org/lkml/20200219150744.428764577@infradead.org/ Link: https://lore.kernel.org/lkml/87mu8p797b.fsf@nanos.tec.linutronix.de/ Link: https://lore.kernel.org/lkml/20200225221305.605144982@linutronix.de/ Cc: Alexei Starovoitov <alexei.starovoitov@gmail.com> Cc: Andrii Nakryiko <andriin@fb.com> [ paulmck: Apply feedback from Steve Rostedt and Joel Fernandes. ] [ paulmck: Decrement trc_n_readers_need_end upon IPI failure. ] [ paulmck: Fix locking issue reported by rcutorture. ] Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
2020-03-10 10:56:53 +08:00
config RCU_STALL_COMMON
def_bool TREE_RCU
help
This option enables RCU CPU stall code that is common between
the TINY and TREE variants of RCU. The purpose is to allow
the tiny variants to disable RCU CPU stall warnings, while
making these warnings mandatory for the tree variants.
config RCU_NEED_SEGCBLIST
def_bool ( TREE_RCU || TREE_SRCU || TASKS_RCU_GENERIC )
config RCU_FANOUT
int "Tree-based hierarchical RCU fanout value"
range 2 64 if 64BIT
range 2 32 if !64BIT
depends on TREE_RCU && RCU_EXPERT
default 64 if 64BIT
default 32 if !64BIT
help
This option controls the fanout of hierarchical implementations
of RCU, allowing RCU to work efficiently on machines with
large numbers of CPUs. This value must be at least the fourth
root of NR_CPUS, which allows NR_CPUS to be insanely large.
The default value of RCU_FANOUT should be used for production
systems, but if you are stress-testing the RCU implementation
itself, small RCU_FANOUT values allow you to test large-system
code paths on small(er) systems.
Select a specific number if testing RCU itself.
Take the default if unsure.
config RCU_FANOUT_LEAF
int "Tree-based hierarchical RCU leaf-level fanout value"
range 2 64 if 64BIT && !RCU_STRICT_GRACE_PERIOD
range 2 32 if !64BIT && !RCU_STRICT_GRACE_PERIOD
range 2 3 if RCU_STRICT_GRACE_PERIOD
depends on TREE_RCU && RCU_EXPERT
default 16 if !RCU_STRICT_GRACE_PERIOD
default 2 if RCU_STRICT_GRACE_PERIOD
help
This option controls the leaf-level fanout of hierarchical
implementations of RCU, and allows trading off cache misses
against lock contention. Systems that synchronize their
scheduling-clock interrupts for energy-efficiency reasons will
want the default because the smaller leaf-level fanout keeps
lock contention levels acceptably low. Very large systems
(hundreds or thousands of CPUs) will instead want to set this
value to the maximum value possible in order to reduce the
number of cache misses incurred during RCU's grace-period
initialization. These systems tend to run CPU-bound, and thus
are not helped by synchronized interrupts, and thus tend to
skew them, which reduces lock contention enough that large
leaf-level fanouts work well. That said, setting leaf-level
fanout to a large number will likely cause problematic
lock contention on the leaf-level rcu_node structures unless
you boot with the skew_tick kernel parameter.
Select a specific number if testing RCU itself.
Select the maximum permissible value for large systems, but
please understand that you may also need to set the skew_tick
kernel boot parameter to avoid contention on the rcu_node
structure's locks.
Take the default if unsure.
config RCU_BOOST
bool "Enable RCU priority boosting"
depends on (RT_MUTEXES && PREEMPT_RCU && RCU_EXPERT) || PREEMPT_RT
default y if PREEMPT_RT
help
This option boosts the priority of preempted RCU readers that
block the current preemptible RCU grace period for too long.
This option also prevents heavy loads from blocking RCU
callback invocation.
Say Y here if you are working with real-time apps or heavy loads
Say N here if you are unsure.
config RCU_BOOST_DELAY
int "Milliseconds to delay boosting after RCU grace-period start"
range 0 3000
depends on RCU_BOOST
default 500
help
This option specifies the time to wait after the beginning of
a given grace period before priority-boosting preempted RCU
readers blocking that grace period. Note that any RCU reader
blocking an expedited RCU grace period is boosted immediately.
