Re: [PATCH 05/19] context_tracking: Split user tracking Kconfig

From: Paul E. McKenney
Date: Thu Mar 10 2022 - 14:43:52 EST

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On Wed, Mar 02, 2022 at 04:47:56PM +0100, Frederic Weisbecker wrote:
> Context tracking is going to be used not only to track user transitions
> but also idle/IRQs/NMIs. The user tracking part will then become a
> seperate feature. Prepare Kconfig for that.

s/seperate/separate/ # nit

> Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
> Cc: Paul E. McKenney <paulmck@xxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Neeraj Upadhyay <quic_neeraju@xxxxxxxxxxx>
> Cc: Uladzislau Rezki <uladzislau.rezki@xxxxxxxx>
> Cc: Joel Fernandes <joel@xxxxxxxxxxxxxxxxx>
> Cc: Boqun Feng <boqun.feng@xxxxxxxxx>
> Cc: Nicolas Saenz Julienne <nsaenz@xxxxxxxxxx>
> Cc: Marcelo Tosatti <mtosatti@xxxxxxxxxx>
> Cc: Xiongfeng Wang <wangxiongfeng2@xxxxxxxxxx>
> Cc: Yu Liao<liaoyu15@xxxxxxxxxx>
> Cc: Phil Auld <pauld@xxxxxxxxxx>
> Cc: Paul Gortmaker<paul.gortmaker@xxxxxxxxxxxxx>
> Cc: Alex Belits <abelits@xxxxxxxxxxx>
> ---
> .../time/context-tracking/arch-support.txt | 6 ++---
> arch/Kconfig | 4 ++--
> arch/arm/Kconfig | 2 +-
> arch/arm/kernel/entry-common.S | 4 ++--
> arch/arm/kernel/entry-header.S | 4 ++--
> arch/arm64/Kconfig | 2 +-
> arch/csky/Kconfig | 2 +-
> arch/csky/kernel/entry.S | 4 ++--
> arch/mips/Kconfig | 2 +-
> arch/powerpc/Kconfig | 2 +-
> arch/powerpc/include/asm/context_tracking.h | 2 +-
> arch/riscv/Kconfig | 2 +-
> arch/riscv/kernel/entry.S | 6 ++---
> arch/sparc/Kconfig | 2 +-
> arch/sparc/kernel/rtrap_64.S | 2 +-
> arch/x86/Kconfig | 4 ++--
> include/linux/context_tracking.h | 12 +++++-----
> include/linux/context_tracking_state.h | 4 ++--
> init/Kconfig | 4 ++--
> kernel/context_tracking.c | 6 ++++-
> kernel/sched/core.c | 2 +-
> kernel/time/Kconfig | 22 +++++++++++--------
> 22 files changed, 54 insertions(+), 46 deletions(-)
>
> diff --git a/Documentation/features/time/context-tracking/arch-support.txt b/Documentation/features/time/context-tracking/arch-support.txt
> index 4ed116c2ec39..0696fd08429e 100644
> --- a/Documentation/features/time/context-tracking/arch-support.txt
> +++ b/Documentation/features/time/context-tracking/arch-support.txt
> @@ -1,7 +1,7 @@
> #
> -# Feature name: context-tracking
> -# Kconfig: HAVE_CONTEXT_TRACKING
> -# description: arch supports context tracking for NO_HZ_FULL
> +# Feature name: user-context-tracking
> +# Kconfig: HAVE_CONTEXT_TRACKING_USER
> +# description: arch supports user context tracking for NO_HZ_FULL
> #
> -----------------------
> | arch |status|
> diff --git a/arch/Kconfig b/arch/Kconfig
> index 678a80713b21..1a3b79cfc9e3 100644
> --- a/arch/Kconfig
> +++ b/arch/Kconfig
> @@ -762,7 +762,7 @@ config HAVE_ARCH_WITHIN_STACK_FRAMES
> and similar) by implementing an inline arch_within_stack_frames(),
> which is used by CONFIG_HARDENED_USERCOPY.
>
> -config HAVE_CONTEXT_TRACKING
> +config HAVE_CONTEXT_TRACKING_USER

Just checking... This means that only some configs will see userland
execution as being different than kernel execution, correct? (Which
is the case today, to be fair.)

