| # SPDX-License-Identifier: GPL-2.0-only |
| # |
| # RCU-related configuration options |
| # |
| |
| menu "RCU Subsystem" |
| |
| config TREE_RCU |
| bool |
| default y if SMP |
| 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 SRCU |
| bool |
| help |
| This option selects the sleepable version of RCU. This version |
| permits arbitrary sleeping or blocking within RCU read-side critical |
| sections. |
| |
| config TINY_SRCU |
| bool |
| default y if SRCU && TINY_RCU |
| help |
| This option selects the single-CPU non-preemptible version of SRCU. |
| |
| config TREE_SRCU |
| bool |
| default y if SRCU && !TINY_RCU |
| help |
| This option selects the full-fledged version of SRCU. |
| |
| config TASKS_RCU_GENERIC |
| def_bool TASKS_RCU || TASKS_RUDE_RCU || TASKS_TRACE_RCU |
| select SRCU |
| 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 |
| 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 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). 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 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. |
| |
| endmenu # "RCU Subsystem" |
