context-tracking: Introduce CONFIG_HAVE_TIF_NOHZ

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>

diff --git a/arch/Kconfig b/arch/Kconfig
index 98de654..dbf420a 100644 (file)
--- 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 97864aa..38b764c 100644 (file)
--- 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 0b30e88..5c945fa 100644 (file)
--- 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 797d7f1..2589d47 100644 (file)
--- 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 497b7d0..6f40af2 100644 (file)
--- 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 c1dd6dd..9cc9ab0 100644 (file)
--- 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 beea770..549eed3 100644 (file)
--- 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 0296b4b..ce43088 100644 (file)
--- 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;