Thanx, Paul

> bool
> help
> Provide kernel/user boundaries probes necessary for subsystems
> @@ -773,7 +773,7 @@ config HAVE_CONTEXT_TRACKING
> protected inside rcu_irq_enter/rcu_irq_exit() but preemption or signal
> handling on irq exit still need to be protected.
>
> -config HAVE_CONTEXT_TRACKING_OFFSTACK
> +config HAVE_CONTEXT_TRACKING_USER_OFFSTACK
> bool
> help
> Architecture neither relies on exception_enter()/exception_exit()
> diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
> index fabe39169b12..2c5688f20421 100644
> --- a/arch/arm/Kconfig
> +++ b/arch/arm/Kconfig
> @@ -81,7 +81,7 @@ config ARM
> select HAVE_ARCH_TRANSPARENT_HUGEPAGE if ARM_LPAE
> select HAVE_ARM_SMCCC if CPU_V7
> select HAVE_EBPF_JIT if !CPU_ENDIAN_BE32
> - select HAVE_CONTEXT_TRACKING
> + select HAVE_CONTEXT_TRACKING_USER
> select HAVE_C_RECORDMCOUNT
> select HAVE_DEBUG_KMEMLEAK if !XIP_KERNEL
> select HAVE_DMA_CONTIGUOUS if MMU
> diff --git a/arch/arm/kernel/entry-common.S b/arch/arm/kernel/entry-common.S
> index ac86c34682bb..5be34b7fe41e 100644
> --- a/arch/arm/kernel/entry-common.S
> +++ b/arch/arm/kernel/entry-common.S
> @@ -26,7 +26,7 @@
> #include "entry-header.S"
>
> saved_psr .req r8
> -#if defined(CONFIG_TRACE_IRQFLAGS) || defined(CONFIG_CONTEXT_TRACKING)
> +#if defined(CONFIG_TRACE_IRQFLAGS) || defined(CONFIG_CONTEXT_TRACKING_USER)
> saved_pc .req r9
> #define TRACE(x...) x
> #else
> @@ -36,7 +36,7 @@ saved_pc .req lr
>
> .section .entry.text,"ax",%progbits
> .align 5
> -#if !(IS_ENABLED(CONFIG_TRACE_IRQFLAGS) || IS_ENABLED(CONFIG_CONTEXT_TRACKING) || \
> +#if !(IS_ENABLED(CONFIG_TRACE_IRQFLAGS) || IS_ENABLED(CONFIG_CONTEXT_TRACKING_USER) || \
> IS_ENABLED(CONFIG_DEBUG_RSEQ))
> /*
> * This is the fast syscall return path. We do as little as possible here,
> diff --git a/arch/arm/kernel/entry-header.S b/arch/arm/kernel/entry-header.S
> index 3af2a521e1d6..cd1ce0a9c652 100644
> --- a/arch/arm/kernel/entry-header.S
> +++ b/arch/arm/kernel/entry-header.S
> @@ -361,7 +361,7 @@
> * between user and kernel mode.
