| # SPDX-License-Identifier: GPL-2.0-only |
| # |
| # RCU-related configuration options |
| # |
| |
| menu "RCU Subsystem" |
| |
| config TREE_RCU |
| bool |
| default y if !PREEMPT && 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 PREEMPT |
| 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 && !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 |
| def_bool PREEMPT |
| select SRCU |
| help |
| This option enables a task-based RCU implementation that uses |
| only voluntary context switch (not preemption!), idle, and |
| user-mode execution as quiescent states. |
| |
| config RCU_STALL_COMMON |
| def_bool ( TREE_RCU || PREEMPT_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 || PREEMPT_RCU || TREE_SRCU ) |
| |
| config RCU_FANOUT |
| int "Tree-based hierarchical RCU fanout value" |
| range 2 64 if 64BIT |
| range 2 32 if !64BIT |
| depends on (TREE_RCU || PREEMPT_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 |
| range 2 32 if !64BIT |
| depends on (TREE_RCU || PREEMPT_RCU) && RCU_EXPERT |
| default 16 |
| 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_FAST_NO_HZ |
| bool "Accelerate last non-dyntick-idle CPU's grace periods" |
| depends on NO_HZ_COMMON && SMP && RCU_EXPERT |
| default n |
| help |
| This option permits CPUs to enter dynticks-idle state even if |
| they have RCU callbacks queued, and prevents RCU from waking |
| these CPUs up more than roughly once every four jiffies (by |
| default, you can adjust this using the rcutree.rcu_idle_gp_delay |
| parameter), thus improving energy efficiency. On the other |
| hand, this option increases the duration of RCU grace periods, |
| for example, slowing down synchronize_rcu(). |
| |
| Say Y if energy efficiency is critically important, and you |
| don't care about increased grace-period durations. |
| |
| Say N if you are unsure. |
| |
| config RCU_BOOST |
| bool "Enable RCU priority boosting" |
| depends on RT_MUTEXES && PREEMPT_RCU && RCU_EXPERT |
| default n |
| 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_NOCB_CPU |
| bool "Offload RCU callback processing from boot-selected CPUs" |
| depends on TREE_RCU || PREEMPT_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. |
| |
| 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 "p" for RCU-preempt (PREEMPT kernels) and "s" for RCU-sched |
| (!PREEMPT 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 want to help to debug reduced OS jitter. |
| Say N here if you are unsure. |
| |
| endmenu # "RCU Subsystem" |
