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Subject[PATCH 2/5] context-tracking: Introduce CONFIG_HAVE_TIF_NOHZ
Date
A few archs (x86, arm, arm64) don't rely anymore on TIF_NOHZ to call
into context tracking on user entry/exit but instead use static keys
(or not) to optimize those calls. Ideally every arch should migrate to
that behaviour in the long run.

Settle a config option to let those archs remove their TIF_NOHZ
definitions.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Russell King <linux@armlinux.org.uk>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Paul Burton <paulburton@kernel.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: David S. Miller <davem@davemloft.net>
---
arch/Kconfig | 16 +++++++++++-----
arch/arm/Kconfig | 1 +
arch/arm64/Kconfig | 1 +
arch/mips/Kconfig | 1 +
arch/powerpc/Kconfig | 1 +
arch/sparc/Kconfig | 1 +
arch/x86/Kconfig | 1 +
kernel/context_tracking.c | 2 ++
8 files changed, 19 insertions(+), 5 deletions(-)

diff --git a/arch/Kconfig b/arch/Kconfig
index 98de654b79b3..dbf420a9f87b 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -540,11 +540,17 @@ config HAVE_CONTEXT_TRACKING
help
Provide kernel/user boundaries probes necessary for subsystems
that need it, such as userspace RCU extended quiescent state.
- Syscalls need to be wrapped inside user_exit()-user_enter() through
- the slow path using TIF_NOHZ flag. Exceptions handlers must be
- wrapped as well. Irqs are already protected inside
- rcu_irq_enter/rcu_irq_exit() but preemption or signal handling on
- irq exit still need to be protected.
+ Syscalls need to be wrapped inside user_exit()-user_enter(), either
+ optimized behind static key or through the slow path using TIF_NOHZ
+ flag. Exceptions handlers must be wrapped as well. Irqs are already
+ protected inside rcu_irq_enter/rcu_irq_exit() but preemption or signal
+ handling on irq exit still need to be protected.
+
+config HAVE_TIF_NOHZ
+ bool
+ help
+ Arch relies on TIF_NOHZ and syscall slow path to implement context
+ tracking calls to user_enter()/user_exit().

config HAVE_VIRT_CPU_ACCOUNTING
bool
diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index 97864aabc2a6..38b764cae559 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -72,6 +72,7 @@ config ARM
select HAVE_ARM_SMCCC if CPU_V7
select HAVE_EBPF_JIT if !CPU_ENDIAN_BE32
select HAVE_CONTEXT_TRACKING
+ select HAVE_TIF_NOHZ
select HAVE_COPY_THREAD_TLS
select HAVE_C_RECORDMCOUNT
select HAVE_DEBUG_KMEMLEAK if !XIP_KERNEL
diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 0b30e884e088..5c945fa3df26 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -140,6 +140,7 @@ config ARM64
select HAVE_CMPXCHG_DOUBLE
select HAVE_CMPXCHG_LOCAL
select HAVE_CONTEXT_TRACKING
+ select HAVE_TIF_NOHZ
select HAVE_COPY_THREAD_TLS
select HAVE_DEBUG_BUGVERBOSE
select HAVE_DEBUG_KMEMLEAK
diff --git a/arch/mips/Kconfig b/arch/mips/Kconfig
index 797d7f1ad5fe..2589d4760e45 100644
--- a/arch/mips/Kconfig
+++ b/arch/mips/Kconfig
@@ -51,6 +51,7 @@ config MIPS
select HAVE_ASM_MODVERSIONS
select HAVE_CBPF_JIT if !64BIT && !CPU_MICROMIPS
select HAVE_CONTEXT_TRACKING
+ select HAVE_TIF_NOHZ
select HAVE_COPY_THREAD_TLS
select HAVE_C_RECORDMCOUNT
select HAVE_DEBUG_KMEMLEAK
diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index 497b7d0b2d7e..6f40af294685 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig
@@ -182,6 +182,7 @@ config PPC
select HAVE_STACKPROTECTOR if PPC64 && $(cc-option,-mstack-protector-guard=tls -mstack-protector-guard-reg=r13)
select HAVE_STACKPROTECTOR if PPC32 && $(cc-option,-mstack-protector-guard=tls -mstack-protector-guard-reg=r2)
select HAVE_CONTEXT_TRACKING if PPC64
+ select HAVE_TIF_NOHZ if PPC64
select HAVE_COPY_THREAD_TLS
select HAVE_DEBUG_KMEMLEAK
select HAVE_DEBUG_STACKOVERFLOW
diff --git a/arch/sparc/Kconfig b/arch/sparc/Kconfig
index c1dd6dd642f4..9cc9ab04bd99 100644
--- a/arch/sparc/Kconfig
+++ b/arch/sparc/Kconfig
@@ -71,6 +71,7 @@ config SPARC64
select HAVE_FTRACE_MCOUNT_RECORD
select HAVE_SYSCALL_TRACEPOINTS
select HAVE_CONTEXT_TRACKING
+ select HAVE_TIF_NOHZ
select HAVE_DEBUG_KMEMLEAK
select IOMMU_HELPER
select SPARSE_IRQ
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index beea77046f9b..549eed3460c9 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -211,6 +211,7 @@ config X86
select HAVE_STACK_VALIDATION if X86_64
select HAVE_RSEQ
select HAVE_SYSCALL_TRACEPOINTS
+ select HAVE_TIF_NOHZ if X86_64
select HAVE_UNSTABLE_SCHED_CLOCK
select HAVE_USER_RETURN_NOTIFIER
select HAVE_GENERIC_VDSO
diff --git a/kernel/context_tracking.c b/kernel/context_tracking.c
index 0296b4bda8f1..ce430885c26c 100644
--- a/kernel/context_tracking.c
+++ b/kernel/context_tracking.c
@@ -198,11 +198,13 @@ void __init context_tracking_cpu_set(int cpu)
if (initialized)
return;

+#ifdef CONFIG_HAVE_TIF_NOHZ
/*
* Set TIF_NOHZ to init/0 and let it propagate to all tasks through fork
* This assumes that init is the only task at this early boot stage.
*/
set_tsk_thread_flag(&init_task, TIF_NOHZ);
+#endif
WARN_ON_ONCE(!tasklist_empty());

initialized = true;
--
2.25.0
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