> */
> .macro ct_user_exit, save = 1
> -#ifdef CONFIG_CONTEXT_TRACKING
> +#ifdef CONFIG_CONTEXT_TRACKING_USER
> .if \save
> stmdb sp!, {r0-r3, ip, lr}
> bl user_exit_callable
> @@ -373,7 +373,7 @@
> .endm
>
> .macro ct_user_enter, save = 1
> -#ifdef CONFIG_CONTEXT_TRACKING
> +#ifdef CONFIG_CONTEXT_TRACKING_USER
> .if \save
> stmdb sp!, {r0-r3, ip, lr}
> bl user_enter_callable
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index 6978140edfa4..96e75d7fa0a3 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -169,7 +169,7 @@ config ARM64
> select HAVE_C_RECORDMCOUNT
> select HAVE_CMPXCHG_DOUBLE
> select HAVE_CMPXCHG_LOCAL
> - select HAVE_CONTEXT_TRACKING
> + select HAVE_CONTEXT_TRACKING_USER
> select HAVE_DEBUG_KMEMLEAK
> select HAVE_DMA_CONTIGUOUS
> select HAVE_DYNAMIC_FTRACE
> diff --git a/arch/csky/Kconfig b/arch/csky/Kconfig
> index 132f43f12dd8..c94cc907b828 100644
> --- a/arch/csky/Kconfig
> +++ b/arch/csky/Kconfig
> @@ -42,7 +42,7 @@ config CSKY
> select HAVE_ARCH_AUDITSYSCALL
> select HAVE_ARCH_MMAP_RND_BITS
> select HAVE_ARCH_SECCOMP_FILTER
> - select HAVE_CONTEXT_TRACKING
> + select HAVE_CONTEXT_TRACKING_USER
> select HAVE_VIRT_CPU_ACCOUNTING_GEN
> select HAVE_DEBUG_BUGVERBOSE
> select HAVE_DEBUG_KMEMLEAK
> diff --git a/arch/csky/kernel/entry.S b/arch/csky/kernel/entry.S
> index bc734d17c16f..547b4cd1b24b 100644
> --- a/arch/csky/kernel/entry.S
> +++ b/arch/csky/kernel/entry.S
> @@ -19,7 +19,7 @@
> .endm
>
> .macro context_tracking
> -#ifdef CONFIG_CONTEXT_TRACKING
> +#ifdef CONFIG_CONTEXT_TRACKING_USER
> mfcr a0, epsr
> btsti a0, 31
> bt 1f
> @@ -159,7 +159,7 @@ ret_from_exception:
> and r10, r9
> cmpnei r10, 0
> bt exit_work
> -#ifdef CONFIG_CONTEXT_TRACKING
> +#ifdef CONFIG_CONTEXT_TRACKING_USER
> jbsr user_enter_callable
> #endif
> 1:
> diff --git a/arch/mips/Kconfig b/arch/mips/Kconfig
> index 058446f01487..efcab39667ea 100644
> --- a/arch/mips/Kconfig
> +++ b/arch/mips/Kconfig
> @@ -55,7 +55,7 @@ config MIPS
> select HAVE_ARCH_TRACEHOOK
> select HAVE_ARCH_TRANSPARENT_HUGEPAGE if CPU_SUPPORTS_HUGEPAGES
> select HAVE_ASM_MODVERSIONS
> - select HAVE_CONTEXT_TRACKING
> + select HAVE_CONTEXT_TRACKING_USER
> select HAVE_TIF_NOHZ
> select HAVE_C_RECORDMCOUNT
> select HAVE_DEBUG_KMEMLEAK
> diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
> index b779603978e1..9a889f919fed 100644
> --- a/arch/powerpc/Kconfig
> +++ b/arch/powerpc/Kconfig
> @@ -192,7 +192,7 @@ config PPC
> select HAVE_ARCH_SECCOMP_FILTER
> select HAVE_ARCH_TRACEHOOK
> select HAVE_ASM_MODVERSIONS
> - select HAVE_CONTEXT_TRACKING if PPC64
> + select HAVE_CONTEXT_TRACKING_USER if PPC64
> select HAVE_C_RECORDMCOUNT
> select HAVE_DEBUG_KMEMLEAK
> select HAVE_DEBUG_STACKOVERFLOW
> diff --git a/arch/powerpc/include/asm/context_tracking.h b/arch/powerpc/include/asm/context_tracking.h
> index f2682b28b050..4b63931c49e0 100644
> --- a/arch/powerpc/include/asm/context_tracking.h
> +++ b/arch/powerpc/include/asm/context_tracking.h
> @@ -2,7 +2,7 @@
> #ifndef _ASM_POWERPC_CONTEXT_TRACKING_H
> #define _ASM_POWERPC_CONTEXT_TRACKING_H
>
> -#ifdef CONFIG_CONTEXT_TRACKING
> +#ifdef CONFIG_CONTEXT_TRACKING_USER
> #define SCHEDULE_USER bl schedule_user
> #else
> #define SCHEDULE_USER bl schedule
> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> index 5adcbd9b5e88..36953ec26294 100644
> --- a/arch/riscv/Kconfig
> +++ b/arch/riscv/Kconfig
> @@ -80,7 +80,7 @@ config RISCV
> select HAVE_ARCH_THREAD_STRUCT_WHITELIST
> select HAVE_ARCH_VMAP_STACK if MMU && 64BIT
> select HAVE_ASM_MODVERSIONS
> - select HAVE_CONTEXT_TRACKING
> + select HAVE_CONTEXT_TRACKING_USER
> select HAVE_DEBUG_KMEMLEAK
> select HAVE_DMA_CONTIGUOUS if MMU
> select HAVE_EBPF_JIT if MMU
> diff --git a/arch/riscv/kernel/entry.S b/arch/riscv/kernel/entry.S
> index 5fbaa7be18a2..a773526fb3cc 100644
> --- a/arch/riscv/kernel/entry.S
> +++ b/arch/riscv/kernel/entry.S
> @@ -111,7 +111,7 @@ _save_context:
> call trace_hardirqs_off
> #endif
>
> -#ifdef CONFIG_CONTEXT_TRACKING
> +#ifdef CONFIG_CONTEXT_TRACKING_USER
> /* If previous state is in user mode, call user_exit_callable(). */
> li a0, SR_PP
> and a0, s1, a0
> @@ -176,7 +176,7 @@ handle_syscall:
> */
> csrs CSR_STATUS, SR_IE
> #endif
> -#if defined(CONFIG_TRACE_IRQFLAGS) || defined(CONFIG_CONTEXT_TRACKING)
> +#if defined(CONFIG_TRACE_IRQFLAGS) || defined(CONFIG_CONTEXT_TRACKING_USER)
> /* Recover a0 - a7 for system calls */
> REG_L a0, PT_A0(sp)
> REG_L a1, PT_A1(sp)
> @@ -251,7 +251,7 @@ resume_userspace:
> andi s1, s0, _TIF_WORK_MASK
> bnez s1, work_pending
>
> -#ifdef CONFIG_CONTEXT_TRACKING
> +#ifdef CONFIG_CONTEXT_TRACKING_USER
> call user_enter_callable
> #endif
>
> diff --git a/arch/sparc/Kconfig b/arch/sparc/Kconfig
> index 1cab1b284f1a..e736120f4333 100644
> --- a/arch/sparc/Kconfig
> +++ b/arch/sparc/Kconfig
> @@ -71,7 +71,7 @@ config SPARC64
> select HAVE_DYNAMIC_FTRACE
> select HAVE_FTRACE_MCOUNT_RECORD
> select HAVE_SYSCALL_TRACEPOINTS
> - select HAVE_CONTEXT_TRACKING
> + select HAVE_CONTEXT_TRACKING_USER
> select HAVE_TIF_NOHZ
> select HAVE_DEBUG_KMEMLEAK
> select IOMMU_HELPER
> diff --git a/arch/sparc/kernel/rtrap_64.S b/arch/sparc/kernel/rtrap_64.S
> index c5fd4b450d9b..eef102765a7e 100644
> --- a/arch/sparc/kernel/rtrap_64.S
> +++ b/arch/sparc/kernel/rtrap_64.S
> @@ -15,7 +15,7 @@
> #include <asm/visasm.h>
> #include <asm/processor.h>
>
> -#ifdef CONFIG_CONTEXT_TRACKING
> +#ifdef CONFIG_CONTEXT_TRACKING_USER
> # define SCHEDULE_USER schedule_user
> #else
> # define SCHEDULE_USER schedule
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index ebe8fc76949a..fbda20f6cf08 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -182,8 +182,8 @@ config X86
> select HAVE_ASM_MODVERSIONS
> select HAVE_CMPXCHG_DOUBLE
> select HAVE_CMPXCHG_LOCAL
> - select HAVE_CONTEXT_TRACKING if X86_64
> - select HAVE_CONTEXT_TRACKING_OFFSTACK if HAVE_CONTEXT_TRACKING
> + select HAVE_CONTEXT_TRACKING_USER if X86_64
> + select HAVE_CONTEXT_TRACKING_USER_OFFSTACK if HAVE_CONTEXT_TRACKING_USER
> select HAVE_C_RECORDMCOUNT
> select HAVE_OBJTOOL_MCOUNT if STACK_VALIDATION
> select HAVE_DEBUG_KMEMLEAK
> diff --git a/include/linux/context_tracking.h b/include/linux/context_tracking.h
> index 40badd62ad56..75738f20e111 100644
> --- a/include/linux/context_tracking.h
> +++ b/include/linux/context_tracking.h
> @@ -10,7 +10,7 @@
> #include <asm/ptrace.h>
>
>
> -#ifdef CONFIG_CONTEXT_TRACKING
> +#ifdef CONFIG_CONTEXT_TRACKING_USER
