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Linux kernel mirror (for testing) git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
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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 (PREEMPT || PREEMPT_RT || PREEMPT_DYNAMIC) 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 !PREEMPT_RCU && !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.
config FORCE_NEED_SRCU_NMI_SAFE bool "Force selection of NEED_SRCU_NMI_SAFE" depends on !TINY_SRCU depends on RCU_EXPERT depends on ARCH_HAS_NMI_SAFE_THIS_CPU_OPS select NEED_SRCU_NMI_SAFE default n help This option forces selection of the NEED_SRCU_NMI_SAFE Kconfig option, allowing testing of srcu_read_lock_nmisafe() and srcu_read_unlock_nmisafe() on architectures (like x86) that select the ARCH_HAS_NMI_SAFE_THIS_CPU_OPS Kconfig option.
config NEED_SRCU_NMI_SAFE def_bool HAVE_NMI && !ARCH_HAS_NMI_SAFE_THIS_CPU_OPS && !TINY_SRCU
config TASKS_RCU_GENERIC def_bool TASKS_RCU || TASKS_RUDE_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 NEED_TASKS_RCU bool default n
config TASKS_RCU bool default NEED_TASKS_RCU && PREEMPTION 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 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
config TASKS_TRACE_RCU_NO_MB bool "Override RCU Tasks Trace inclusion of read-side memory barriers" depends on RCU_EXPERT && TASKS_TRACE_RCU default ARCH_WANTS_NO_INSTR help This option prevents the use of read-side memory barriers in rcu_read_lock_tasks_trace() and rcu_read_unlock_tasks_trace() even in kernels built with CONFIG_ARCH_WANTS_NO_INSTR=n, that is, in kernels that do not have noinstr set up in entry/exit code. By setting this option, you are promising to carefully review use of ftrace, BPF, and friends to ensure that no tracing operation is attached to a function that runs in that portion of the entry/exit code that RCU does not watch, that is, where rcu_is_watching() returns false. Alternatively, you might choose to never remove traces except by rebooting.
Those wishing to disable read-side memory barriers for an entire architecture can select this Kconfig option, hence the polarity.
Say Y here if you need speed and will review use of tracing. Say N here for certain esoteric testing of RCU itself. Take the default if you are unsure.
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.
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). This option also creates another kthread for each sqrt(nr_cpu_ids) CPUs ("rcuog/N", where N is the first CPU in that group to come online), which handles grace periods for its group. Nothing prevents these kthreads 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.
The sqrt(nr_cpu_ids) grouping may be overridden using the rcutree.rcu_nocb_gp_stride kernel boot parameter. This can be especially helpful for smaller numbers of CPUs, where sqrt(nr_cpu_ids) can be a bit of a blunt instrument.
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.
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 RCU_LAZY bool "RCU callback lazy invocation functionality" depends on RCU_NOCB_CPU default n help To save power, batch RCU callbacks and delay starting the corresponding grace period for multiple seconds. The grace period will be started after this delay, in case of memory pressure, or if the corresponding CPU's callback list grows too large.
These delays happen only on rcu_nocbs CPUs, that is, CPUs whose callbacks have been offloaded.
Use the rcutree.enable_rcu_lazy=0 kernel-boot parameter to globally disable these delays.
config RCU_LAZY_DEFAULT_OFF bool "Turn RCU lazy invocation off by default" depends on RCU_LAZY default n help Build the kernel with CONFIG_RCU_LAZY=y, but cause the kernel to boot with these energy-efficiency delays disabled. Use the rcutree.enable_rcu_lazy=0 kernel-boot parameter to override the this option at boot time, thus re-enabling these delays.
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"