2019-05-19 12:07:45 +00:00
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# SPDX-License-Identifier: GPL-2.0-only
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2017-05-17 15:43:40 +00:00
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#
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# RCU-related configuration options
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#
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menu "RCU Subsystem"
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config TREE_RCU
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bool
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2019-10-15 02:55:57 +00:00
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default y if SMP
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2017-05-17 15:43:40 +00:00
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help
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This option selects the RCU implementation that is
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designed for very large SMP system with hundreds or
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thousands of CPUs. It also scales down nicely to
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smaller systems.
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config PREEMPT_RCU
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bool
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2019-07-26 21:19:38 +00:00
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default y if PREEMPTION
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2019-10-15 02:55:57 +00:00
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select TREE_RCU
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2017-05-17 15:43:40 +00:00
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help
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This option selects the RCU implementation that is
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designed for very large SMP systems with hundreds or
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thousands of CPUs, but for which real-time response
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is also required. It also scales down nicely to
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smaller systems.
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Select this option if you are unsure.
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config TINY_RCU
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bool
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2019-07-26 21:19:38 +00:00
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default y if !PREEMPTION && !SMP
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2017-05-17 15:43:40 +00:00
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help
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This option selects the RCU implementation that is
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designed for UP systems from which real-time response
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is not required. This option greatly reduces the
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memory footprint of RCU.
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config RCU_EXPERT
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bool "Make expert-level adjustments to RCU configuration"
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default n
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help
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This option needs to be enabled if you wish to make
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expert-level adjustments to RCU configuration. By default,
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no such adjustments can be made, which has the often-beneficial
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side-effect of preventing "make oldconfig" from asking you all
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sorts of detailed questions about how you would like numerous
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obscure RCU options to be set up.
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Say Y if you need to make expert-level adjustments to RCU.
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Say N if you are unsure.
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config SRCU
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bool
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help
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This option selects the sleepable version of RCU. This version
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permits arbitrary sleeping or blocking within RCU read-side critical
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sections.
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config TINY_SRCU
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bool
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default y if SRCU && TINY_RCU
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help
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This option selects the single-CPU non-preemptible version of SRCU.
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config TREE_SRCU
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bool
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default y if SRCU && !TINY_RCU
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help
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This option selects the full-fledged version of SRCU.
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2020-03-03 19:49:21 +00:00
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config TASKS_RCU_GENERIC
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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 02:56:53 +00:00
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def_bool TASKS_RCU || TASKS_RUDE_RCU || TASKS_TRACE_RCU
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2020-03-03 19:49:21 +00:00
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select SRCU
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help
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This option enables generic infrastructure code supporting
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task-based RCU implementations. Not for manual selection.
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2017-05-17 15:43:40 +00:00
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config TASKS_RCU
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2019-07-26 21:19:38 +00:00
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def_bool PREEMPTION
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2017-05-17 15:43:40 +00:00
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help
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This option enables a task-based RCU implementation that uses
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only voluntary context switch (not preemption!), idle, and
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2020-03-03 19:49:21 +00:00
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user-mode execution as quiescent states. Not for manual selection.
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2017-05-17 15:43:40 +00:00
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2020-03-03 05:06:43 +00:00
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config TASKS_RUDE_RCU
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def_bool 0
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help
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This option enables a task-based RCU implementation that uses
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only context switch (including preemption) and user-mode
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execution as quiescent states. It forces IPIs and context
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switches on all online CPUs, including idle ones, so use
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with caution.
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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 02:56:53 +00:00
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config TASKS_TRACE_RCU
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def_bool 0
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help
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This option enables a task-based RCU implementation that uses
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explicit rcu_read_lock_trace() read-side markers, and allows
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these readers to appear in the idle loop as well as on the CPU
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hotplug code paths. It can force IPIs on online CPUs, including
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idle ones, so use with caution.
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2017-05-17 15:43:40 +00:00
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config RCU_STALL_COMMON
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def_bool TREE_RCU
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help
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This option enables RCU CPU stall code that is common between
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the TINY and TREE variants of RCU. The purpose is to allow
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the tiny variants to disable RCU CPU stall warnings, while
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making these warnings mandatory for the tree variants.
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config RCU_NEED_SEGCBLIST
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def_bool ( TREE_RCU || TREE_SRCU )
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config RCU_FANOUT
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int "Tree-based hierarchical RCU fanout value"
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range 2 64 if 64BIT
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range 2 32 if !64BIT
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depends on TREE_RCU && RCU_EXPERT
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default 64 if 64BIT
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default 32 if !64BIT
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help
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This option controls the fanout of hierarchical implementations
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of RCU, allowing RCU to work efficiently on machines with
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large numbers of CPUs. This value must be at least the fourth
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root of NR_CPUS, which allows NR_CPUS to be insanely large.
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The default value of RCU_FANOUT should be used for production
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systems, but if you are stress-testing the RCU implementation
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itself, small RCU_FANOUT values allow you to test large-system
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code paths on small(er) systems.
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Select a specific number if testing RCU itself.
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Take the default if unsure.