> extern void context_tracking_cpu_track_user(int cpu);
>
> /* Called with interrupts disabled. */
> @@ -52,7 +52,7 @@ static inline enum ctx_state exception_enter(void)
> {
> enum ctx_state prev_ctx;
>
> - if (IS_ENABLED(CONFIG_HAVE_CONTEXT_TRACKING_OFFSTACK) ||
> + if (IS_ENABLED(CONFIG_HAVE_CONTEXT_TRACKING_USER_OFFSTACK) ||
> !context_tracking_enabled())
> return 0;
>
> @@ -65,7 +65,7 @@ static inline enum ctx_state exception_enter(void)
>
> static inline void exception_exit(enum ctx_state prev_ctx)
> {
> - if (!IS_ENABLED(CONFIG_HAVE_CONTEXT_TRACKING_OFFSTACK) &&
> + if (!IS_ENABLED(CONFIG_HAVE_CONTEXT_TRACKING_USER_OFFSTACK) &&
> context_tracking_enabled()) {
> if (prev_ctx != CONTEXT_KERNEL)
> ct_user_enter(prev_ctx);
> @@ -109,14 +109,14 @@ static inline enum ctx_state ct_state(void) { return CONTEXT_DISABLED; }
> static __always_inline bool context_tracking_guest_enter(void) { return false; }
> static inline void context_tracking_guest_exit(void) { }
>
> -#endif /* !CONFIG_CONTEXT_TRACKING */
> +#endif /* !CONFIG_CONTEXT_TRACKING_USER */
>
> #define CT_WARN_ON(cond) WARN_ON(context_tracking_enabled() && (cond))
>
> -#ifdef CONFIG_CONTEXT_TRACKING_FORCE
> +#ifdef CONFIG_CONTEXT_TRACKING_USER_FORCE
> extern void context_tracking_init(void);
> #else
> static inline void context_tracking_init(void) { }
> -#endif /* CONFIG_CONTEXT_TRACKING_FORCE */
> +#endif /* CONFIG_CONTEXT_TRACKING_USER_FORCE */
>
> #endif
> diff --git a/include/linux/context_tracking_state.h b/include/linux/context_tracking_state.h
> index 65a60d3313b0..64dbbb880378 100644
> --- a/include/linux/context_tracking_state.h
> +++ b/include/linux/context_tracking_state.h
> @@ -22,7 +22,7 @@ struct context_tracking {
> } state;
> };
>
> -#ifdef CONFIG_CONTEXT_TRACKING
> +#ifdef CONFIG_CONTEXT_TRACKING_USER
> extern struct static_key_false context_tracking_key;
> DECLARE_PER_CPU(struct context_tracking, context_tracking);
>
> @@ -50,6 +50,6 @@ static inline bool context_tracking_in_user(void) { return false; }
> static inline bool context_tracking_enabled(void) { return false; }
> static inline bool context_tracking_enabled_cpu(int cpu) { return false; }
> static inline bool context_tracking_enabled_this_cpu(void) { return false; }
> -#endif /* CONFIG_CONTEXT_TRACKING */
> +#endif /* CONFIG_CONTEXT_TRACKING_USER */
>
> #endif
> diff --git a/init/Kconfig b/init/Kconfig
> index e9119bf54b1f..22525443de90 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -498,11 +498,11 @@ config VIRT_CPU_ACCOUNTING_NATIVE
>
> config VIRT_CPU_ACCOUNTING_GEN
> bool "Full dynticks CPU time accounting"
> - depends on HAVE_CONTEXT_TRACKING
> + depends on HAVE_CONTEXT_TRACKING_USER
> depends on HAVE_VIRT_CPU_ACCOUNTING_GEN
> depends on GENERIC_CLOCKEVENTS
> select VIRT_CPU_ACCOUNTING
> - select CONTEXT_TRACKING
> + select CONTEXT_TRACKING_USER
> help
> Select this option to enable task and CPU time accounting on full
> dynticks systems. This accounting is implemented by watching every
> diff --git a/kernel/context_tracking.c b/kernel/context_tracking.c
> index 7b6643d2075d..42054841af3f 100644
> --- a/kernel/context_tracking.c
> +++ b/kernel/context_tracking.c
> @@ -22,6 +22,8 @@
> #include <linux/export.h>