Accept the default if unsure.
rcu: Move expedited grace period (GP) work to RT kthread_worker Enabling CONFIG_RCU_BOOST did not reduce RCU expedited grace-period latency because its workqueues run at SCHED_OTHER, and thus can be delayed by normal processes. This commit avoids these delays by moving the expedited GP work items to a real-time-priority kthread_worker. This option is controlled by CONFIG_RCU_EXP_KTHREAD and disabled by default on PREEMPT_RT=y kernels which disable expedited grace periods after boot by unconditionally setting rcupdate.rcu_normal_after_boot=1. The results were evaluated on arm64 Android devices (6GB ram) running 5.10 kernel, and capturing trace data in critical user-level code. The table below shows the resulting order-of-magnitude improvements in synchronize_rcu_expedited() latency: ------------------------------------------------------------------------ | | workqueues | kthread_worker | Diff | ------------------------------------------------------------------------ | Count | 725 | 688 | | ------------------------------------------------------------------------ | Min Duration (ns) | 326 | 447 | 37.12% | ------------------------------------------------------------------------ | Q1 (ns) | 39,428 | 38,971 | -1.16% | ------------------------------------------------------------------------ | Q2 - Median (ns) | 98,225 | 69,743 | -29.00% | ------------------------------------------------------------------------ | Q3 (ns) | 342,122 | 126,638 | -62.98% | ------------------------------------------------------------------------ | Max Duration (ns) | 372,766,967 | 2,329,671 | -99.38% | ------------------------------------------------------------------------ | Avg Duration (ns) | 2,746,353 | 151,242 | -94.49% | ------------------------------------------------------------------------ | Standard Deviation (ns) | 19,327,765 | 294,408 | | ------------------------------------------------------------------------ The below table show the range of maximums/minimums for synchronize_rcu_expedited() latency from all experiments: ------------------------------------------------------------------------ | | workqueues | kthread_worker | Diff | ------------------------------------------------------------------------ | Total No. of Experiments | 25 | 23 | | ------------------------------------------------------------------------ | Largest Maximum (ns) | 372,766,967 | 2,329,671 | -99.38% | ------------------------------------------------------------------------ | Smallest Maximum (ns) | 38,819 | 86,954 | 124.00% | ------------------------------------------------------------------------ | Range of Maximums (ns) | 372,728,148 | 2,242,717 | | ------------------------------------------------------------------------ | Largest Minimum (ns) | 88,623 | 27,588 | -68.87% | ------------------------------------------------------------------------ | Smallest Minimum (ns) | 326 | 447 | 37.12% | ------------------------------------------------------------------------ | Range of Minimums (ns) | 88,297 | 27,141 | | ------------------------------------------------------------------------ Cc: "Paul E. McKenney" <paulmck@kernel.org> Cc: Tejun Heo <tj@kernel.org> Reported-by: Tim Murray <timmurray@google.com> Reported-by: Wei Wang <wvw@google.com> Tested-by: Kyle Lin <kylelin@google.com> Tested-by: Chunwei Lu <chunweilu@google.com> Tested-by: Lulu Wang <luluw@google.com> Signed-off-by: Kalesh Singh <kaleshsingh@google.com> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
2022-04-09 08:35:27 +08:00
config RCU_EXP_KTHREAD
bool "Perform RCU expedited work in a real-time kthread"
depends on RCU_BOOST && RCU_EXPERT
default !PREEMPT_RT && NR_CPUS <= 32
help
Use this option to further reduce the latencies of expedited
grace periods at the expense of being more disruptive.
This option is disabled by default on PREEMPT_RT=y kernels which
disable expedited grace periods after boot by unconditionally
setting rcupdate.rcu_normal_after_boot=1.
Accept the default if unsure.
config RCU_NOCB_CPU
bool "Offload RCU callback processing from boot-selected CPUs"
depends on TREE_RCU
depends on RCU_EXPERT || NO_HZ_FULL
default n
help
Use this option to reduce OS jitter for aggressive HPC or
real-time workloads. It can also be used to offload RCU
callback invocation to energy-efficient CPUs in battery-powered
asymmetric multiprocessors. The price of this reduced jitter
is that the overhead of call_rcu() increases and that some
workloads will incur significant increases in context-switch
rates.
This option offloads callback invocation from the set of CPUs
specified at boot time by the rcu_nocbs parameter. For each
such CPU, a kthread ("rcuox/N") will be created to invoke
callbacks, where the "N" is the CPU being offloaded, and where
the "x" is "p" for RCU-preempt (PREEMPTION kernels) and "s" for
RCU-sched (!PREEMPTION kernels). Nothing prevents this kthread
from running on the specified CPUs, but (1) the kthreads may be
preempted between each callback, and (2) affinity or cgroups can
be used to force the kthreads to run on whatever set of CPUs is
desired.
Say Y here if you need reduced OS jitter, despite added overhead.
Say N here if you are unsure.
config RCU_NOCB_CPU_DEFAULT_ALL
bool "Offload RCU callback processing from all CPUs by default"
depends on RCU_NOCB_CPU
default n
help
Use this option to offload callback processing from all CPUs
by default, in the absence of the rcu_nocbs or nohz_full boot
parameter. This also avoids the need to use any boot parameters
to achieve the effect of offloading all CPUs on boot.