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config RCU_FANOUT_LEAF
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int "Tree-based hierarchical RCU leaf-level fanout value"
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range 2 64 if 64BIT
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range 2 32 if !64BIT
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2019-10-15 02:55:57 +00:00
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depends on TREE_RCU && RCU_EXPERT
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2017-05-17 15:43:40 +00:00
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default 16
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help
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This option controls the leaf-level fanout of hierarchical
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implementations of RCU, and allows trading off cache misses
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against lock contention. Systems that synchronize their
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scheduling-clock interrupts for energy-efficiency reasons will
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want the default because the smaller leaf-level fanout keeps
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lock contention levels acceptably low. Very large systems
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(hundreds or thousands of CPUs) will instead want to set this
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value to the maximum value possible in order to reduce the
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number of cache misses incurred during RCU's grace-period
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initialization. These systems tend to run CPU-bound, and thus
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are not helped by synchronized interrupts, and thus tend to
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skew them, which reduces lock contention enough that large
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leaf-level fanouts work well. That said, setting leaf-level
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fanout to a large number will likely cause problematic
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lock contention on the leaf-level rcu_node structures unless
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you boot with the skew_tick kernel parameter.
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Select a specific number if testing RCU itself.
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Select the maximum permissible value for large systems, but
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please understand that you may also need to set the skew_tick
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kernel boot parameter to avoid contention on the rcu_node
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structure's locks.
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Take the default if unsure.
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config RCU_FAST_NO_HZ
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bool "Accelerate last non-dyntick-idle CPU's grace periods"
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depends on NO_HZ_COMMON && SMP && RCU_EXPERT
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default n
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help
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This option permits CPUs to enter dynticks-idle state even if
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they have RCU callbacks queued, and prevents RCU from waking
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these CPUs up more than roughly once every four jiffies (by
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default, you can adjust this using the rcutree.rcu_idle_gp_delay
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parameter), thus improving energy efficiency. On the other
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hand, this option increases the duration of RCU grace periods,
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for example, slowing down synchronize_rcu().
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Say Y if energy efficiency is critically important, and you
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don't care about increased grace-period durations.
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Say N if you are unsure.
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config RCU_BOOST
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bool "Enable RCU priority boosting"
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depends on RT_MUTEXES && PREEMPT_RCU && RCU_EXPERT
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default n
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help
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This option boosts the priority of preempted RCU readers that
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block the current preemptible RCU grace period for too long.
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This option also prevents heavy loads from blocking RCU
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2018-07-08 01:12:26 +00:00
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callback invocation.
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2017-05-17 15:43:40 +00:00
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Say Y here if you are working with real-time apps or heavy loads
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Say N here if you are unsure.
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config RCU_BOOST_DELAY
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int "Milliseconds to delay boosting after RCU grace-period start"
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range 0 3000
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depends on RCU_BOOST
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default 500
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help
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This option specifies the time to wait after the beginning of
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a given grace period before priority-boosting preempted RCU
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readers blocking that grace period. Note that any RCU reader
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blocking an expedited RCU grace period is boosted immediately.
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Accept the default if unsure.
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config RCU_NOCB_CPU
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bool "Offload RCU callback processing from boot-selected CPUs"
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depends on TREE_RCU
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2017-05-17 15:43:40 +00:00
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depends on RCU_EXPERT || NO_HZ_FULL
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default n
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help
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Use this option to reduce OS jitter for aggressive HPC or
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real-time workloads. It can also be used to offload RCU
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callback invocation to energy-efficient CPUs in battery-powered
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asymmetric multiprocessors.
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2018-07-08 01:12:26 +00:00
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This option offloads callback invocation from the set of CPUs
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specified at boot time by the rcu_nocbs parameter. For each
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such CPU, a kthread ("rcuox/N") will be created to invoke
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callbacks, where the "N" is the CPU being offloaded, and where
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2019-10-15 19:18:14 +00:00
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the "p" for RCU-preempt (PREEMPTION kernels) and "s" for RCU-sched
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(!PREEMPTION kernels). Nothing prevents this kthread from running
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2018-07-08 01:12:26 +00:00
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on the specified CPUs, but (1) the kthreads may be preempted
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between each callback, and (2) affinity or cgroups can be used
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to force the kthreads to run on whatever set of CPUs is desired.
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2017-05-17 15:43:40 +00:00
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Say Y here if you want to help to debug reduced OS jitter.
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Say N here if you are unsure.
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2020-03-19 00:16:37 +00:00
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config TASKS_TRACE_RCU_READ_MB
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bool "Tasks Trace RCU readers use memory barriers in user and idle"
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depends on RCU_EXPERT
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default PREEMPT_RT || NR_CPUS < 8
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help
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Use this option to further reduce the number of IPIs sent
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to CPUs executing in userspace or idle during tasks trace
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RCU grace periods. Given that a reasonable setting of
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the rcupdate.rcu_task_ipi_delay kernel boot parameter
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eliminates such IPIs for many workloads, proper setting
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of this Kconfig option is important mostly for aggressive
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real-time installations and for battery-powered devices,
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hence the default chosen above.
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Say Y here if you hate IPIs.
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Say N here if you hate read-side memory barriers.
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Take the default if you are unsure.
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2017-05-17 15:43:40 +00:00
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endmenu # "RCU Subsystem"
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