> #include <linux/kprobes.h>
>
> +#ifdef CONFIG_CONTEXT_TRACKING_USER
> +
> #define CREATE_TRACE_POINTS
> #include <trace/events/context_tracking.h>
>
> @@ -222,7 +224,7 @@ void __init context_tracking_cpu_track_user(int cpu)
> initialized = true;
> }
>
> -#ifdef CONFIG_CONTEXT_TRACKING_FORCE
> +#ifdef CONFIG_CONTEXT_TRACKING_USER_FORCE
> void __init context_tracking_init(void)
> {
> int cpu;
> @@ -231,3 +233,5 @@ void __init context_tracking_init(void)
> context_tracking_cpu_track_user(cpu);
> }
> #endif
> +
> +#endif /* #ifdef CONFIG_CONTEXT_TRACKING_USER */
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 2e4ae00e52d1..e79485afb58c 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -6398,7 +6398,7 @@ void __sched schedule_idle(void)
> } while (need_resched());
> }
>
> -#if defined(CONFIG_CONTEXT_TRACKING) && !defined(CONFIG_HAVE_CONTEXT_TRACKING_OFFSTACK)
> +#if defined(CONFIG_CONTEXT_TRACKING_USER) && !defined(CONFIG_HAVE_CONTEXT_TRACKING_USER_OFFSTACK)
> asmlinkage __visible void __sched schedule_user(void)
> {
> /*
> diff --git a/kernel/time/Kconfig b/kernel/time/Kconfig
> index 27b7868b5c30..aad89cc96787 100644
> --- a/kernel/time/Kconfig
> +++ b/kernel/time/Kconfig
> @@ -111,7 +111,7 @@ config NO_HZ_FULL
> # NO_HZ_COMMON dependency
> # We need at least one periodic CPU for timekeeping
> depends on SMP
> - depends on HAVE_CONTEXT_TRACKING
> + depends on HAVE_CONTEXT_TRACKING_USER
> # VIRT_CPU_ACCOUNTING_GEN dependency
> depends on HAVE_VIRT_CPU_ACCOUNTING_GEN
> select NO_HZ_COMMON
> @@ -140,28 +140,32 @@ endchoice
> config CONTEXT_TRACKING
> bool
>
> -config CONTEXT_TRACKING_FORCE
> - bool "Force context tracking"
> - depends on CONTEXT_TRACKING
> +config CONTEXT_TRACKING_USER
> + select CONTEXT_TRACKING
> + bool
> +
> +config CONTEXT_TRACKING_USER_FORCE
> + bool "Force user context tracking"
> + depends on CONTEXT_TRACKING_USER
> default y if !NO_HZ_FULL
> help
> The major pre-requirement for full dynticks to work is to
> - support the context tracking subsystem. But there are also
> + support the user context tracking subsystem. But there are also
> other dependencies to provide in order to make the full
> dynticks working.
>
> This option stands for testing when an arch implements the
> - context tracking backend but doesn't yet fulfill all the
> + user context tracking backend but doesn't yet fulfill all the
> requirements to make the full dynticks feature working.
> Without the full dynticks, there is no way to test the support
> - for context tracking and the subsystems that rely on it: RCU
> + for user context tracking and the subsystems that rely on it: RCU
> userspace extended quiescent state and tickless cputime
> accounting. This option copes with the absence of the full
> - dynticks subsystem by forcing the context tracking on all
> + dynticks subsystem by forcing the user context tracking on all
> CPUs in the system.
>
> Say Y only if you're working on the development of an
> - architecture backend for the context tracking.
> + architecture backend for the user context tracking.
>
> Say N otherwise, this option brings an overhead that you
> don't want in production.
> --
> 2.25.1
>

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