[PATCH 2/5] context-tracking: Introduce CONFIG_HAVE_TIF_NOHZ

[Frederic Weisbecker]

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@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Russell King <linux@xxxxxxxxxxxxxxx>
Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
Cc: Will Deacon <will@xxxxxxxxxx>
Cc: Ralf Baechle <ralf@xxxxxxxxxxxxxx>
Cc: Paul Burton <paulburton@xxxxxxxxxx>
Cc: Benjamin Herrenschmidt <benh@xxxxxxxxxxxxxxxxxxx>
Cc: Paul Mackerras <paulus@xxxxxxxxx>
Cc: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
Cc: David S. Miller <davem@xxxxxxxxxxxxx>
---
 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