Say Y here if you want offload all CPUs by default on boot.
Say N here if you are unsure.
rcu/nocb: Add option to opt rcuo kthreads out of RT priority This commit introduces a RCU_NOCB_CPU_CB_BOOST Kconfig option that prevents rcuo kthreads from running at real-time priority, even in kernels built with RCU_BOOST. This capability is important to devices needing low-latency (as in a few milliseconds) response from expedited RCU grace periods, but which are not running a classic real-time workload. On such devices, permitting the rcuo kthreads to run at real-time priority results in unacceptable latencies imposed on the application tasks, which run as SCHED_OTHER. See for example the following trace output: <snip> <...>-60 [006] d..1 2979.028717: rcu_batch_start: rcu_preempt CBs=34619 bl=270 <snip> If that rcuop kthread were permitted to run at real-time SCHED_FIFO priority, it would monopolize its CPU for hundreds of milliseconds while invoking those 34619 RCU callback functions, which would cause an unacceptably long latency spike for many application stacks on Android platforms. However, some existing real-time workloads require that callback invocation run at SCHED_FIFO priority, for example, those running on systems with heavy SCHED_OTHER background loads. (It is the real-time system's administrator's responsibility to make sure that important real-time tasks run at a higher priority than do RCU's kthreads.) Therefore, this new RCU_NOCB_CPU_CB_BOOST Kconfig option defaults to "y" on kernels built with PREEMPT_RT and defaults to "n" otherwise. The effect is to preserve current behavior for real-time systems, but for other systems to allow expedited RCU grace periods to run with real-time priority while continuing to invoke RCU callbacks as SCHED_OTHER. As you would expect, this RCU_NOCB_CPU_CB_BOOST Kconfig option has no effect except on CPUs with offloaded RCU callbacks. Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com> Signed-off-by: Paul E. McKenney <paulmck@kernel.org> Acked-by: Joel Fernandes (Google) <joel@joelfernandes.org> Reviewed-by: Neeraj Upadhyay <quic_neeraju@quicinc.com>
2022-05-11 16:57:03 +08:00
config RCU_NOCB_CPU_CB_BOOST
bool "Offload RCU callback from real-time kthread"
depends on RCU_NOCB_CPU && RCU_BOOST
default y if PREEMPT_RT
help
Use this option to invoke offloaded callbacks as SCHED_FIFO
to avoid starvation by heavy SCHED_OTHER background load.
Of course, running as SCHED_FIFO during callback floods will
cause the rcuo[ps] kthreads to monopolize the CPU for hundreds
of milliseconds or more. Therefore, when enabling this option,
it is your responsibility to ensure that latency-sensitive
tasks either run with higher priority or run on some other CPU.
Say Y here if you want to set RT priority for offloading kthreads.
Say N here if you are building a !PREEMPT_RT kernel and are unsure.
config TASKS_TRACE_RCU_READ_MB
bool "Tasks Trace RCU readers use memory barriers in user and idle"
depends on RCU_EXPERT && TASKS_TRACE_RCU
default PREEMPT_RT || NR_CPUS < 8
help
Use this option to further reduce the number of IPIs sent
to CPUs executing in userspace or idle during tasks trace
RCU grace periods. Given that a reasonable setting of
the rcupdate.rcu_task_ipi_delay kernel boot parameter
eliminates such IPIs for many workloads, proper setting
of this Kconfig option is important mostly for aggressive
real-time installations and for battery-powered devices,
hence the default chosen above.
Say Y here if you hate IPIs.
Say N here if you hate read-side memory barriers.
Take the default if you are unsure.
config RCU_LAZY
bool "RCU callback lazy invocation functionality"
depends on RCU_NOCB_CPU
default n
help
To save power, batch RCU callbacks and flush after delay, memory
pressure, or callback list growing too big.
config RCU_DOUBLE_CHECK_CB_TIME
bool "RCU callback-batch backup time check"
depends on RCU_EXPERT
default n
help
Use this option to provide more precise enforcement of the
rcutree.rcu_resched_ns module parameter in situations where
a single RCU callback might run for hundreds of microseconds,
thus defeating the 32-callback batching used to amortize the
cost of the fine-grained but expensive local_clock() function.
This option rounds rcutree.rcu_resched_ns up to the next
jiffy, and overrides the 32-callback batching if this limit
is exceeded.
Say Y here if you need tighter callback-limit enforcement.
Say N here if you are unsure.
endmenu # "RCU Subsystem"